Biology Questionnaire

PhysiologyPhysiology
SIXTH EDITION
LINDA S. COSTANZO, PhD
Professor of Physiology and Biophysics
Virginia Commonwealth University School of Medicine
Richmond, Virginia
1600 John F. Kennedy Blvd.
Ste 1800
Philadelphia, PA 19103-2899
PHYSIOLOGY, SIXTH EDITION
Copyright
ISBN: 978-0-323-47881-6
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Notices
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Previous editions copyrighted 2014, 2010, 2006, 2002, and 1998.
Library of Congress Cataloging-in-Publication Data
Names: Costanzo, Linda S., 1947- author.
Title: Physiology / Linda S. Costanzo.
Other titles: Physiology (Elsevier)
Description: Sixth edition. | Philadelphia, PA : Elsevier, [2018] | Includes index.
Identifiers: LCCN 2017002153 | ISBN 9780323478816 (pbk.)
Subjects: | MESH: Physiological Phenomena | Physiology
Classification: LCC QP31.2 | NLM QT 104 | DDC 612–dc23
LC record available at https://lccn.loc.gov/2017002153
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Cover image: Laguna Design/Nerve Cell, abstract artwork/Getty Images
Printed in China.
Last digit is the print number:
9
8 7 6
5 4 3
2 1
To
Heinz Valtin and Arthur C. Guyton,
who have written so well for students of physiology
Richard, Dan, Rebecca, Sheila, Elise, and Max,
who make everything worthwhile
Preface
Physiology is the foundation of medical practice. A firm grasp of its principles is essential
for the medical student and the practicing physician. This book is intended for students
of medicine and related disciplines who are engaged in the study of physiology. It can
be used either as a companion to lectures and syllabi in discipline-based curricula or as
a primary source in integrated or problem-based curricula. For advanced students, the
book can serve as a reference in pathophysiology courses and in clinical clerkships.
In the sixth edition of this book, as in the previous editions, the important concepts
in physiology are covered at the organ system and cellular levels. Chapters 1 and 2
present the underlying principles of cellular physiology and the autonomic nervous
system. Chapters 3 through 10 present the major organ systems: neurophysiology and
cardiovascular, respiratory, renal, acid-base, gastrointestinal, endocrine, and reproductive physiology. The relationships between organ systems are emphasized to underscore
the integrative mechanisms for homeostasis.
This edition includes the following features designed to facilitate the study of
physiology:
♦ Text that is easy to read and concise: Clear headings orient the student to the organization and hierarchy of the material. Complex physiologic information is presented
systematically, logically, and in a stepwise manner. When a process occurs in a
specific sequence, the steps are numbered in the text and often correlate with numbers
shown in a companion figure. Bullets are used to separate and highlight the features
of a process. Rhetorical questions are posed throughout the text to anticipate the
questions that students may be asking; by first contemplating and then answering
these questions, students learn to explain difficult concepts and rationalize unexpected
or paradoxical findings. Chapter summaries provide a brief overview.
♦ Tables and illustrations that can be used in concert with the text or, because they
are designed to stand alone, as a review: The tables summarize, organize, and make
comparisons. Examples are (1) a table that compares the gastrointestinal hormones
with respect to hormone family, site of and stimuli for secretion, and hormone
actions; (2) a table that compares the pathophysiologic features of disorders of
Ca2+ homeostasis; and (3) a table that compares the features of the action potential
in different cardiac tissues. The illustrations are clearly labeled, often with main
headings, and include simple diagrams, complex diagrams with numbered steps, and
flow charts.
♦ Equations and sample problems that are integrated into the text: All terms and units
in equations are defined, and each equation is restated in words to place it in a
physiologic context. Sample problems are followed by complete numerical solutions
and explanations that guide students through the proper steps in reasoning; by following the steps provided, students acquire the skills and confidence to solve similar
or related problems.
♦ Clinical physiology presented in boxes: Each box features a fictitious patient with a
classic disorder. The clinical findings and proposed treatment are explained in terms
of underlying physiologic principles. An integrative approach to the patient is used
to emphasize the relationships between organ systems. For example, the case of type
I diabetes mellitus involves a disorder not only of the endocrine system but also of
the renal, acid-base, respiratory, and cardiovascular systems.
vii
viii • Preface
♦ Practice questions in “Challenge Yourself” sections
at the end of each chapter: Practice questions, which
are designed for short answers (a word, a phrase, or
a numerical solution), challenge the student to apply
principles and concepts in problem solving rather
than to recall isolated facts. The questions are posed
in varying formats and are given in random order.
They will be most helpful when used as a tool after
studying each chapter and without referring to the
text. In that way, the student can confirm his or her
understanding of the material and can determine
areas of weakness. Answers are provided at the end
of the book.
♦ Teaching videos on selected topics: Because students may benefit from oral explanation of complex
principles, brief teaching videos on selected topics
are included to complement the written text.
♦ Abbreviations and normal values presented in
appendices: As students refer to and use these
common abbreviations and values throughout the
book, they will find that their use becomes second
nature.
This book embodies three beliefs that I hold about
teaching: (1) even complex information can be transmitted clearly if the presentation is systematic, logical,
and stepwise; (2) the presentation can be just as effective in print as in person; and (3) beginning medical
students wish for nonreference teaching materials that
are accurate and didactically strong but without the
details that primarily concern experts. In essence, a
book can “teach” if the teacher’s voice is present, if the
material is carefully selected to include essential information, and if great care is given to logic and sequence.
This text offers a down-to-earth and professional presentation written to students and for students.
I hope that the readers of this book enjoy their study
of physiology. Those who learn its principles well will
be rewarded throughout their professional careers!
Linda S. Costanzo
Acknowledgments
I gratefully acknowledge the contributions of Elyse O’Grady, Jennifer Ehlers, and Dan
Fitzgerald at Elsevier in preparing the sixth edition of Physiology. The artist, Matthew
Chansky, revised existing figures and created new figures—all of which beautifully
complement the text.
Colleagues at Virginia Commonwealth University have faithfully answered my questions, especially Drs. Clive Baumgarten, Diomedes Logothetis, Roland Pittman, and
Raphael Witorsch. Sincere thanks also go to the medical students worldwide who have
generously written to me about their experiences with earlier editions of the book.
My husband, Richard; our children, Dan and Rebecca; our daughter-in-law, Sheila;
and our grandchildren, Elise and Max, have provided enthusiastic support and unqualified love, which give the book its spirit.
ix
CHAPTER
1
Cellular Physiology
Understanding the functions of the organ systems
Volume and Composition of Body Fluids, 1
requires profound knowledge of basic cellular mechanisms. Although each organ system differs in its overall
Characteristics of Cell Membranes, 4
function, all are undergirded by a common set of physiTransport Across Cell Membranes, 5
ologic principles.
The following basic principles of physiology are
Diffusion Potentials and Equilibrium
introduced in this chapter: body fluids, with particular
Potentials, 14
emphasis on the differences in composition of intracelResting Membrane Potential, 18
lular fluid and extracellular fluid; creation of these
concentration differences by transport processes in cell
Action Potentials, 19
membranes; the origin of the electrical potential differSynaptic and Neuromuscular Transmission, 26
ence across cell membranes, particularly in excitable
cells such as nerve and muscle; generation of action
Skeletal Muscle, 34
potentials and their propagation in excitable cells;
Smooth Muscle, 40
transmission of information between cells across synapses and the role of neurotransmitters; and the
Summary, 43
mechanisms that couple the action potentials to conChallenge Yourself, 44
traction in muscle cells.
These principles of cellular physiology constitute a
set of recurring and interlocking themes. Once these principles are understood, they can
be applied and integrated into the function of each organ system.
VOLUME AND COMPOSITION OF BODY FLUIDS
Distribution of Water in the Body Fluid Compartments
In the human body, water constitutes a high proportion of body weight. The total
amount of fluid or water is called total body water, which accounts for 50% to 70%
of body weight. For example, a 70-kilogram (kg) man whose total body water is 65%
of his body weight has 45.5 kg or 45.5 liters (L) of water (1 kg water ≈ 1 L water). In
general, total body water correlates inversely with body fat. Thus total body water is a
higher percentage of body weight when body fat is low and a lower percentage when
body fat is high. Because females have a higher percentage of adipose tissue than males,
they tend to have less body water. The distribution of water among body fluid compartments is described briefly in this chapter and in greater detail in Chapter 6.
Total body water is distributed between two major body fluid compartments: intracellular fluid (ICF) and extracellular fluid (ECF) (Fig. 1.1). The ICF is contained within the
cells and is two-thirds of total body water; the ECF is outside the cells and is one-third
of total body water. ICF and ECF are separated by the cell membranes.
ECF is further divided into two compartments: plasma and interstitial fluid. Plasma
is the fluid circulating in the blood vessels and is the smaller of the two ECF
1
2 • Physiology
TOTAL BODY WATER
Intracellular fluid
Extracellular fluid
Interstitial fluid Plasma
Cell membrane
Fig. 1.1
Capillary wall
Body fluid compartments.
subcompartments. Interstitial fluid is the fluid that
actually bathes the cells and is the larger of the two
subcompartments. Plasma and interstitial fluid are
separated by the capillary wall. Interstitial fluid is an
ultrafiltrate of plasma, formed by filtration processes
across the capillary wall. Because the capillary wall is
virtually impermeable to large molecules such as
plasma proteins, interstitial fluid contains little, if any,
protein.
The method for estimating the volume of the body
fluid compartments is presented in Chapter 6.
Composition of Body Fluid Compartments
The composition of the body fluids is not uniform. ICF
and ECF have vastly different concentrations of various
solutes. There are also certain predictable differences
in solute concentrations between plasma and interstitial
fluid that occur as a result of the exclusion of protein
from interstitial fluid.
Units for Measuring Solute Concentrations
Typically, amounts of solute are expressed in moles,
equivalents, or osmoles. Likewise, concentrations of
solutes are expressed in moles per liter (mol/L),
equivalents per liter (Eq/L), or osmoles per liter
(Osm/L). In biologic solutions, concentrations of
solutes are usually quite low and are expressed in
millimoles per liter (mmol/L), milliequivalents per liter
(mEq/L), or milliosmoles per liter (mOsm/L).
One mole is 6 × 1023 molecules of a substance. One
millimole is 1/1000 or 10−3 moles. A glucose concentration of 1 mmol/L has 1 × 10−3 moles of glucose in 1 L
of solution.
An equivalent is used to describe the amount of
charged (ionized) solute and is the number of moles
of the solute multiplied by its valence. For example,
one mole of potassium chloride (KCl) in solution dissociates into one equivalent of potassium (K+) and one
equivalent of chloride (Cl−). Likewise, one mole of
calcium chloride (CaCl2) in solution dissociates into
two equivalents of calcium (Ca2+) and two equivalents
of chloride (Cl−); accordingly, a Ca2+ concentration of
1 mmol/L corresponds to 2 mEq/L.
One osmole is the number of particles into which a
solute dissociates in solution. Osmolarity is the concentration of particles in solution expressed as osmoles
per liter. If a solute does not dissociate in solution (e.g.,
glucose), then its osmolarity is equal to its molarity. If
a solute dissociates into more than one particle in
solution (e.g., NaCl), then its osmolarity equals the
molarity multiplied by the number of particles in solution. For example, a solution containing 1 mmol/L
NaCl is 2 mOsm/L because NaCl dissociates into two
particles.
pH is a logarithmic term that is used to express
hydrogen (H+) concentration. Because the H+ concentration of body fluids is very low (e.g., 40 × 10−9 Eq/L
in arterial blood), it is more conveniently expressed as
a logarithmic term, pH. The negative sign means that
pH decreases as the concentration of H+ increases, and
pH increases as the concentration of H+ decreases. Thus
pH = − log10[H + ]
SAMPLE PROBLEM. Two men, Subject A and
Subject B, have disorders that cause excessive acid
production in the body. The laboratory reports the
acidity of Subject A’s blood in terms of [H+] and the
acidity of Subject B’s blood in terms of pH. Subject
A has an arterial [H+] of 65 × 10−9 Eq/L, and Subject
B has an arterial pH of 7.3. Which subject has the
higher concentration of H+ in his blood?
SOLUTION. To compare the acidity of the blood of
each subject, convert the [H+] for Subject A to pH
as follows:
pH = − log10[H + ]
= − log10(65 × 10−9 Eq/L)
= − log10(6.5 × 10−8 Eq/L)
log10 6.5 = 0.81
log10 10−8 = −8.0
log10 6.5 × 10−8 = 0.81 + ( −8.0) = −7.19
pH = −( −7.19) = 7.19
Thus Subject A has a blood pH of 7.19 computed
from the [H+], and Subject B has a reported blood
pH of 7.3. Subject A has a lower blood pH, reflecting
a higher [H+] and a more acidic condition.
Electroneutrality of Body Fluid Compartments
Each body fluid compartment must obey the principle
of macroscopic electroneutrality; that is, each
1—Cellular Physiology
compartment must have the same concentration, in
mEq/L, of positive charges (cations) as of negative
charges (anions). There can be no more cations than
anions, or vice versa. Even when there is a potential
difference across the cell membrane, charge balance
still is maintained in the bulk (macroscopic) solutions.
(Because potential differences are created by the separation of just a few charges adjacent to the membrane,
this small separation of charges is not enough to
measurably change bulk concentrations.)
Composition of Intracellular Fluid and
Extracellular Fluid
The compositions of ICF and ECF are strikingly different, as shown in Table 1.1. The major cation in ECF is
sodium (Na+), and the balancing anions are chloride
(Cl−) and bicarbonate (HCO3−). The major cations in
ICF are potassium (K+) and magnesium (Mg2+), and the
balancing anions are proteins and organic phosphates.
Other notable differences in composition involve Ca2+
and pH. Typically, ICF has a very low concentration of
ionized Ca2+ (≈10−7 mol/L), whereas the Ca2+ concentration in ECF is higher by approximately four orders of
magnitude. ICF is more acidic (has a lower pH) than
ECF. Thus substances found in high concentration in
ECF are found in low concentration in ICF, and vice
versa.
Remarkably, given all of the concentration differences for individual solutes, the total solute concentration (osmolarity) is the same in ICF and ECF. This
equality is achieved because water flows freely across
cell membranes. Any transient differences in osmolarity that occur between ICF and ECF are quickly dissipated by water movement into or out of cells to
reestablish the equality.
TABLE 1.1 Approximate Compositions of Extracellular
and Intracellular Fluids
Substance and Units
Na+ (mEq/L)
+
K (mEq/L)
2+
Ca , ionized (mEq/L)
−
Cl (mEq/L)
−
HCO3 (mEq/L)
pHc
Osmolarity (mOsm/L)
a
Extracellular
Fluid
140
14
4
2.5
Intracellular
Fluida
120
b
1 × 10−4
105
10
24
10
7.4
290
7.1
290
The major anions of intracellular fluid are proteins and organic
phosphates.
b
The corresponding total [Ca2+] in extracellular fluid is 5 mEq/L
or 10 mg/dL.
c
pH is −log10 of the [H+]; pH 7.4 corresponds to [H+] of 40 ×
10−9 Eq/L.
•
3
Creation of Concentration Differences
Across Cell Membranes
The differences in solute concentration across cell
membranes are created and maintained by energyconsuming transport mechanisms in the cell membranes.
The best known of these transport mechanisms is
the Na+-K+ ATPase (Na+-K+ pump), which transports
Na+ from ICF to ECF and simultaneously transports K+
from ECF to ICF. Both Na+ and K+ are transported
against their respective electrochemical gradients;
therefore an energy source, adenosine triphosphate
(ATP), is required. The Na+-K+ ATPase is responsible
for creating the large concentration gradients for Na+
and K+ that exist across cell membranes (i.e., the low
intracellular Na+ concentration and the high intracellular K+ concentration).
Similarly, the intracellular Ca2+ concentration is
maintained at a level much lower than the extracellular
Ca2+ concentration. This concentration difference is
established, in part, by a cell membrane Ca2+ ATPase
that pumps Ca2+ against its electrochemical gradient.
Like the Na+-K+ ATPase, the Ca2+ ATPase uses ATP as a
direct energy source.
In addition to the transporters that use ATP directly,
other transporters establish concentration differences
across the cell membrane by utilizing the transmembrane Na+ concentration gradient (established by the
Na+-K+ ATPase) as an energy source. These transporters
create concentration gradients for glucose, amino acids,
Ca2+, and H+ without the direct utilization of ATP.
Clearly, cell membranes have the machinery to
establish large concentration gradients. However, if
cell membranes were freely permeable to all solutes,
these gradients would quickly dissipate. Thus it is
critically important that cell membranes are not freely
permeable to all substances but, rather, have selective permeabilities that maintain the concentration
gradients established by energy-consuming transport
processes.
Directly or indirectly, the differences in composition
between ICF and ECF underlie every important physiologic function, as the following examples illustrate: (1)
The resting membrane potential of nerve and muscle
critically depends on the difference in concentration of
K+ across the cell membrane; (2) The upstroke of the
action potential of these same excitable cells depends
on the differences in Na+ concentration across the cell
membrane; (3) Excitation-contraction coupling in
muscle cells depends on the differences in Ca2+ concentration across the cell membrane and the membrane of
the sarcoplasmic reticulum (SR); and (4) Absorption of
essential nutrients depends on the transmembrane Na+
concentration gradient (e.g., glucose absorption in the
small intestine or glucose reabsorption in the renal
proximal tubule).
4 • Physiology
Concentration Differences Between
Plasma and Interstitial Fluids
As previously discussed, ECF consists of two subcompartments: interstitial fluid and plasma. The most significant difference in composition between these two
compartments is the presence of proteins (e.g., albumin)
in the plasma compartment. Plasma proteins do not
readily cross capillary walls because of their large
molecular size and therefore are excluded from interstitial fluid.
The exclusion of proteins from interstitial fluid has
secondary consequences. The plasma proteins are
negatively charged, and this negative charge causes a
redistribution of small, permeant cations and anions
across the capillary wall, called a Gibbs-Donnan equil­
ibrium. The redistribution can be explained as follows:
The plasma compartment contains the impermeant,
negatively charged proteins. Because of the requirement
for electroneutrality, the plasma compartment must
have a slightly lower concentration of small anions
(e.g., Cl−) and a slightly higher concentration of small
cations (e.g., Na+ and K+) than that of interstitial fluid.
The small concentration difference for permeant ions
is expressed in the Gibbs-Donnan ratio, which gives
the plasma concentration relative to the interstitial fluid
concentration for anions and interstitial fluid relative to
plasma for cations. For example, the Cl− concentration
in plasma is slightly less than the Cl− concentration in
interstitial fluid (due to the effect of the impermeant
plasma proteins); the Gibbs-Donnan ratio for Cl− is
0.95, meaning that [Cl−]plasma/[Cl−]interstitial fluid equals 0.95.
For Na+, the Gibbs-Donnan ratio is also 0.95, but Na+,
being positively charged, is oriented the opposite way,
and [Na+]interstitial fluid/[Na+]plasma equals 0.95. Generally,
these minor differences in concentration for small
cations and anions between plasma and interstitial
fluid are ignored.
CHARACTERISTICS OF CELL
MEMBRANES
Cell membranes are composed primarily of lipids and
proteins. The lipid component consists of phospholipids, cholesterol, and glycolipids and is responsible for
the high permeability of cell membranes to lipid-soluble
substances such as carbon dioxide, oxygen, fatty acids,
and steroid hormones. The lipid component of cell
membranes is also responsible for the low permeability
of cell membranes to water-soluble substances such as
ions, glucose, and amino acids. The protein component
of the membrane consists of transporters, enzymes,
hormone receptors, cell-surface antigens, and ion and
water channels.
Water
Oil
Water
A
Water
B
Fig. 1.2 Orientation of phospholipid molecules at oil and
water interfaces. Depicted are the orientation of phospholipid
at an oil-water interface (A) and the orientation of phospholipid
in a bilayer, as occurs in the cell membrane (B).
Phospholipid Component of Cell Membranes
Phospholipids consist of a phosphorylated glycerol
backbone (“head”) and two fatty acid “tails” (Fig. 1.2).
The glycerol backbone is hydrophilic (water soluble),
and the fatty acid tails are hydrophobic (water insoluble). Thus phospholipid molecules have both hydrophilic and hydrophobic properties and are called
amphipathic. At an oil-water interface (see Fig. 1.2A),
molecules of phospholipids form a monolayer and
orient themselves so that the glycerol backbone dissolves in the water phase and the fatty acid tails dissolve in the oil phase. In cell membranes (see Fig.
1.2B), phospholipids orient so that the lipid-soluble
fatty acid tails face each other and the water-soluble
glycerol heads point away from each other, dissolving
in the aqueous solutions of the ICF or ECF. This orientation creates a lipid bilayer.
Protein Component of Cell Membranes
Proteins in cell membranes may be either integral or
peripheral, depending on whether they span the membrane or whether they are present on only one side.
The distribution of proteins in a phospholipid bilayer
is illustrated in the fluid mosaic model, shown in
Figure 1.3.
♦ Integral membrane proteins are embedded in, and
anchored to, the cell membrane by hydrophobic
interactions. To remove an integral protein from the
cell membrane, its attachments to the lipid bilayer
must be disrupted (e.g., by detergents). Some integral proteins are transmembrane proteins, meaning
they span the lipid bilayer one or more times; thus
1—Cellular Physiology
•
5
Intracellular fluid
Lipid
bilayer
Peripheral
protein
Integral
protein
Gated ion
channel
Extracellular fluid
Fig. 1.3
TABLE 1.2
Fluid mosaic model for cell membranes.
Summary of Membrane Transport
Type of Transport
Active or Passive
CarrierMediated
Uses Metabolic
Energy
Dependent on Na+ Gradient
Simple diffusion
Passive; downhill
No
No
No
Facilitated diffusion
Passive; downhill
Yes
No
No
Primary active transport
Active; uphill
Yes
Yes; direct
No
Cotransport
a
Secondary active
Yes
Yes; indirect
Yes (solutes move in same direction
as Na+ across cell membrane)
Countertransport
Secondary activea
Yes
Yes; indirect
Yes (solutes move in opposite
direction as Na+ across cell
membrane)
Na+ is transported downhill, and one or more solutes are transported uphill.
a
transmembrane proteins are in contact with both
ECF and ICF. Examples of transmembrane integral
proteins are ligand-binding receptors (e.g., for hormones or neurotransmitters), transport proteins
(e.g., Na+-K+ ATPase), pores, ion channels, cell
adhesion molecules, and GTP-binding proteins (G
proteins). A second category of integral proteins is
embedded in the lipid bilayer of the membrane but
does not span it. A third category of integral proteins
is associated with membrane proteins but is not
embedded in the lipid bilayer.
♦ Peripheral membrane proteins are not embedded
in the membrane and are not covalently bound to
cell membrane components. They are loosely
attached to either the intracellular or extracellular
side of the cell membrane by electrostatic interactions (e.g., with integral proteins) and can be
removed with mild treatments that disrupt ionic or
hydrogen bonds. One example of a peripheral membrane protein is ankyrin, which “anchors” the
cytoskeleton of red blood cells to an integral membrane transport protein, the Cl−-HCO3− exchanger
(also called band 3 protein).
TRANSPORT ACROSS CELL
MEMBRANES
Several types of mechanisms are responsible for transport of substances across cell membranes (Table 1.2).
Substances may be transported down an electrochemical gradient (downhill) or against an electrochemical gradient (uphill). Downhill transport occurs
by diffusion, either simple or facilitated, and requires
no input of metabolic energy. Uphill transport occurs
by active transport, which may be primary or secondary. Primary and secondary active transport processes
6 • Physiology
Membrane
Transport rate
Tm
Carrier-mediated
transport
Simple
diffusion
A
Concentration
Fig. 1.4 Kinetics of carrier-mediated transport. Tm, Transport maximum.
are distinguished by their energy source. Primary active
transport requires a direct input of metabolic energy;
secondary active transport utilizes an indirect input of
metabolic energy.
Further distinctions among transport mechanisms
are based on whether the process involves a protein
carrier. Simple diffusion is the only form of transport
that is not carrier mediated. Facilitated diffusion,
primary active transport, and secondary active transport all involve integral membrane proteins and are
called carrier-mediated transport. All forms of carriermediated transport share the following three features:
saturation, stereospecificity, and competition.
♦ Saturation. Saturability is based on the concept that
carrier proteins have a limited number of binding
sites for the solute. Figure 1.4 shows the relationship
between the rate of carrier-mediated transport and
solute concentration. At low solute concentrations,
many binding sites are available and the rate of
transport increases steeply as the concentration
increases. However, at high solute concentrations,
the available binding sites become scarce and the
rate of transport levels off. Finally, when all of the
binding sites are occupied, saturation is achieved at
a point called the transport maximum, or Tm. The
kinetics of carrier-mediated transport are similar to
Michaelis-Menten enzyme kinetics—both involve
proteins with a limited number of binding sites. (The
Tm is analogous to the Vmax of enzyme kinetics.)
Tm-limited glucose transport in the proximal tubule
of the kidney is an example of saturable transport.
B
Fig. 1.5 Simple diffusion. The two solutions, A and B, are
separated by a membrane, which is permeable to the solute
(circles). Solution A initially contains a higher concentration of the
solute than does Solution B.
♦ Stereospecificity. The binding sites for solute on the
transport proteins are stereospecific. For example,
the transporter for glucose in the renal proximal
tubule recognizes and transports the natural isomer
D-glucose, but it does not recognize or transport the
unnatural isomer L-glucose. In contrast, simple diffusion does not distinguish between the two glucose
isomers because no protein carrier is involved.
♦ Competition. Although the binding sites for transported solutes are quite specific, they may recognize,
bind, and even transport chemically related solutes.
For example, the transporter for glucose is specific
for D-glucose, but it also recognizes and transports
a closely related sugar, D-galactose. Therefore the
presence of D-galactose inhibits the transport of
D-glucose by occupying some of the binding sites
and making them unavailable for glucose.
Simple Diffusion
Diffusion of Nonelectrolytes
Simple diffusion occurs as a result of the random
thermal motion of molecules, as shown in Figure 1.5.
Two solutions, A and B, are separated by a membrane
that is permeable to the solute. The solute concentration in A is initially twice that of B. The solute molecules
are in constant motion, with equal probability that a
given molecule will cross the membrane to the other
solution. However, because there are twice as many
solute molecules in Solution A as in Solution B, there
will be greater movement of molecules from A to B
than from B to A. In other words, there will be net
diffusion of the solute from A to B, which will continue
until the solute concentrations of the two solutions
become equal (although the random movement of
molecules will go on forever).
1—Cellular Physiology
•
7
Net diffusion of the solute is called flux, or flow (J),
and depends on the following variables: size of the
concentration gradient, partition coefficient, diffusion
coefficient, thickness of the membrane, and surface
area available for diffusion.
THICKNESS OF THE MEMBRANE (ΔX)
CONCENTRATION GRADIENT (CA − CB)
The greater the surface area of membrane available, the
higher the rate of diffusion. For example, lipid-soluble
gases such as oxygen and carbon dioxide have particularly high rates of diffusion across cell membranes.
These high rates can be attributed to the large surface
area for diffusion provided by the lipid component of
the membrane.
The concentration gradient across the membrane is the
driving force for net diffusion. The larger the difference
in solute concentration between Solution A and Solution B, the greater the driving force and the greater the
net diffusion. It also follows that, if the concentrations
in the two solutions are equal, there is no driving force
and no net diffusion.
PARTITION COEFFICIENT (K)
The partition coefficient, by definition, describes the
solubility of a solute in oil relative to its solubility in
water. The greater the relative solubility in oil, the
higher the partition coefficient and the more easily the
solute can dissolve in the cell membrane’s lipid bilayer.
Nonpolar solutes tend to be soluble in oil and have
high values for partition coefficient, whereas polar
solutes tend to be insoluble in oil and have low values
for partition coefficient. The partition coefficient can be
measured by adding the solute to a mixture of olive oil
and water and then measuring its concentration in the
oil phase relative to its concentration in the water
phase. Thus
The thicker the cell membrane, the greater the distance
the solute must diffuse and the lower the rate of
diffusion.
SURFACE AREA (A)
To simplify the description of diffusion, several of
the previously cited characteristics can be combined
into a single term called permeability (P). Permeability
includes the partition coefficient, the diffusion coefficient, and the membrane thickness. Thus
P=
KD
∆x
By combining several variables into permeability, the
rate of net diffusion is simplified to the following
expression:
J = PA(C A − CB )
where
Concentration in olive oil
K=
Concentration in water
DIFFUSION COEFFICIENT (D)
The diffusion coefficient depends on such characteristics as size of the solute molecule and the viscosity of
the medium. It is defined by the Stokes-Einstein equation (see later). The diffusion coefficient correlates
inversely with the molecular radius of the solute and
the viscosity of the medium. Thus small solutes in
nonviscous solutions have the largest diffusion coefficients and diffuse most readily; large solutes in viscous
solutions have the smallest diffusion coefficients and
diffuse least readily. Thus
D=
KT
6πrη
where
D = Diffusion coefficient
K = Boltzmann constant
T = Absolute temperature (K)
r = Molecular radius
η = Viscosity of the medium
J = Net rate of diffusion (mmol/s)
P = Permeability (cm/s)
A = Surface area for diffusion (cm 2 )
C A = Concentration in Solution A (mmol/L)
CB = Concentration in Solution B (mmol/L)
SAMPLE PROBLEM. Solution A and Solution B are
separated by a membrane whose permeability to
urea is 2 × 10−5 cm/s and whose surface area is
1 cm2. The concentration of urea in A is 10 mg/mL,
and the concentration of urea in B is 1 mg/mL. The
partition coefficient for urea is 10−3, as measured in
an oil-water mixture. What are the initial rate and
direction of net diffusion of urea?
SOLUTION. Note that the partition coefficient is
extraneous information because the value for permeability, which already includes the partition
coefficient, is given. Net flux can be calculated by
substituting the following values in the equation for
net diffusion: Assume that 1 mL of water = 1 cm3.
Thus
J = PA(C A − CB )
8 • Physiology
where
J = 2 × 10−5 cm/s × 1 cm 2 × (10 mg/mL − 1 mg/mL)
J = 2 × 10−5 cm/s × 1 cm 2 × (10 mg/cm3 − 1 mg/cm3 )
= 1.8 × 10−4 mg/s
The magnitude of net flux has been calculated as
1.8 × 10−4 mg/s. The direction of net flux can be
determined intuitively because net flux will occur
from the area of high concentration (Solution A) to
the area of low concentration (Solution B). Net diffusion will continue until the urea concentrations of
the two solutions become equal, at which point the
driving force will be zero.
Diffusion of Electrolytes
Thus far, the discussion concerning diffusion has
assumed that the solute is a nonelectrolyte (i.e., it is
uncharged). However, if the diffusing solute is an ion
or an electrolyte, there are two additional consequences
of the presence of charge on the solute.
First, if there is a potential difference across the
membrane, that potential difference will alter the net
rate of diffusion of a charged solute. (A potential difference does not alter the rate of diffusion of a nonelectrolyte.) For example, the diffusion of K+ ions will be
slowed if K+ is diffusing into an area of positive charge,
and it will be accelerated if K+ is diffusing into an area
of negative charge. This effect of potential difference
can either add to or negate the effects of differences in
concentrations, depending on the orientation of the
potential difference and the charge on the diffusing ion.
If the concentration gradient and the charge effect are
oriented in the same direction across the membrane,
they will combine; if they are oriented in opposite
directions, they may cancel each other out.
Second, when a charged solute diffuses down a
concentration gradient, that diffusion can itself generate a potential difference across a membrane called a
diffusion potential. The concept of diffusion potential
will be discussed more fully in a following section.
Facilitated Diffusion
Like simple diffusion, facilitated diffusion occurs down
an electrochemical potential gradient; thus it requires
no input of metabolic energy. Unlike simple diffusion,
however, facilitated diffusion uses a membrane carrier
and exhibits all the characteristics of carrier-mediated
transport: saturation, stereospecificity, and competition. At low solute concentration, facilitated diffusion
typically proceeds faster than simple diffusion (i.e., is
facilitated) because of the function of the carrier.
However, at higher concentrations, the carriers will
become saturated and facilitated diffusion will level off.
(In contrast, simple diffusion will proceed as long as
there is a concentration gradient for the solute.)
An excellent example of facilitated diffusion is the
transport of D-glucose into skeletal muscle and adipose
cells by the GLUT4 transporter. Glucose transport
can proceed as long as the blood concentration of
glucose is higher than the intracellular concentration of
glucose and as long as the carriers are not saturated.
Other monosaccharides such as D-galactose, 3-O-methyl
glucose, and phlorizin competitively inhibit the transport of glucose because they bind to transport sites on
the carrier. The competitive solute may itself be transported (e.g., D-galactose), or it may simply occupy the
binding sites and prevent the attachment of glucose
(e.g., phlorizin). As noted previously, the nonphysiologic stereoisomer, L-glucose, is not recognized by the
carrier for facilitated diffusion and therefore is not
bound or transported.
Primary Active Transport
In active transport, one or more solutes are moved
against an electrochemical potential gradient (uphill).
In other words, solute is moved from an area of low
concentration (or low electrochemical potential) to an
area of high concentration (or high electrochemical
potential). Because movement of a solute uphill is
work, metabolic energy in the form of ATP must be
provided. In the process, ATP is hydrolyzed to adenosine diphosphate (ADP) and inorganic phosphate (Pi),
releasing energy from the terminal high-energy phosphate bond of ATP. When the terminal phosphate is
released, it is transferred to the transport protein, initiating a cycle of phosphorylation and dephosphorylation. When the ATP energy source is directly coupled
to the transport process, it is called primary active
transport. Three examples of primary active transport
in physiologic systems are the Na+-K+ ATPase present
in all cell membranes, the Ca2+ ATPase present in SR
and endoplasmic reticulum, and the H+-K+ ATPase
present in gastric parietal cells and renal α-intercalated
cells.
Na+-K+ ATPase (Na+-K+ Pump)
Na+-K+ ATPase is present in the membranes of all cells.
It pumps Na+ from ICF to ECF and K+ from ECF to ICF
(Fig. 1.6). Each ion moves against its respective electrochemical gradient. The stoichiometry can vary but,
typically, for every three Na+ ions pumped out of the
cell, two K+ ions are pumped into the cell. This stoichiometry of three Na+ ions per two K+ ions means that,
for each cycle of the Na+-K+ ATPase, more positive
charge is pumped out of the cell than is pumped into
the cell. Thus the transport process is termed electrogenic because it creates a charge separation and a
potential difference. The Na+-K+ ATPase is responsible
1—Cellular Physiology
•
9
Extracellular fluid
Intracellular fluid
Intracellular fluid
E1~P
3Na+
Extracellular fluid
E2~P
Na+
ADP + Pi
ATP
3Na+
2K+
Cardiac
glycosides
2K+
ATP
K+
E1
E2
Cardiac
glycosides
Fig. 1.6 Na+-K+ pump of cell membranes. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; E, Na+-K+ ATPase; E~P, phosphorylated Na+-K+ ATPase; Pi, inorganic phosphate.
for maintaining concentration gradients for both Na+
and K+ across cell membranes, keeping the intracellular
Na+ concentration low and the intracellular K+ concentration high.
The Na+-K+ ATPase consists of α and β subunits. The
α subunit contains the ATPase activity, as well as
the binding sites for the transported ions, Na+ and K+.
The Na+-K+ ATPase switches between two major conformational states, E1 and E2. In the E1 state, the binding
sites for Na+ and K+ face the ICF and the enzyme has
a high affinity for Na+. In the E2 state, the binding sites
for Na+ and K+ face the ECF and the enzyme has a high
affinity for K+. The enzyme’s ion-transporting function
(i.e., pumping Na+ out of the cell and K+ into the cell)
is based on cycling between the E1 and E2 states and is
powered by ATP hydrolysis.
The transport cycle is illustrated in Figure 1.6. The
cycle begins with the enzyme in the E1 state, bound to
ATP. In the E1 state, the ion-binding sites face the ICF,
and the enzyme has a high affinity for Na+; three Na+
ions bind, ATP is hydrolyzed, and the terminal phosphate of ATP is transferred to the enzyme, producing a
high-energy state, E1~P. Now, a major conformational
change occurs, and the enzyme switches from E1~P to
E2~P. In the E2 state, the ion-binding sites face the ECF,
the affinity for Na+ is low, and the affinity for K+ is high.
The three Na+ ions are released from the enzyme to
ECF, two K+ ions are bound, and inorganic phosphate
is released from E2. The enzyme now binds intracellular
ATP, and another major conformational change occurs
that returns the enzyme to the E1 state; the two K+
ions are released to ICF, and the enzyme is ready for
another cycle.
Cardiac glycosides (e.g., ouabain and digitalis) are
a class of drugs that inhibits Na+-K+ ATPase. Treatment with this class of drugs causes certain predictable changes in intracellular ionic concentration: The
intracellular Na+ concentration will increase, and the
intracellular K+ concentration will decrease. Cardiac
glycosides inhibit the Na+-K+ ATPase by binding to the
E2~P form near the K+-binding site on the extracellular
side, thereby preventing the conversion of E2~P back
to E1. By disrupting the cycle of phosphorylationdephosphorylation, these drugs disrupt the entire
enzyme cycle and its transport functions.
Ca2+ ATPase (Ca2+ Pump)
Most cell (plasma) membranes contain a Ca2+ ATPase,
or plasma-membrane Ca2+ ATPase (PMCA), whose
function is to extrude Ca2+ from the cell against an
electrochemical gradient; one Ca2+ ion is extruded for
each ATP hydrolyzed. PMCA is responsible, in part, for
maintaining the very low intracellular Ca2+ concentration. In addition, the sarcoplasmic reticulum (SR) of
muscle cells and the endoplasmic reticulum of other
cells contain variants of Ca2+ ATPase that pump two
Ca2+ ions (for each ATP hydrolyzed) from ICF into the
interior of the SR or endoplasmic reticulum (i.e., Ca2+
sequestration). These variants are called SR and endoplasmic reticulum Ca2+ ATPase (SERCA). Ca2+ ATPase
functions similarly to Na+-K+ ATPase, with E1 and E2
states that have, respectively, high and low affinities
for Ca2+. For PMCA, the E1 state binds Ca2+ on the
intracellular side, a conformational change to the E2
state occurs, and the E2 state releases Ca2+ to ECF. For
SERCA, the E1 state binds Ca2+ on the intracellular side
and the E2 state releases Ca2+ to the lumen of the SR or
endoplasmic reticulum.
H+-K+ ATPase (H+-K+ Pump)
H+-K+ ATPase is found in the parietal cells of the gastric
mucosa and in the α-intercalated cells of the renal
collecting duct. In the stomach, it pumps H+ from the
ICF of the parietal cells into the lumen of the stomach,
where it acidifies the gastric contents. Omeprazole,
an inhibitor of gastric H+-K+ ATPase, can be used therapeutically to reduce the secretion of H+ in the treatment
of some types of peptic ulcer disease.
10 • Physiology
Secondary Active Transport
Secondary active transport processes are those in which
the transport of two or more solutes is coupled. One
of the solutes, usually Na+, moves down its electrochemical gradient (downhill), and the other solute
moves against its electrochemical gradient (uphill). The
downhill movement of Na+ provides energy for the
uphill movement of the other solute. Thus metabolic
energy, as ATP, is not used directly, but it is supplied
indirectly in the Na+ concentration gradient across the
cell membrane. (The Na+-K+ ATPase, utilizing ATP,
creates and maintains this Na+ gradient.) The name
secondary active transport therefore refers to the indirect utilization of ATP as an energy source.
Inhibition of the Na+-K+ ATPase (e.g., by treatment
with ouabain) diminishes the transport of Na+ from ICF
to ECF, causing the intracellular Na+ concentration to
increase and thereby decreasing the size of the transmembrane Na+ gradient. Thus indirectly, all secondary
active transport processes are diminished by inhibitors
of the Na+-K+ ATPase because their energy source, the
Na+ gradient, is diminished.
There are two types of secondary active transport,
distinguishable by the direction of movement of the
uphill solute. If the uphill solute moves in the same
direction as Na+, it is called cotransport, or symport.
If the uphill solute moves in the opposite direction
of Na+, it is called countertransport, antiport, or
exchange.
Cotransport
Cotransport (symport) is a form of secondary active
transport in which all solutes are transported in the
same direction across the cell membrane. Na+ moves
into the cell on the carrier down its electrochemical
gradient; the solutes, cotransported with Na+, also
move into the cell. Cotransport is involved in several
critical physiologic processes, particularly in the
absorbing epithelia of the small intestine and the
renal tubule. For example, Na+-glucose cotransport
(SGLT) and Na+-amino acid cotransport are present
in the luminal membranes of the epithelial cells of
both small intestine and renal proximal tubule. Another
example of cotransport involving the renal tubule is
Na+-K+-2Cl− cotransport, which is present in the luminal
membrane of epithelial cells of the thick ascending
limb. In each example, the Na+ gradient established by
the Na+-K+ ATPase is used to transport solutes such as
glucose, amino acids, K+, or Cl− against electrochemical
gradients.
Figure 1.7 illustrates the principles of cotransport
using the example of Na+-glucose cotransport (SGLT1,
or Na+-glucose transport protein 1) in intestinal epithelial cells. The cotransporter is present in the luminal
membrane of these cells and can be visualized as having
Lumen
Intestinal epithelial cell
Blood
3Na+
ATP
Na+
2K+
SGLT1
Glucose
Luminal or
apical membrane
Glucose
Basolateral
membrane
Fig. 1.7 Na+-glucose cotransport in an intestinal epithelial
cell. ATP, Adenosine triphosphate; SGLT1, Na+-glucose transport
protein 1.
two specific recognition sites, one for Na+ ions and the
other for glucose. When both Na+ and glucose are
present in the lumen of the small intestine, they bind
to the transporter. In this configuration, the cotransport
protein rotates and releases both Na+ and glucose to
the interior of the cell. (Subsequently, both solutes
are transported out of the cell across the basolateral
membrane—Na+ by the Na+-K+ ATPase and glucose by
facilitated diffusion.) If either Na+ or glucose is missing
from the intestinal lumen, the cotransporter cannot
rotate. Thus both solutes are required, and neither can
be transported in the absence of the other (Box 1.1).
Finally, the role of the intestinal Na+-glucose cotransport process can be understood in the context of overall
intestinal absorption of carbohydrates. Dietary carbohydrates are digested by gastrointestinal enzymes to an
absorbable form, the monosaccharides. One of these
monosaccharides is glucose, which is absorbed across
the intestinal epithelial cells by a combination of Na+glucose cotransport in the luminal membrane and
facilitated diffusion of glucose in the basolateral membrane. Na+-glucose cotransport is the active step, allowing glucose to be absorbed into the blood against an
electrochemical gradient.
Countertransport
Countertransport (antiport or exchange) is a form of
secondary active transport in which solutes move in
opposite directions across the cell membrane. Na+ moves
into the cell on the carrier down its electrochemical
gradient; the solutes that are countertransported or
exchanged for Na+ move out of the cell. Countertransport is illustrated by Ca2+-Na+ exchange (Fig. 1.8) and
by Na+-H+ exchange. As with cotransport, each process
1—Cellular Physiology
BOX 1.1 Clinical Physiology: Glucosuria Due to
Diabetes Mellitus
DESCRIPTION OF CASE. At his annual physical
examination, a 14-year-old boy reports symptoms of
frequent urination and severe thirst. A dipstick test
of his urine shows elevated levels of glucose. The
physician orders a glucose tolerance test, which
indicates that the boy has type I diabetes mellitus.
He is treated with insulin by injection, and his
dipstick test is subsequently normal.
EXPLANATION OF CASE. Although type I diabetes
mellitus is a complex disease, this discussion is
limited to the symptom of frequent urination and
the finding of glucosuria (glucose in the urine).
Glucose is normally handled by the kidney in the
following manner: Glucose in the blood is filtered
across the glomerular capillaries. The epithelial
cells, which line the renal proximal tubule, then
reabsorb all of the filtered glucose so that no glucose
is excreted in the urine. Thus a normal dipstick test
would show no glucose in the urine. If the epithelial
cells in the proximal tubule do not reabsorb all of
the filtered glucose back into the blood, the glucose
that escapes reabsorption is excreted. The cellular
mechanism for this glucose reabsorption is the Na+glucose cotransporter in the luminal membrane of
the proximal tubule cells. Because this is a carriermediated transporter, there is a finite number of
binding sites for glucose. Once these binding sites
are fully occupied, saturation of transport occurs
(transport maximum).
In this patient with type I diabetes mellitus, the
hormone insulin is not produced in sufficient
amounts by the pancreatic β cells. Insulin is required
for normal uptake of glucose into liver, muscle, and
other cells. Without insulin, the blood glucose
concentration increases because glucose is not taken
up by the cells. When the blood glucose concentration increases to high levels, more glucose is filtered
by the renal glomeruli and the amount of glucose
filtered exceeds the capacity of the Na+-glucose
cotransporter. The glucose that cannot be reabsorbed
because of saturation of this transporter is then
“spilled” in the urine.
TREATMENT. Treatment of the patient with
type I diabetes mellitus consists of administering
exogenous insulin by injection. Whether secreted
normally from the pancreatic β cells or administered by injection, insulin lowers the blood glucose
concentration by promoting glucose uptake into
cells. When this patient received insulin, his blood
glucose concentration was reduced; thus the amount
of glucose filtered was reduced, and the Na+-glucose
cotransporters were no longer saturated. All of the
filtered glucose could be reabsorbed, and therefore no glucose was excreted, or “spilled,” in the
urine.
•
11
Muscle cell
3Na+
3Na+
ATP
Ca2+
2K+
Fig. 1.8 Ca2+-Na+ countertransport (exchange) in a muscle
cell. ATP, Adenosine triphosphate.
uses the Na+ gradient established by the Na+-K+ ATPase
as an energy source; Na+ moves downhill and Ca2+ or
H+ moves uphill.
Ca2+-Na+ exchange is one of the transport mechanisms, along with the Ca2+ ATPase, that helps maintain
the intracellular Ca2+ concentration at very low levels
(≈10−7 molar). To accomplish Ca2+-Na+ exchange, active
transport must be involved because Ca2+ moves out of
the cell against its electrochemical gradient. Figure 1.8
illustrates the concept of Ca2+-Na+ exchange in a muscle
cell membrane. The exchange protein has recognition
sites for both Ca2+ and Na+. The protein must bind Ca2+
on the intracellular side of the membrane and, simultaneously, bind Na+ on the extracellular side. In this
configuration, the exchange protein rotates and delivers
Ca2+ to the exterior of the cell and Na+ to the interior
of the cell.
The stoichiometry of Ca2+-Na+ exchange varies
between different cell types and may even vary for a
single cell type under different conditions. Usually,
however, three Na+ ions enter the cell for each Ca2+ ion
extruded from the cell. With this stoichiometry of three
Na+ ions per one Ca2+ ion, three positive charges move
into the cell in exchange for two positive charges
leaving the cell, making the Ca2+-Na+ exchanger
electrogenic.
Osmosis
Osmosis is the flow of water across a semipermeable
membrane because of differences in solute concentration. Concentration differences of impermeant solutes
establish osmotic pressure differences, and this osmotic
pressure difference causes water to flow by osmosis.
Osmosis of water is not diffusion of water: Osmosis
occurs because of a pressure difference, whereas diffusion occurs because of a concentration (or activity)
difference of water.
12 • Physiology
Osmolarity
The osmolarity of a solution is its concentration of
osmotically active particles, expressed as osmoles per
liter or milliosmoles per liter. To calculate osmolarity,
it is necessary to know the concentration of solute and
whether the solute dissociates in solution. For example,
glucose does not dissociate in solution; theoretically,
NaCl dissociates into two particles and CaCl2 dissociates into three particles. The symbol “g” gives the
number of particles in solution and also takes into
account whether there is complete or only partial dissociation. Thus if NaCl is completely dissociated into
two particles, g equals 2.0; if NaCl dissociates only
partially, then g falls between 1.0 and 2.0. Osmolarity
is calculated as follows:
Osmolarity = g C
where
Osmolarity = Concentration of particles (mOsm/L)
g = Number of particles per mole in
solution (Osm/mol)
C = Concentration (mmol/L)
If two solutions have the same calculated osmolarity,
they are called isosmotic. If two solutions have different calculated osmolarities, the solution with the higher
osmolarity is called hyperosmotic and the solution
with the lower osmolarity is called hyposmotic.
Osmolality
Osmolality is similar to osmolarity, except that it is the
concentration of osmotically active particles, expressed
as osmoles (or milliosmoles) per kilogram of water.
Because 1 kg of water is approximately equivalent to
1 L of water, osmolarity and osmolality will have
essentially the same numerical value.
The two solutions do not have the same calculated osmolarity; therefore they are not isosmotic.
Solution A has a higher osmolarity than Solution B
and is hyperosmotic; Solution B is hyposmotic.
Osmotic Pressure
Osmosis is the flow of water across a semipermeable
membrane due to a difference in solute concentration.
The difference in solute concentration creates an
osmotic pressure difference across the membrane and
that pressure difference is the driving force for osmotic
water flow.
Figure 1.9 illustrates the concept of osmosis. Two
aqueous solutions, open to the atmosphere, are shown
in Figure 1.9A. The membrane separating the solutions
is permeable to water but is impermeable to the solute.
Initially, solute is present only in Solution 1. The solute
in Solution 1 produces an osmotic pressure and causes,
by the interaction of solute with pores in the membrane,
a reduction in hydrostatic pressure of Solution 1. The
resulting hydrostatic pressure difference across the
membrane then causes water to flow from Solution 2
into Solution 1. With time, water flow causes the
volume of Solution 1 to increase and the volume of
Solution 2 to decrease.
Figure 1.9B shows a similar pair of solutions;
however, the preparation has been modified so that
water flow into Solution 1 is prevented by applying
pressure to a piston. The pressure required to stop the
flow of water is the osmotic pressure of Solution 1.
The osmotic pressure (π) of Solution 1 depends on
two factors: the concentration of osmotically active
particles and whether the solute remains in Solution 1
(i.e., whether the solute can cross the membrane or
not). Osmotic pressure is calculated by the van’t Hoff
equation (as follows), which converts the concentration of particles to a pressure, taking into account
whether the solute is retained in the original solution.
Thus
π=gCσRT
SAMPLE PROBLEM. Solution A is 2 mmol/L urea,
and Solution B is 1 mmol/L NaCl. Assume that gNaCl
= 1.85. Are the two solutions isosmotic?
SOLUTION. Calculate the osmolarities of both solutions to compare them. Solution A contains urea,
which does not dissociate in solution. Solution B
contains NaCl, which dissociates partially in solution but not completely (i.e., g < 2.0). Thus Osmolarity A = 1 Osm/mol × 2 mmol/L = 2 mOsm/L OsmolarityB = 1.85 Osm/mol × 1 mmol/L = 1.85 mOsm/L where π = Osmotic pressure (atm or mm Hg ) g = Number of particles per mole in solution (Osm/mol) C = Concentration (mmol/L) σ = Reflection coefficient (varies from 0 to 1) R = Gas constant (0.082 L − atm/mol − K) T = Absolute temperature (K) The reflection coefficient (σ) is a dimensionless number ranging between 0 and 1 that describes the 1—Cellular Physiology • 13 Semipermeable membrane Time A 1 2 1 2 atm Time 1 2 Piston applies pressure to stop water flow 1 2 B Fig. 1.9 Osmosis across a semipermeable membrane. A, Solute (circles) is present on one side of a semipermeable membrane; with time, the osmotic pressure created by the solute causes water to flow from Solution 2 to Solution 1. The resulting volume changes are shown. B, The solutions are closed to the atmosphere, and a piston is applied to stop the flow of water into Solution 1. The pressure needed to stop the flow of water is the effective osmotic pressure of Solution 1. atm, Atmosphere. ease with which a solute crosses a membrane. Reflection coefficients can be described for the following three conditions (Fig. 1.10): ♦ σ = 1.0 (see Fig. 1.10A). If the membrane is impermeable to the solute, σ is 1.0, and the solute will be retained in the original solution and exert its full osmotic effect. In this case, the effective osmotic pressure will be maximal and will cause maximal water flow. For example, serum albumin and intracellular proteins are solutes where σ = 1. ♦ σ = 0 (see Fig. 1.10C). If the membrane is freely permeable to the solute, σ is 0, and the solute will diffuse across the membrane down its concentration gradient until the solute concentrations of the two solutions are equal. In other words, the solute behaves as if it were water. In this case, there will be no effective osmotic pressure difference across the membrane and therefore no driving force for osmotic water flow. Refer again to the van’t Hoff equation and notice that, when σ = 0, the calculated effective osmotic pressure becomes zero. Urea is an example of a solute where σ = 0 (or nearly 0). ♦ σ = a value between 0 and 1 (see Fig. 1.10B). Most solutes are neither impermeant (σ = 1) nor freely permeant (σ = 0) across membranes, but the reflection coefficient falls somewhere between 0 and 1. In such cases, the effective osmotic pressure lies between its maximal possible value (when the solute is completely impermeable) and zero (when the solute is freely permeable). Refer once again to the van’t Hoff equation and notice that, when σ is between 0 and 1, the calculated effective osmotic pressure will be less than its maximal possible value but greater than zero. When two solutions separated by a semipermeable membrane have the same effective osmotic pressure, 14 • Physiology σ=1 σ = between 0 and 1 σ=0 Membrane A B Fig. 1.10 C Reflection coefficient (σ). they are isotonic; that is, no water will flow between them because there is no effective osmotic pressure difference across the membrane. When two solutions have different effective osmotic pressures, the solution with the lower effective osmotic pressure is hypotonic and the solution with the higher effective osmotic pressure is hypertonic. Water will flow from the hypotonic solution into the hypertonic solution (Box 1.2). SAMPLE PROBLEM. A solution of 1 mol/L NaCl is separated from a solution of 2 mol/L urea by a semipermeable membrane. Assume that NaCl is completely dissociated, that σNaCl = 0.3, and σurea = 0.05. Are the two solutions isosmotic and/or isotonic? Is there net water flow, and what is its direction? SOLUTION Step 1. To determine whether the solutions are isosmotic, simply calculate the osmolarity of each solution (g × C) and compare the two values. It was stated that NaCl is completely dissociated (i.e., separated into two particles); thus for NaCl, g = 2.0. Urea does not dissociate in solution; thus for urea, g = 1.0. NaCl: Osmolarity = g C = 2.0 × 1 mol/L = 2 Osm/L Urea: Osmolarity = g C = 1.0 × 2 mol/L = 2 Osm/L Each solution has an osmolarity of 2 Osm/L— they are indeed isosmotic. Step 2. To determine whether the solutions are isotonic, the effective osmotic pressure of each solution must be determined. Assume that at 37°C (310 K), RT = 25.45 L-atm/mol. Thus NaCl: π = g C σ RT = 2 × 1 mol/L × 0.3 × RT = 0.6 RT = 15.3 atm Urea: π = g C σ RT = 1 × 2 mol/L × 0.05 × RT = 0.1 RT = 2.5 atm Although the two solutions have the same calculated osmolarities and are isosmotic (Step 1), they have different effective osmotic pressures and they are not isotonic (Step 2). This difference occurs because the reflection coefficient for NaCl is much higher than the reflection coefficient for urea and, thus NaCl creates the greater effective osmotic pressure. Water will flow from the urea solution into the NaCl solution, from the hypotonic solution to the hypertonic solution. DIFFUSION POTENTIALS AND EQUILIBRIUM POTENTIALS Ion Channels Ion channels are integral, membrane-spanning proteins that, when open, permit the passage of certain ions. Thus ion channels are selective and allow ions with specific characteristics to move through them. This selectivity is based on both the size of the channel and the charges lining it. For example, channels lined with negative charges typically permit the passage of cations but exclude anions; channels lined with positive charges permit the passage of anions but exclude cations. Channels also discriminate on the basis of size. For example, a cation-selective channel lined with negative charges might permit the passage of Na+ but exclude K+; another 1—Cellular Physiology BOX 1.2 Clinical Physiology: Hyposmolarity With Brain Swelling DESCRIPTION OF CASE. A 72-year-old man was diagnosed recently with oat cell carcinoma of the lung. He tried to stay busy with consulting work, but the disease sapped his energy. One evening, his wife noticed that he seemed confused and lethargic, and suddenly he suffered a grand mal seizure. In the emergency department, his plasma Na+ concentration was 113 mEq/L (normal, 140 mEq/L) and his plasma osmolarity was 230 mOsm/L (normal, 290 mOsm/L). He was treated immediately with an infusion of hypertonic NaCl and was released from the hospital a few days later, with strict instructions to limit his water intake. EXPLANATION OF CASE. The man’s oat cell carcinoma autonomously secretes antidiuretic hormone (ADH), which causes syndrome of inappropriate antidiuretic hormone (SIADH). In SIADH, the high circulating levels of ADH cause excessive water reabsorption by the principal cells of the late distal tubule and collecting ducts. The excess water that is reabsorbed and retained in the body dilutes the Na+ concentration and osmolarity of the ECF. The decreased osmolarity means there is also decreased effective osmotic pressure of ECF and, briefly, osmotic pressure of ECF is less than osmotic pressure of ICF. The effective osmotic pressure difference across cell membranes causes osmotic water flow from ECF to ICF, which results in cell swelling. Because the brain is contained in a fixed structure (the skull), swelling of brain cells can cause seizure. TREATMENT. Treatment of the patient with hypertonic NaCl infusion was designed to quickly raise his ECF osmolarity and osmotic pressure, which would eliminate the effective osmotic pressure difference across the brain cell membranes and stop osmotic water flow and brain cell swelling. cation-selective channel (e.g., nicotinic receptor on the motor end plate) might have less selectivity and permit the passage of several different small cations. Ion channels are controlled by gates, and, depending on the position of the gates, the channels may be open or closed. When a channel is open, the ions for which it is selective can flow through it by passive diffusion, down the existing electrochemical gradient. In the open state, there is a continuous path between ECF and ICF, through which ions can flow. When the channel is closed, the ions cannot flow through it, no matter what the size of the electrochemical gradient. The conductance of a channel depends on the probability that it is open. The higher the probability that the channel is open, the higher is its conductance or permeability. • 15 The gates on ion channels are controlled by three types of sensors. One type of gate has sensors that respond to changes in membrane potential (i.e., voltage-gated channels); a second type of gate responds to changes in signaling molecules (i.e., second messenger–gated channels); and a third type of gate responds to changes in ligands such as hormones or neurotransmitters (i.e., ligand-gated channels). ♦ Voltage-gated channels have gates that are controlled by changes in membrane potential. For example, the activation gate on the nerve Na+ channel is opened by depolarization of the nerve cell membrane; opening of this channel is responsible for the upstroke of the action potential. Interestingly, another gate on the Na+ channel, an inactivation gate, is closed by depolarization. Because the activation gate responds more rapidly to depolarization than the inactivation gate, the Na+ channel first opens and then closes. This difference in response times of the two gates accounts for the shape and time course of the action potential. ♦ Second messenger–gated channels have gates that are controlled by changes in levels of intracellular signaling molecules such as cyclic adenosine monophosphate (cAMP) or inositol 1,4,5-triphosphate (IP3). Thus the sensors for these gates are on the intracellular side of the ion channel. For example, the gates on Na+ channels in cardiac sinoatrial node are opened by increased intracellular cAMP. ♦ Ligand-gated channels have gates that are controlled by hormones and neurotransmitters. The sensors for these gates are located on the extracellular side of the ion channel. For example, the nicotinic receptor on the motor end plate is actually an ion channel that opens when acetylcholine (ACh) binds to it; when open, it is permeable to Na+ and K+ ions. Diffusion Potentials A diffusion potential is the potential difference generated across a membrane when a charged solute (an ion) diffuses down its concentration gradient. Therefore a diffusion potential is caused by diffusion of ions. It follows, then, that a diffusion potential can be generated only if the membrane is permeable to that ion. Furthermore, if the membrane is not permeable to the ion, no diffusion potential will be generated no matter how large a concentration gradient is present. The magnitude of a diffusion potential, measured in millivolts (mV), depends on the size of the concentration gradient, where the concentration gradient is the driving force. The sign of the diffusion potential depends on the charge of the diffusing ion. Finally, as noted, diffusion potentials are created by the 16 • Physiology movement of only a few ions, and they do not cause changes in the concentration of ions in bulk solution. Equilibrium Potentials The concept of equilibrium potential is simply an extension of the concept of diffusion potential. If there is a concentration difference for an ion across a membrane and the membrane is permeable to that ion, a potential difference (the diffusion potential) is created. Eventually, net diffusion of the ion slows and then stops because of that potential difference. In other words, if a cation diffuses down its concentration gradient, it carries a positive charge across the membrane, which will retard and eventually stop further diffusion of the cation. If an anion diffuses down its concentration gradient, it carries a negative charge, which will retard and then stop further diffusion of the anion. The equilibrium potential is the diffusion potential that exactly balances or opposes the tendency for diffusion down the concentration difference. At electrochemical equilibrium, the chemical and electrical driving forces acting on an ion are equal and opposite, and no further net diffusion occurs. The following examples of a diffusing cation and a diffusing anion illustrate the concepts of equilibrium potential and electrochemical equilibrium. Example of Na+ Equilibrium Potential Figure 1.11 shows two solutions separated by a theoretical membrane that is permeable to Na+ but not to Cl−. The NaCl concentration is higher in Solution 1 than in Solution 2. The permeant ion, Na+, will diffuse down its concentration gradient from Solution 1 to Solution 2, but the impermeant ion, Cl−, will not accompany it. As a result of the net movement of positive charge to Solution 2, an Na+ diffusion potential develops and Solution 2 becomes positive with respect to Solution 1. The positivity in Solution 2 opposes further diffusion of Na+, and eventually it is large enough to prevent further net diffusion. The potential difference that exactly balances the tendency of Na+ to diffuse down its concentration gradient is the Na+ equilibrium potential. When the chemical and electrical driving forces on Na+ are equal and opposite, Na+ is said to be at electrochemical equilibrium. This diffusion of a few Na+ ions, sufficient to create the diffusion potential, does not produce any change in Na+ concentration in the bulk solutions. Example of Cl− Equilibrium Potential Figure 1.12 shows the same pair of solutions as in Figure 1.11; however, in Figure 1.12, the theoretical membrane is permeable to Cl− rather than to Na+. Cl− will diffuse from Solution 1 to Solution 2 down its concentration gradient, but Na+ will not accompany it. A diffusion potential will be established, and Solution 2 will become negative relative to Solution 1. The potential difference that exactly balances the tendency of Cl− to diffuse down its concentration gradient is the Cl− equilibrium potential. When the chemical and electrical driving forces on Cl− are equal and opposite, then Cl− is at electrochemical equilibrium. Again, diffusion of these few Cl− ions will not change the Cl− concentration in the bulk solutions. Nernst Equation The Nernst equation is used to calculate the equilibrium potential for an ion at a given concentration difference across a membrane, assuming that the membrane is permeable to that ion. By definition, the equilibrium potential is calculated for one ion at a time. Thus Ex = −2.3RT [C ] log10 i [Ce ] zF Na+-selective membrane Na+ Cl– Time Na+ Na+ Cl– Cl– 1 2 Fig. 1.11 1 Generation of an Na+ diffusion potential. – – – – + + + + Na+ Cl– 2 1—Cellular Physiology • Cl–-selective membrane Na+ Time Na+ Cl– Na+ Cl– Cl– 1 2 Fig. 1.12 + + + + – – – – 1 Na+ Cl– 2 Generation of a Cl− diffusion potential. where E X = Equilibrium potential (mV ) for a given ion, X 2.3RT = Constant (60 mV at 37°C ) F z = Charge on the ion ( +1 for Na + ; + 2 for Ca 2+ ; − 1 for Cl − ) Ci = Intracellular concentration of X (mmol/L) Ce = Extracellular concentration of X (mmol/L) In words, the Nernst equation converts a concentration difference for an ion into a voltage. This conversion is accomplished by the various constants: R is the gas constant, T is the absolute temperature, and F is Faraday constant; multiplying by 2.3 converts natural logarithm to log10. By convention, membrane potential is expressed as intracellular potential relative to extracellular potential. Hence, a transmembrane potential difference of −70 mV means 70 mV, cell interior negative. Typical values for equilibrium potential for common ions in skeletal muscle, calculated as previously described and assuming typical concentration gradients across cell membranes, are as follows: ENa + = +65 mV ECa2+ = +120 mV EK+ = −95 mV ECl − = −90 mV It is useful to keep these values in mind when considering the concepts of resting membrane potential and action potentials. SAMPLE PROBLEM. If the intracellular [Ca2+] is 10−7 mol/L and the extracellular [Ca2+] is 2 × 10−3 mol/L, at what potential difference across the cell membrane will Ca2+ be at electrochemical equilibrium? Assume that 2.3RT/F = 60 mV at body temperature (37°C). SOLUTION. Another way of posing the question is to ask what the membrane potential will be, given this concentration gradient across the membrane, if Ca2+ is the only permeant ion. Remember, Ca2+ is divalent, so z = +2. Thus −60 mV C log10 i z Ce 10−7 mol/L −60 mV log10 = 2 × 10−3 mol/L +2 ECa2+ = = −30 mV log10 5 × 10−5 = −30 mV ( −4.3) = +129 mV Because this is a log function, it is not necessary to remember which concentration goes in the numerator. Simply complete the calculation either way to arrive at 129 mV, and then determine the correct sign with an intuitive approach. The intuitive approach depends on the knowledge that, because the [Ca2+] is much higher in ECF than in ICF, Ca2+ will tend to diffuse down this concentration gradient from ECF into ICF, making the inside of the cell positive. Thus Ca2+ will be at electrochemical equilibrium when the membrane potential is +129 mV (cell interior positive). Be aware that the equilibrium potential has been calculated at a given concentration gradient for Ca2+ ions. With a different concentration gradient, the calculated equilibrium potential would be different. 17 18 • Physiology I X = G X (E m − E X ) Driving Force When dealing with uncharged solutes, the driving force for net diffusion is simply the concentration difference of the solute across the cell membrane. However, when dealing with charged solutes (i.e., ions), the driving force for net diffusion must consider both concentration difference and electrical potential difference across the cell membrane. The driving force on a given ion is the difference between the actual, measured membrane potential (Em) and the ion’s calculated equilibrium potential (EX). In other words, it is the difference between the actual Em and the value the ion would “like” the membrane potential to be. (The ion would “like” the membrane potential to be its equilibrium potential, as calculated by the Nernst equation.) The driving force on a given ion, X, is therefore calculated as: Net driving force (mV ) = E m − E x where Driving force = Driving force (mV ) E m = Actual membrane potential (mV ) E X = Equilibrium potential for X (mV ) When the driving force is negative (i.e., Em is more negative than the ion’s equilibrium potential), that ion X will enter the cell if it is a cation and will leave the cell if it is an anion. In other words, ion X “thinks” the membrane potential is too negative and tries to bring the membrane potential toward its equilibrium potential by diffusing in the appropriate direction across the cell membrane. Conversely, if the driving force is positive (Em is more positive than the ion’s equilibrium potential), then ion X will leave the cell if it is a cation and will enter the cell if it is an anion; in this case, ion X “thinks” the membrane potential is too positive and tries to bring the membrane potential toward its equilibrium potential by diffusing in the appropriate direction across the cell membrane. Finally, if Em is equal to the ion’s equilibrium potential, then the driving force on the ion is zero, and the ion is, by definition, at electrochemical equilibrium; since there is no driving force, there will be no net movement of the ion in either direction. Ionic Current Ionic current (IX), or current flow, occurs when there is movement of an ion across the cell membrane. Ions will move across the cell membrane through ion channels when two conditions are met: (1) there is a driving force on the ion, and (2) the membrane has a conductance to that ion (i.e., its ion channels are open). Thus where I X = ionic current (mAmp) G X = ionic conductance (1/ohm), where conductance is the reciprocal of resistance E m − E X = driving force on ion X (mV ) You will notice that the equation for ionic current is simply a rearrangement of Ohm’s law, where V = IR or I = V/R (where V is the same thing as E). Because conductance (G) is the reciprocal of resistance (R), I = G × V. The direction of ionic current is determined by the direction of the driving force, as described in the previous section. The magnitude of ionic current is determined by the size of the driving force and the conductance of the ion. For a given conductance, the greater the driving force, the greater the current flow. For a given driving force, the greater the conductance, the greater the current flow. Lastly, if either the driving force or the conductance of an ion is zero, there can be no net diffusion of that ion across the cell membrane and no current flow. RESTING MEMBRANE POTENTIAL The resting membrane potential is the potential difference that exists across the membrane of excitable cells such as nerve and muscle in the period between action potentials (i.e., at rest). As stated previously, in expressing the membrane potential, it is conventional to refer the intracellular potential to the extracellular potential. The resting membrane potential is established by diffusion potentials, which result from the concentration differences for various ions across the cell membrane. (Recall that these concentration differences have been established by primary and secondary active transport mechanisms.) Each permeant ion attempts to drive the membrane potential toward its own equilibrium potential. Ions with the highest permeabilities or conductances at rest will make the greatest contributions to the resting membrane potential, and those with the lowest permeabilities will make little or no contribution. The resting membrane potential of most excitable cells falls in the range of −70 to −80 mV. These values can best be explained by the concept of relative permeabilities of the cell membrane. Thus the resting membrane potential is close to the equilibrium potentials for K+ and Cl− because the permeability to these ions at rest is high. The resting membrane potential is far from 1—Cellular Physiology the equilibrium potentials for Na+ and Ca2+ because the permeability to these ions at rest is low. One way of evaluating the contribution each ion makes to the membrane potential is by using the chord conductance equation, which weights the equilibrium potential for each ion (calculated by the Nernst equation) by its relative conductance. Ions with the highest conductance drive the membrane potential toward their equilibrium potentials, whereas those with low conductance have little influence on the membrane potential. (An alternative approach to the same question applies the Goldman equation, which considers the contribution of each ion by its relative permeability rather than by its conductance.) The chord conductance equation is written as follows: Em = gK+ g + g − g 2+ E + + Na ENa + + Cl ECl − + Ca ECa2+ gT K gT gT gT where E m = Membrane potential (mV ) g K+ etc. = K + conductance etc. (mho, reciprocal of resistance) g T = Total conductance (mho) EK+ etc. = K + equilibrium potential etc. (mV ) At rest, the membranes of excitable cells are far more permeable to K+ and Cl− than to Na+ and Ca2+. These differences in permeability account for the resting membrane potential. What role, if any, does the Na+-K+ ATPase play in creating the resting membrane potential? The answer has two parts. First, there is a small direct electrogenic contribution of the Na+-K+ ATPase, which is based on the stoichiometry of three Na+ ions pumped out of the cell for every two K+ ions pumped into the cell. Second, the more important indirect contribution is in maintaining the concentration gradient for K+ across the cell membrane, which then is responsible for the K+ diffusion potential that drives the membrane potential toward the K+ equilibrium potential. Thus the Na+-K+ ATPase is necessary to create and maintain the K+ concentration gradient, which establishes the resting membrane potential. (A similar argument can be made for the role of the Na+-K+ ATPase in the upstroke of the action potential, where it maintains the ionic gradient for Na+ across the cell membrane.) ACTION POTENTIALS The action potential is a phenomenon of excitable cells such as nerve and muscle and consists of a rapid depolarization (upstroke) followed by repolarization of • 19 the membrane potential. Action potentials are the basic mechanism for transmission of information in the nervous system and in all types of muscle. Terminology The following terminology will be used for discussion of the action potential, the refractory periods, and the propagation of action potentials: ♦ Depolarization is the process of making the membrane potential less negative. As noted, the usual resting membrane potential of excitable cells is oriented with the cell interior negative. Depolarization makes the interior of the cell less negative, or it may even cause the cell interior to become positive. Such a change in membrane potential should not be described as “increasing” or “decreasing” because those terms are ambiguous. (For example, when the membrane potential depolarizes, or becomes less negative, has the membrane potential increased or decreased?) ♦ Hyperpolarization is the process of making the membrane potential more negative. As with depolarization, the terms “increasing” or “decreasing” should not be used to describe a change that makes the membrane potential more negative. ♦ Inward current is the flow of positive charge into the cell. Thus inward currents depolarize the membrane potential. An example of an inward current is the flow of Na+ into the cell during the upstroke of the action potential. ♦ Outward current is the flow of positive charge out of the cell. Outward currents hyperpolarize the membrane potential. An example of an outward current is the flow of K+ out of the cell during the repolarization phase of the action potential. ♦ Threshold potential is the membrane potential at which occurrence of the action potential is inevitable. Because the threshold potential is less negative than the resting membrane potential, an inward current is required to depolarize the membrane potential to threshold. At threshold potential, net inward current (e.g., inward Na+ current) becomes larger than net outward current (e.g., outward K+ current), and the resulting depolarization becomes self-sustaining, giving rise to the upstroke of the action potential. If net inward current is less than net outward current, the membrane will not be depolarized to threshold and no action potential will occur (see all-or-none response). ♦ Overshoot is that portion of the action potential where the membrane potential is positive (cell interior positive). 20 • Physiology ♦ Undershoot, or hyperpolarizing afterpotential, is that portion of the action potential, following repolarization, where the membrane potential is actually more negative than it is at rest. ♦ Refractory period is a period during which another normal action potential cannot be elicited in an excitable cell. Refractory periods can be absolute or relative. (In cardiac muscle cells, there is an additional category called effective refractory period.) Characteristics of Action Potentials Action potentials have three basic characteristics: stereotypical size and shape, propagation, and all-or-none response. ♦ Stereotypical size and shape. Each normal action potential for a given cell type looks identical, depolarizes to the same potential, and repolarizes back to the same resting potential. ♦ Propagation. An action potential at one site causes depolarization at adjacent sites, bringing those adjacent sites to threshold. Propagation of Absolute refractory period action potentials from one site to the next is nondecremental. ♦ All-or-none response. An action potential either occurs or does not occur. If an excitable cell is depolarized to threshold in a normal manner, then the occurrence of an action potential is inevitable. On the other hand, if the membrane is not depolarized to threshold, no action potential can occur. Indeed, if the stimulus is applied during the refractory period, then either no action potential occurs, or the action potential will occur but not have the stereotypical size and shape. Ionic Basis of the Action Potential The action potential is a fast depolarization (the upstroke), followed by repolarization back to the resting membrane potential. Figure 1.13 illustrates the events of the action potential in nerve and skeletal muscle, which occur in the following steps: 1. Resting membrane potential. At rest, the membrane potential is approximately −70 mV (cell interior Relative refractory period Na+ equilibrium potential +65 mV Voltage or conductance Action potential 0 mV Na+ conductance K+ conductance –70 mV Resting membrane potential –85 mV K+ equilibrium potential 1.0 2.0 Time (milliseconds) Fig. 1.13 Time course of voltage and conductance changes during the action potential of nerve. 1—Cellular Physiology negative). The K+ conductance or permeability is high and K+ channels are almost fully open, allowing K+ ions to diffuse out of the cell down the existing concentration gradient. This diffusion creates a K+ diffusion potential, which drives the membrane potential toward the K+ equilibrium potential. The conductance to Cl− (not shown) also is high, and, at rest, Cl− also is near electrochemical equilibrium. At rest, the Na+ conductance is low, and thus the resting membrane potential is far from the Na+ equilibrium potential, and Na+ is far from electrochemical equilibrium. 3. Repolarization of the action potential. The upstroke is terminated, and the membrane potential repolarizes to the resting level as a result of two events. First, the inactivation gates on the Na+ channels respond to depolarization by closing, but their response is slower than the opening of the activation gates. Thus after a delay, the inactivation gates close, which closes the Na+ channels and terminates the upstroke. Second, depolarization opens K+ channels and increases K+ conductance to a value even higher than occurs at rest. The combined effect of closing of the Na+ channels and greater opening of the K+ channels makes the K+ conductance much higher than the Na+ conductance. Thus an outward K+ current results, and the membrane is repolarized. Tetraethylammonium (TEA) blocks these voltagegated K+ channels, the outward K+ current, and repolarization. Activation gate Inactivation gate Na+ Closed, but available 2 21 but does not quite reach, the Na+ equilibrium potential of +65 mV. Tetrodotoxin (a toxin from the Japanese puffer fish) and the local anesthetic lidocaine block these voltage-sensitive Na+ channels and prevent the occurrence of nerve action potentials. 2. Upstroke of the action potential. An inward current, usually the result of current spread from action potentials at neighboring sites, causes depolarization of the nerve cell membrane to threshold, which occurs at approximately −60 mV. This initial depolarization causes rapid opening of the activation gates of the Na+ channel, and the Na+ conductance promptly increases and becomes even higher than the K+ conductance (Fig. 1.14). The increase in Na+ conductance results in an inward Na+ current; the membrane potential is further depolarized toward, 1 • Open 3 Inactivated Fig. 1.14 States of activation and inactivation gates on the nerve Na+ channel. 1, In the closed but available state, at the resting membrane potential, the activation gate is closed, the inactivation gate is open, and the channel is closed (but available, if depolarization occurs). 2, In the open state, during the upstroke of the action potential, both the activation and inactivation gates are open and the channel is open. 3, In the inactivated state, at the peak of the action potential, the activation gate is open, the inactivation gate is closed, and the channel is closed. 22 • Physiology 4. Hyperpolarizing afterpotential (undershoot). For a brief period following repolarization, the K+ conductance is higher than at rest and the membrane potential is driven even closer to the K+ equilibrium potential (hyperpolarizing afterpotential). Eventually, the K+ conductance returns to the resting level, and the membrane potential depolarizes slightly, back to the resting membrane potential. The membrane is now ready, if stimulated, to generate another action potential. The Nerve Na+ Channel A voltage-gated Na+ channel is responsible for the upstroke of the action potential in nerve and skeletal muscle. This channel is an integral membrane protein, consisting of a large α subunit and two β subunits. The α subunit has four domains, each of which has six transmembrane α-helices. The repeats of transmembrane α-helices surround a central pore, through which Na+ ions can flow (if the channel’s gates are open). A conceptual model of the Na+ channel demonstrating the function of the activation and inactivation gates is shown in Figure 1.14. The basic assumption of this model is that in order for Na+ to move through the channel, both gates on the channel must be open. Recall how these gates respond to changes in voltage. The activation gates open quickly in response to depolarization. The inactivation gates close in response to depolarization, but slowly, after a time delay. Thus when depolarization occurs, the activation gates open quickly, followed by slower closing of the inactivation gates. The figure shows three combinations of the gates’ positions and the resulting effect on Na+ channel opening. 1. Closed, but available. At the resting membrane potential, the activation gates are closed and the inactivation gates are open. Thus the Na+ channels are closed. However, they are “available” to fire an action potential if depolarization occurs. (Depolarization would open the activation gates and, because the inactivation gates are already open, the Na+ channels would then be open.) 2. Open. During the upstroke of the action potential, depolarization quickly opens the activation gates and both the activation and inactivation gates are briefly open. Na+ can flow through the channels into the cell, causing further depolarization. 3. Inactivated. At the peak of the action potential, the slow inactivation gates finally close in response to depolarization; now the Na+ channels are closed, the upstroke is terminated, and repolarization begins. How do the Na+ channels return to the closed, but available state? In other words, how do they recover, so that they are ready to fire another action potential? Repolarization back to the resting membrane potential causes the inactivation gates to open. The Na+ channels now return to the closed, but available state and are ready and “available” to fire another action potential if depolarization occurs. Refractory Periods During the refractory periods, excitable cells are incapable of producing normal action potentials (see Fig. 1.13). The refractory period includes an absolute refractory period and a relative refractory period. Absolute Refractory Period The absolute refractory period overlaps with almost the entire duration of the action potential. During this period, no matter how great the stimulus, another action potential cannot be elicited. The basis for the absolute refractory period is closure of the inactivation gates of the Na+ channel in response to depolarization. These inactivation gates are in the closed position until the cell is repolarized back to the resting membrane potential and the Na+ channels have recovered to the “closed, but available” state (see Fig. 1.14). Relative Refractory Period The relative refractory period begins at the end of the absolute refractory period and overlaps primarily with the period of the hyperpolarizing afterpotential. During this period, an action potential can be elicited, but only if a greater than usual depolarizing (inward) current is applied. The basis for the relative refractory period is the higher K+ conductance than is present at rest. Because the membrane potential is closer to the K+ equilibrium potential, more inward current is needed to bring the membrane to threshold for the next action potential to be initiated. Accommodation When a nerve or muscle cell is depolarized slowly or is held at a depolarized level, the usual threshold potential may pass without an action potential having been fired. This process, called accommodation, occurs because depolarization closes inactivation gates on the Na+ channels. If depolarization occurs slowly enough, the Na+ channels close and remain closed. The upstroke of the action potential cannot occur because there are insufficient available Na+ channels to carry inward current. An example of accommodation is seen in persons who have an elevated serum K+ concentration, or hyperkalemia. At rest, nerve and muscle cell membranes are very permeable to K+; an increase in extracellular K+ concentration causes depolarization of the resting membrane (as dictated by the Nernst equation). This depolarization brings the cell membrane closer to 1—Cellular Physiology BOX 1.3 • 23 Clinical Physiology: Hyperkalemia With Muscle Weakness DESCRIPTION OF CASE. A 48-year-old woman with insulin-dependent diabetes mellitus reports to her physician that she is experiencing severe muscle weakness. She is being treated for hypertension with propranolol, a β-adrenergic blocking agent. Her physician immediately orders blood studies, which reveal a serum [K+] of 6.5 mEq/L (normal, 4.5 mEq/L) and elevated BUN (blood urea nitrogen). The physician tapers off the dosage of propranolol, with eventual discontinuation of the drug. He adjusts her insulin dosage. Within a few days, the patient’s serum [K+] has decreased to 4.7 mEq/L, and she reports that her muscle strength has returned to normal. EXPLANATION OF CASE. This diabetic patient has severe hyperkalemia caused by several factors: (1) Because her insulin dosage is insufficient, the lack of adequate insulin has caused a shift of K+ out of cells into blood (insulin promotes K+ uptake into cells). (2) Propranolol, the β-blocking agent used to treat the woman’s hypertension, also shifts K+ out of cells into blood. (3) Elevated BUN suggests that the woman is developing renal failure; her failing kidneys are unable to excrete the extra K+ that is accumulating in her blood. These mechanisms involve concepts related to renal physiology and endocrine physiology. It is important to understand that this woman has a severely elevated blood [K+] (hyperkalemia) and that her muscle weakness results from this hyperkalemia. The basis for this weakness can be explained as follows: The resting membrane potential of muscle cells is threshold and would seem to make it more likely to fire an action potential. However, the cell is actually less likely to fire an action potential because this sustained depolarization closes the inactivation gates on the Na+ channels (Box 1.3). Propagation of Action Potentials Propagation of action potentials down a nerve or muscle fiber occurs by the spread of local currents from active regions to adjacent inactive regions. Figure 1.15 shows a nerve cell body with its dendritic tree and an axon. At rest, the entire nerve axon is at the resting membrane potential, with the cell interior negative. Action potentials are initiated in the initial segment of the axon, nearest the nerve cell body. They propagate down the axon by spread of local currents, as illustrated in the figure. In Figure 1.15A the initial segment of the nerve axon is depolarized to threshold and fires an action potential (the active region). As the result of an inward Na+ current, at the peak of the action potential, the polarity of the membrane potential is reversed and determined by the concentration gradient for K+ across the cell membrane (Nernst equation). At rest, the cell membrane is very permeable to K+, and K+ diffuses out of the cell down its concentration gradient, creating a K+ diffusion potential. This K+ diffusion potential is responsible for the resting membrane potential, which is cell interior negative. The larger the K+ concentration gradient, the greater the negativity in the cell. When the blood [K+] is elevated, the concentration gradient across the cell membrane is less than normal; resting membrane potential will therefore be less negative (i.e., depolarized). It might b...

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