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[ Pobierz całość w formacie PDF ]Acid–Base Balance
27
Did you know...?
Electrolyte levels often change when water
levels in the body change. Exhaustive exercise
can cause potentially dangerous disruptions of
fluid and electrolyte balance.
Fluid, Electrolyte, and
Learning Outcomes
After completing this chapter, you should be able to do the following:
27-1
Explain what is meant by the terms fluid balance, electrolyte
balance, and acid–base balance, and discuss their importance
for homeostasis.
27-2
Compare the composition of intracellular and extracellular
fluids, explain the basic concepts involved in the regulation of
fluids and electrolytes, and identify the hormones that play
important roles in fluid and electrolyte regulation.
27-3
Describe the movement of fluid within the ECF, between the
ECF and the ICF, and between the ECF and the environment.
27-4
Discuss the mechanisms by which sodium, potassium, calcium,
and chloride ion concentrations are regulated to maintain
electrolyte balance.
27-5
Explain the buffering systems that balance the pH of the
intracellular and extracellular fluids, and describe the
compensatory mechanisms involved in the maintenance of
acid–base balance.
27-6
Identify the most frequent disturbances of acid–base balance,
and explain how the body responds when the pH of body fluids
varies outside normal limits.
27-7
Describe the effects of aging on fluid, electrolyte, and acid–base
balance.
Clinical Notes
Water and Weight Loss p. 1017
Athletes and Salt Loss p. 1020
1010
Unit 5
Environmental Exchange
An Introduction to Fluid,
Electrolyte, and Acid-Base Balance
In this chapter, we will consider the dynamics of ex-
change among the various body fluids, and between the body
and the external environment. Stabilizing the volumes, solute
concentrations, and pH of the ECF and the ICF involves three
interrelated processes:
The next time you see a small pond, think about the fish it
contains. They live out their lives totally dependent on the
quality of that isolated environment. Severe water pollution
will kill them, but even subtle changes can have equally grave
effects. Changes in the volume of the pond, for example, can
be quite important. If evaporation removes too much of the
water, the fish become overcrowded; oxygen and food sup-
plies run out, and the fish suffocate or starve. The ionic con-
centration of the water is also crucial. Most of the fish in a
freshwater pond will die if the water becomes too salty; those
in a saltwater pond will die if their environment becomes too
dilute. The pH of the pond water, too, is a vital factor; that is
one reason acid rain is such a problem.
Your cells live in a pond whose shores are the exposed sur-
faces of your skin. Most of your body weight is water. Water ac-
counts for up to 99 percent of the volume of the fluid outside
cells, and it is an essential ingredient of cytoplasm. All of a cell’s
operations rely on water as a diffusion medium for the distribu-
tion of gases, nutrients, and waste products. If the water content
of the body changes, cellular activities are jeopardized. For ex-
ample, when the water content reaches very low levels, proteins
denature, enzymes cease functioning, and cells die. This chap-
ter discusses the homeostatic mechanisms that regulate ion
concentrations, volume, and pH in the fluid surrounding cells.
1.
Fluid Balance.
Yo u a re i n
fluid balance
when the amount
of water you gain each day is equal to the amount you lose
to the environment. The maintenance of normal fluid bal-
ance involves regulating the content and distribution of
body water in the ECF and the ICF. The digestive system is
the primary source of water gains; a small amount of addi-
tional water is generated by metabolic activity. The urinary
system is the primary route for water loss under normal
conditions, but as we saw in Chapter 25, sweat gland ac-
tivity can become important when body temperature is el-
evated.
l
p. 958
Although cells and tissues cannot
transport water, they can transport ions and create concen-
tration gradients that are then eliminated by osmosis.
2.
Electrolyte Balance.
Electrolytes
are ions released
through the dissociation of inorganic compounds; they
are so named because they can conduct an electrical cur-
rent in a solution.
l
p. 41
Each day, your body fluids
gain electrolytes from the food and drink you consume,
and lose electrolytes in urine, sweat, and feces. For each
ion, daily gains must balance daily losses. For example, if
you lose 500 mg of Na
in urine and insensible perspira-
tion, you need to gain 500 mg of Na
from food and drink
to remain in sodium balance. If the gains and losses for
every electrolyte are in balance, you are said to be in
electrolyte balance
. Electrolyte balance primarily in-
volves balancing the rates of absorption across the diges-
tive tract with rates of loss at the kidneys, although losses
at sweat glands and other sites can play a secondary role.
3.
Acid–Base Balance.
You are in
acid–base balance
when the
production of hydrogen ions in your body is precisely off-
set by their loss. When acid–base balance exists, the pH of
body fluids remains within normal limits.
l
p. 43
Pre-
venting a reduction in pH is the primary problem, because
your body generates a variety of acids during normal meta-
bolic operations. The kidneys play a major role by secreting
hydrogen ions into the urine and generating buffers that en-
ter the bloodstream. Such secretion occurs primarily in the
distal segments of the distal convoluted tubule (DCT) and
along the collecting system.
l
p. 989
The lungs also play
a key role through the elimination of carbon dioxide.
27-1
Fluid balance, electrolyte
balance, and acid–base balance
are interrelated and essential
to homeostasis
To survive, we must maintain a normal volume and composi-
tion of both the
extracellular fluid
or
ECF
(the interstitial
fluid, plasma, and other body fluids) and the
intracellular
fluid
or
ICF
(the cytosol). The ionic concentrations and pH
(hydrogen ion concentration) of these fluids are as important as
their absolute quantities. If concentrations of calcium or potas-
sium ions in the ECF become too high, cardiac arrhythmias de-
velop and death can result. A pH outside the normal range can
also lead to a variety of serious problems. Low pH is especially
dangerous, because hydrogen ions break chemical bonds,
change the shapes of complex molecules, disrupt plasma mem-
branes, and impair tissue functions.
Much of the material in this chapter was introduced in ear-
lier chapters, in discussions considering aspects of fluid, elec-
trolyte, or acid–base balance that affect specific systems. This
chapter provides an overview that integrates those discussions
to highlight important functional patterns. Few other chapters
have such wide-ranging clinical importance: The treatment of
Tips
&
Tricks
The “p” in pH refers to power. Hence, pH refers to the
p
ower
of
H
ydrogen.
1011
Chapter 27
Fluid, Electrolyte, and Acid–Base Balance
27-2
The ECF and ICF make up the
fluid compartments, which also
contain cations and anions
any serious illness affecting the nervous, cardiovascular, respi-
ratory, urinary, or digestive system must include steps to restore
normal fluid, electrolyte, and acid–base balances. Because this
chapter builds on information presented in earlier chapters, you
will encounter many references to relevant discussions and fig-
ures that can provide a quick review.
Figure 27–1a
presents an overview of the body composition of
a 70-kg (154-pound) individual with a minimum of body fat.
The distribution was obtained by averaging values for males
and females ages 18–40 years. Water accounts for roughly
60 percent of the total body weight of an adult male, and
50 percent of that of an adult female (
Figure 27–1b
). This dif-
ference between the sexes primarily reflects the proportion-
ately larger mass of adipose tissue in adult females, and the
greater average muscle mass in adult males. (Adipose tissue is
CHECKPOINT
1. Identify the three interrelated processes essential to
stabilizing body fluid volumes.
2. List the components of extracellular fluid (ECF) and
intracellular fluid (ICF), respectively.
See the blue Answers tab at the end of the book.
WATER
(38.5 kg; 84.7 lbs)
SOLIDS
(31.5 kg; 69.3 lbs)
20
15
Other
15
Plasma
10
Kg
Liters
10
Interstitial
fluid
5
5
0
Proteins
Lipids
Minerals
Carbohydrates Miscellaneous
Intracellular fluid
Extracellular fluid
(a)
TE
T
E
Interstitial
fluid 18%
Intracellular
fluid 33%
Interstitial
fluid 21.5%
Intracellular
fluid 27%
Other
Solids 50%
(proteins, lipids, minerals,
carbohydrates, organic
and inorganic materials)
Solids 40%
(proteins, lipids, minerals,
carbohydrates, organic
and inorganic materials)
Other
S
S
Adult male
Adult female
(b)
Figure 27–1
The Composition of the Human Body.
(a)
The body composition (by weight, averaged for both sexes) and major body fluid
compartments of a 70-kg individual. For technical reasons, it is extremely difficult to determine the precise size of any of these compartments;
estimates of their relative sizes vary widely.
(b)
A comparison of the body compositions of adult males and females, ages 18–40 years.
1012
Unit 5
Environmental Exchange
only 10 percent water, whereas skeletal muscle is 75 percent
water.) In both sexes, intracellular fluid contains a greater
proportion of total body water than does extracellular fluid.
Exchange between the ICF and the ECF occurs across plasma
membranes
bicarbonate. The ICF contains an abundance of potassium,
magnesium, and phosphate ions, plus large numbers of neg-
atively charged proteins.
Figure 27–2
compares the ICF with
the two major subdivisions of the ECF.
If the plasma membrane were freely permeable, diffusion
would continue until these ions were evenly distributed
across the membrane. But it does not, because plasma mem-
branes are selectively permeable: Ions can enter or leave the
cell only via specific membrane channels. In addition, carrier
mechanisms move specific ions into or out of the cell.
Despite the differences in the concentration of specific sub-
stances, the osmotic concentrations of the ICF and ECF are
identical. Osmosis eliminates minor differences in concentra-
tion almost at once, because most plasma membranes are freely
permeable to water. (The only noteworthy exceptions are the
apical surfaces of epithelial cells along the ascending limb of
the nephron loop, the distal convoluted tubule, and the collect-
ing system.) Because changes in solute concentrations lead to
immediate changes in water distribution, the regulation of fluid
balance and that of electrolyte balance are tightly intertwined.
Physiologists and clinicians pay particular attention to
ionic distributions across membranes and to the electrolyte
composition of body fluids. The Appendix reports normal
values in the units most often used in clinical reports.
by
osmosis,
diffusion,
and
carrier-mediated
transport.
(To
review
the
mechanisms
involved,
see
Table 3–3, p. 99.)
The ECF and the ICF
The largest subdivisions of the ECF are the interstitial fluid of
peripheral tissues and the plasma of circulating blood
(
Figure 27–1a
). Minor components of the ECF include
lymph, cerebrospinal fluid (CSF), synovial fluid, serous flu-
ids (pleural, pericardial, and peritoneal fluids), aqueous hu-
mor, perilymph, and endolymph. More precise measurements
of total body water provide additional information on sex-
linked differences in the distribution of body water
(
Figure 27–1b
). The greatest variation is in the ICF, as a result
of differences in the intracellular water content of fat versus
muscle. Less striking differences occur in the ECF values, due
to variations in the interstitial fluid volume of various tissues
and the larger blood volume in males versus females.
In clinical situations, it is customary to estimate that two-
thirds of the total body water is in the ICF and one-third in the
ECF. This ratio underestimates the real volume of the ECF, but
that underestimation is appropriate because portions of the
ECF—including the water in bone, in many dense connective
tissues, and in many of the minor ECF components—are rel-
atively isolated. Exchange between these fluid volumes and
the rest of the ECF occurs more slowly than does exchange
between plasma and other interstitial fluids. As a result, they
can be safely ignored in many cases. Clinical attention is usu-
ally focused on the rapid fluid and solute movements associ-
ated with the administration of blood, plasma, or saline
solutions to counteract blood loss or dehydration.
Exchange among the subdivisions of the ECF occurs pri-
marily across the endothelial lining of capillaries. Fluid may
also travel from the interstitial spaces to plasma through lym-
phatic vessels that drain into the venous system.
l
p. 779
The identities and quantities of dissolved electrolytes, pro-
teins, nutrients, and waste products in the ECF vary region-
ally. (For a chemical analysis of the composition of ECF
compartments, see the Appendix.) Still, the variations among
the segments of the ECF seem minor compared with the ma-
jor differences between the ECF and the ICF.
The ECF and ICF are called
fluid compartments
, be-
cause they commonly behave as distinct entities. The pres-
ence of a plasma membrane and active transport at the
membrane surface enable cells to maintain internal environ-
ments with a composition that differs from their surround-
ings. The principal ions in the ECF are sodium, chloride, and
Basic Concepts in the Regulation
of Fluids and Electrolytes
Before we can proceed to a discussion of fluid balance and elec-
trolyte balance, you must understand four basic principles:
1.
All the Homeostatic Mechanisms That Monitor and Adjust
the Composition of Body Fluids Respond to Changes in the
ECF, Not in the ICF.
Receptors monitoring the composi-
tion of two key components of the ECF—plasma and
cerebrospinal fluid—detect significant changes in their
composition or volume and trigger appropriate neural
and endocrine responses. This arrangement makes func-
tional sense, because a change in one ECF component
will spread rapidly throughout the extracellular compart-
ment and affect all the body’s cells. In contrast, the ICF is
contained within trillions of individual cells that are
physically and chemically isolated from one another by
their plasma membranes. Thus, changes in the ICF in one
cell have no direct effect on the composition of the ICF in
distant cells and tissues, unless those changes also affect
the ECF.
2.
No Receptors Directly Monitor Fluid or Electrolyte Balance.
In other words, receptors cannot detect how many liters
of water or grams of sodium, chloride, or potassium the
body contains, or count how many liters or grams we
gain or lose in the course of a day. But receptors
can
mon-
1013
Chapter 27
Fluid, Electrolyte, and Acid–Base Balance
CATIONS
ANIONS
ECF
ICF
ECF
ICF
200
200
KEY
Cations
Na
+
HCO
3
–
Na
+
Cl
–
K
+
150
150
Ca
2
+
HCO
3
–
HCO
3
–
Mg
2
+
HPO
4
2
–
K
+
Anions
100
100
HCO
3
–
Cl
–
Na
+
Cl
–
Na
+
SO
4
2
–
Cl
–
HPO
4
2
–
SO
4
2
–
50
50
HPO
4
2
–
Organic
acid
Proteins
Org. acid
HPO
4
2
–
SO
4
2
–
Mg
2
+
Proteins
K
+
Proteins
K
+
Ca
2
+
0
0
Plasma
Plasma
Interstitial
fluid
Intracellular
fluid
Interstitial
fluid
Intracellular
fluid
Figure 27–2
Cations and Anions in Body Fluids.
Notice the differences in cation and anion concentrations in the various body fluid
compartments. For information about the composition of other body fluids, see the Appendix.
itor
plasma volume
and
osmotic concentration
. Because
fluid continuously circulates between interstitial fluid
and plasma, and because exchange occurs between the
ECF and the ICF, the plasma volume and osmotic con-
centration are good indicators of the state of fluid balance
and electrolyte balance for the body as a whole.
3.
Cells Cannot Move Water Molecules by Active Transport.
All
movement of water across plasma membranes and ep-
ithelia occurs passively, in response to osmotic gradients
established by the active transport of specific ions, such
as sodium and chloride. You may find it useful to remem-
ber that
“water follows salt
.
”
As we saw in earlier chapters,
when sodium and chloride ions (or other solutes) are ac-
tively transported across a membrane or epithelium, wa-
ter follows by osmosis.
l
p. 986
This basic principle
accounts for water absorption across the digestive epithe-
lium, and for water conservation in the kidneys.
4.
The Body’s Content of Water or Electrolytes Will Rise if Di-
etary Gains Exceed Losses to the Environment, and Will Fall
if Losses Exceed Gains.
This basic rule is important when
you consider the mechanics of fluid balance and elec-
trolyte balance. Homeostatic adjustments generally affect
the balance between urinary excretion and dietary ab-
sorption. As we saw in Chapter 26, the physiological ad-
justments in renal function are regulated primarily by
circulating hormones. These hormones can also produce
complementary changes in behavior. For example, the
combination of angiotensin II and aldosterone can give
you a sensation of thirst—which stimulates you to drink
fluids—and a taste for heavily salted foods.
An Overview of the Primary
Regulatory Hormones
Major physiological adjustments affecting fluid balance and
electrolyte balance are mediated by three hormones: (1)
anti-
diuretic hormone (ADH)
, (2)
aldosterone
, and (3) the
natri-
uretic peptides (ANP and BNP)
. These hormones were
introduced and discussed in earlier chapters; we will summa-
rize their effects next. Those interested in a more detailed re-
view should refer to the appropriate sections of Chapters 18,
21, and 26. The interactions among these hormones were il-
lustrated in
Figures 18–17b, 21–16, 21–17
, and
26–11
(pp. 636,
743, 746, 983).
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