Page 1056 - Fundamentals of anatomy physiology

Chapter 27 Fluid, Electrolyte, and Acid–Base Balance 1043
external environment. Stabilizing the volumes, solute concen-
trations, and pH of the ECF and the ICF involves three inter-
related processes:
1. Fluid Balance. You are in 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 main source of water gains. Metabolic activity generates
a small amount of additional water. The urinary system is
the primary route for water loss under normal conditions,
but as we saw in Chapter 25, sweating can become impor-
tant when body temperature is elevated. p. 991 Although
cells and tissues cannot transport water, they can transport
ions and create concentration gradients that are then elimi-
nated by osmosis.
2. Electrolyte Balance. Electrolytes are ions released when
inorganic compounds dissociate. They are so named be-
cause they can conduct an electrical current in a solution.
p. 67 Each day, your body fluids gain electrolytes from
the food and drink you consume. Your body fluids also
lose electrolytes in urine, sweat, and feces. For each ion,
daily gains must balance daily losses. For example, if you
lose 500 mg of Na1 in urine and sweat, you need to gain
500 mg of Na1 from food and drink to remain in sodium
balance. If the gains and losses for every electrolyte are in distribution is based on overall average values for males and
balance, you are said to be in electrolyte balance. Elec-
trolyte balance primarily involves balancing the rates of
absorption across the digestive tract with rates of loss by
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. p. 67 Prevent-
ing a decrease in pH is the primary problem, because your
body generates a variety of acids during normal metabolic
operations. The kidneys play a major role by secreting hy-
drogen ions into the urine and generating buffers that enter
the bloodstream. Such secretion takes place primarily in
the distal segments of the distal convoluted tubule (DCT)
and along the collecting system. p. 1022 The lungs also
play a key role by eliminating carbon dioxide.
Much of the information in this chapter should be famil-
iar to you from discussions in earlier chapters about aspects
of fluid, electrolyte, or acid–base balance that affect specific
systems. In this chapter we provide an overview that integrates
those discussions to highlight important functional patterns.
This chapter has wide-ranging clinical importance: Steps
to restore normal fluid, electrolyte, and acid–base balances
must be part of the treatment of any serious illness affecting
the nervous, cardiovascular, respiratory, urinary, or digestive
system. Because this chapter builds on information in earlier
chapters, we have included many references to relevant discus-
sions and figures that can provide a quick review.
Ch e c k p o i n t
1. Identify the three interrelated processes essential to
stabilizing body fluid volumes.
2. List the components of extracellular fluid (ECF) and
intracellular fluid (ICF).
See the blue Answers tab at the back of the book.
27-2 The ECF and ICF make up the
fluid compartments, which also contain
cations and anions
Learning Outcome 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.
Figure 27–1a presents an overview of the body makeup of
a 70-kg (154-lb) person with a minimum of body fat. The
females ages 18–40 years. Water makes up about 60 percent of
the total body weight of an adult male, and 50 percent of that
of an adult female (Figure 27–1b). This difference between the
sexes reflects the proportionately larger mass of adipose tissue
in adult females, and the greater average muscle mass in adult
males. (Adipose tissue is 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 be-
tween the ICF and the ECF takes place across plasma membranes
by osmosis, diffusion, and carrier-mediated transport. (To review
the mechanisms involved, see Spotlight Figure 3–22, p. 124.)
The ECF and the ICF 27
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 fluids (pleural,
pericardial, and peritoneal fluids), aqueous humor, perilymph,
and endolymph. More precise measurements of total body
water provide additional information on differences between
the sexes 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.