Maintenance of the Body
more body water than adults, this does not protect them
from excessive fluid shiFs. Even slight alterations in fluid
balance can cause serious problems. ±urther, although adults
can live without water for about ten days, infants can survive
for only three to four days.
Te relatively high infant metabolic rate
(about twice that
of adults). Te higher metabolic rate yields much larger
amounts of metabolic wastes and acids that the kidneys must
excrete. Tis, along with buffer systems that are not yet fully
effective, results in a tendency toward acidosis.
Te high rate of insensible water loss in infants because of their
larger surface area relative to body volume
(about three times
that of adults). Infants lose substantial amounts of water
through their skin.
Te inefficiency of infant kidneys.
At birth, the kidneys are
immature, only about half as proficient as adult kidneys at
concentrating urine, and notably inefficient at ridding the
body of acids.
All these factors put newborns at risk for dehydration and
acidosis, at least until the end of the first month when the kid-
neys achieve reasonable efficiency. Bouts of vomiting or di-
arrhea greatly amplify the risk.
In old age, total body water oFen decreases (the loss is largely
from the intracellular compartment) because muscle mass pro-
gressively declines and body fat rises. ±ew changes occur in the
solute concentrations of body fluids, but the speed with which
homeostasis is restored aFer being disrupted declines with age.
Elders may be unresponsive to thirst cues and thus are at risk
for dehydration. Additionally, they are the most frequent prey of
diseases that lead to severe fluid, electrolyte, or acid-base prob-
lems, such as congestive heart failure (and its attendant edema)
and diabetes mellitus. Because most fluid, electrolyte, and acid-
base imbalances occur when body water content is highest or low-
est, the very young and the very old are the most frequent victims.
Check Your Understanding
Infants have a higher urine output than adults relative to
their body weight. In addition to their relatively higher fluid
intake, what are the reasons for this?
For answers, see Appendix H.
In this chapter we have examined the chemical and physi-
ological mechanisms that provide the optimal internal envi-
ronment for survival. Te kidneys are the superstars among
homeostatic organs in regulating water, electrolyte, and acid-
base balance, but they do not and cannot act alone. Rather, their
activity is made possible by a host of hormones and enhanced
both by bloodborne buffers, which give the kidneys time to re-
act, and by the respiratory system, which shoulders a substantial
responsibility for acid-base balance of the blood.
Now that we have discussed the topics relevant to renal func-
tioning, and once you read in
System Connections
how the urinary
system interacts with other body systems, the topics in Chapters
25 and 26 should draw together in an understandable way.
initially), elevated bicarbonate levels (over 26 mEq/L), and a
above 45 mm Hg.
Renal Compensations
When an acid-base imbalance is of res-
piratory origin, renal mechanisms are stepped up to compensate
for the imbalance. ±or example, a hypoventilating individual
will exhibit acidosis. When renal compensation is occurring,
both the P
and the HCO
levels are high. Te high P
causes the acidosis, and the rising HCO
level indicates that
the kidneys are retaining bicarbonate to offset the acidosis.
Conversely, a person with renal-compensated respiratory al-
kalosis will have a high blood pH and a low P
. Bicarbonate
ion levels begin to fall as the kidneys eliminate more HCO
from the body by failing to reclaim it or by actively secreting it.
Note that the kidneys cannot compensate for alkalosis or acido-
sis if that condition reflects a
Check Your Understanding
Which two abnormalities in plasma are key features of an
uncompensated metabolic alkalosis? An uncompensated
respiratory acidosis?
How do the kidneys compensate for respiratory acidosis?
For answers, see Appendix H.
Developmental Aspects of Fluid,
Electrolyte, and
Acid-Base Balance
Explain why infants and the aged are at greater risk for
fluid and electrolyte imbalances than are young adults.
An embryo and a very young fetus are more than 90% water, but
solids accumulate as fetal development continues, and at birth
an infant is “only” 70–80% water. (Te average value for adults
is 50–60%.) Infants have proportionately more EC± than adults
and, consequently, a much higher NaCl content in relation to
, Mg
, and PO
Distribution of body water begins to change about two
months aFer birth and achieves the adult pattern by the time the
child is 2 years old. Plasma electrolyte concentrations are simi-
lar in infants and adults, but K
and Ca
values are higher and
, and total protein levels are lower in the first few
days of life than at any other time. At puberty, sex differences in
body water content become obvious as males develop relatively
greater amounts of skeletal muscle.
Problems with fluid, electrolyte, and particularly acid-base
balance are most common in infancy, reflecting the following
Te very low residual volume of infant lungs
half that of adults relative to body weight). When respiration
is altered, P
can shiF rapidly and dramatically.
Te high rate of fluid intake and output in infants
(about seven
times higher than in adults). Infants may exchange fully half
their EC± daily. Tough infants have proportionately much
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