Water, Electrolyte, and Acid-Base Balance
death in soldiers in Iraq and Afghanistan are due to heatstroke. For this rea-
son, military o±
cials insist that soldiers carry drinking water with them at all
times and drink throughout the day, whether they feel thirsty or not.
Clues to the cause of heatstroke have come from an unexpected source—
mutant mice bred to have a version of a disease that also affects humans,
called malignant hyperthermia. In this condition, a patient receiving general
anesthesia experiences increased heart rate, whole-body muscle rigidity, and
a spike in body temperature up to 112°F. A²
ected mice die upon exposure to
anesthesia—and also when subjected to 105° heat. The mutant gene encodes
a receptor that admits calcium ions into skeletal muscle cells. Under intense
heat, the receptors allow too many calcium ions to enter, causing uncontrolled
contraction. In addition, calcium ions leak from the receptor area and free radi-
cals are released, which causes oxidative damage. Adding an antioxidant drug
to the animals’ water dampened their reaction to heat.
The experiments done with mice that were genetically susceptible to
heatstroke suggest that people, particularly those with malignant hyper-
thermia, might be predisposed to react dangerously to heat, too. Researchers
are using the mice as models to test treatments for heatstroke—but it is still
wise to avoid extreme heat and get medical attention swiftly if the initial
symptoms of heatstroke appear.
eatstroke is a response to extreme environmental heat that
can be quickly fatal. It occurs when the body is exposed
to a heat index (heat considering humidity) of more than
105°F and body temperature exceeds 106°F. Under these
conditions, evaporation of sweat becomes less e±
at cooling the body, and organs begin to fail.
The symptoms of heatstroke happen in a sequence. First come head-
ache, dizziness, and exhaustion. Sweating is profuse, then stops, as the
skin becomes dry, hot, and red. Respiratory rate rises and the pulse may
race up to 180 beats per minute. If the person isn’t cooled with fluids,
water applied to the skin, fanning, and removal of clothing, neurological
symptoms may begin, including disorientation, hallucinations, and odd
behavior. Kidney failure and/or heart arrhythmia prove fatal.
During heat waves, the very young and the very old are more suscep-
tible to heatstroke because their temperature control mechanisms may
be poor. However, heatstroke also a²
ects two groups of young, otherwise
healthy individuals—athletes who work out in extreme heat and soldiers
deployed to hot climates. In the Persian Gulf in July, the temperature may
soar to 122°F at midday, dipping down only to about 100° at night. Given
these conditions it isn’t surprising that some cases of unexpected sudden
The term
suggests a state of equilibrium. For water
and electrolytes, balance means that the quantities enter-
ing the body equal the quantities leaving. Mechanisms that
replace lost water and electrolytes and excrete excesses
maintain this balance. As a result, the levels of water and
electrolytes in the body remain relatively stable at all times.
Water balance and electrolyte balance are interdepen-
dent, because electrolytes are dissolved in the water of body
fl uids. Consequently, anything that alters the concentrations
of the electrolytes will alter the concentration of the water by
adding solutes to it or by removing solutes from it. Likewise,
anything that changes the concentration of the water will
change the concentrations of the electrolytes by concentrat-
ing or diluting them.
How are ³
uid balance and electrolyte balance interdependent?
Body fl uids are not uniformly distributed. Instead, they occupy
regions, or
of different volumes that contain
fl uids of varying compositions. The movement of water and
electrolytes between these compartments is regulated to stabi-
lize their distribution and the composition of body fl uids.
Fluid Compartments
The body of an average adult female is about 52% water by
weight, and that of an average male is about 63% water.
This difference between the sexes is because females gener-
ally have more adipose tissue, which has little water. Males
have more muscle tissue, which contains a great deal of
water. Water in the body (about 40 liters), with its dissolved
electrolytes, is distributed into two major compartments: an
intracellular fl
uid compartment and an extracellular fl
g. 21.1)
uid compartment
includes all the water and electrolytes that cell membranes
enclose. In other words, intracellular fl uid is the fl uid inside
cells, and, in an adult, it accounts for about 63% by volume
of total body water.
To Chapter 1, Homeostasis, page 9.
uid compartment
includes all the fl uid outside cells—in tissue spaces (interstitial
fl uid), blood vessels (plasma), and lymphatic vessels (lymph).
Epithelial layers separate a specialized fraction of the extracel-
lular fl uid from other extracellular fl uids. This
includes cerebrospinal fl uid of the central
nervous system, aqueous and vitreous humors of the eyes,
synovial fl uid of the joints, serous fl uid in the body cavities,
and fl uid secretions of the exocrine glands. The fl uids of the
extracellular compartment constitute about 37% by volume of
the total body water
g. 21.2)
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