Chapter 27: Fluid, Electrolyte, and Acid–Base Balance
Fluid, Electrolyte, and Acid–Base Balance: An Overview, p. 995
Most of your body weight is water.
Water accounts 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 distribution of gases, nutrients, and waste products.
To survive, we must maintain a normal volume and composition 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
Stabilizing the volumes,
solute concentrations, and pH of the ECF and the ICF involves three
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 balance 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 additional water is generated by
metabolic activity. The urinary system is the primary route for water loss under normal
conditions. Although cells and tissues cannot transport water, they can transport ions and create
concentration gradients that are then eliminated by osmosis.
Electrolytes are ions released through the dissociation of inorganic
compounds; they are so named because they can conduct an electrical current in a solution. If the
gains and losses for every electrolyte are in balance, you are said to be in electrolyte balance.
Electrolyte balance primarily involves balancing the rates of absorption across the digestive tract
with rates of loss at the kidneys, although losses at sweat glands and other sites can play a
You are in acid–base balance when the production of hydrogen ions in your
body is precisely offset by their loss. When acid–base balance exists, the pH of body fluids
remains within normal limits. Preventing a reduction 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 hydrogen ions into the urine and generating buffers that enter the bloodstream.
Such secretion occurs primarily in the distal segments of the distal convoluted tubule (DCT) and
along the collecting system. The lungs also play a key role through the elimination of carbon
An Introduction to Fluid and Electrolyte Balance, p. 996
Water accounts for roughly 60 percent of the total body weight of an adult male,
and 50 percent of that
of an adult female. This difference between the sexes primarily reflects the proportionately larger mass
of adipose tissue in adult females, and the greater average muscle mass in adult males In both sexes,
intracellular fluid contains a greater proportion of total body water than does extracellular fluid.