E l e c t r o l y t e a n d p r o t e i n a n i o n c o n c e n t r a t i o n s in b o d y f l u i d s
F i g u r e 1 5 . 1 0
Sodium (Na+), and chloride (Cl-) are the most abundant ions in extracellular
fluid. Potassium ions (K+) are the most abundant ions in intracellular fluid.
1 0 0
Electrolytes and protein anions dissolved in various fluid compartments
Fluid Balance Depends Primarily
on Electrolyte Balance
The solutes or dissolved substances in body fluids consist
of various ions and proteins (
F ig u r e 1 5 . 1 0
). While water
moves freely among the body’s flu-
id compartments, its flow is driven
ions, also called
e l e c t r o ly t e s
The ions formed when elec-
trolytes break apart
general functions in the body:
Certain ions control the osmosis of water between
fluid compartments, for example, sodium ions (Na+),
chloride ions (Cl-), and potassium ions (K+).
Ions maintain acid-base balance required for normal
cellular activities: for example, H+ (acid) and the bicar-
bonate ion, HCO3- (base).
Ions such as Na+, K+, and calcium (Ca2+) carry elec-
trical current for action potentials.
Several ions, such as Ca2+ and magnesium (Mg2+), are
cofactors (nonprotein substances) for enzymes.
Here we’ll focus on the first function just listed, control
of osmosis between fluid compartments. The distribution
of electrolytes in plasma, interstitial fluid, and intracel-
lular fluid regulates the osmotic gradient and therefore
has an impact on movement of water between fluid com-
partments. Na+ plays a pivotal role in fluid and electrolyte
balance because it accounts for almost half of the osmotic
pressure of extracellular fluid. Cl- moves easily between
the intracellular and extracellular compartments and
helps to balance the level of anions in each compartment.
Very small changes in the concentration of any of these
solutes can cause water to move from one compartment
What happens if we lose electrolytes through sweating
or if we do not take in enough? In fact, intake of water and
electrolytes rarely occurs in exactly the same proportion
as their presence in body fluids; the kidneys help the body
to achieve the right balance and maintain homeostasis in
two ways. The kidneys can excrete excess water by produc-
ing dilute urine, and they can excrete excess electrolytes
by producing concentrated urine.
Because water is a primary component of blood, a
change in the water level changes blood volume and,
e le c t r o ly t e
tro-Utz) A compound
that separates into
ions when dissolved
in water and that con-
454 CHAPTER 15
The Urinary System and Fluid, Electrolyte, and Acid-Base Balance