160
U N I T 2
Integrative Body Functions
fluid contained within all of the trillions of cells in the
body, contains about two thirds of the body water in
healthy adults. The remaining one third is in the
extra-
cellular fluid (ECF) compartment
, which contains all
the fluids outside the cells, including those in the inter-
stitial or tissue spaces and the plasma in the blood ves-
sels (Fig. 8-1).
The composition of the ECF and ICF are strikingly
different. The ECF contains large amounts of sodium
and chloride, moderate amounts of bicarbonate, but
only small quantities of potassium, magnesium, cal-
cium, and phosphate. In contrast, the ICF contains
almost no calcium; small amounts of sodium, chloride,
bicarbonate, and phosphate; moderate amounts of mag-
nesium; and large amounts of potassium (Table 8-1).
Although blood levels usually are representative of the
total body levels of an electrolyte, this is not always the
case, particularly with potassium, which is approxi-
mately 28 times more concentrated inside the cell than
outside.
Movement of Body Fluids and
Electrolytes Between Compartments
The lipid bilayer and transport proteins serve as the
primary barriers to the movement of substances
across the cell membrane that separates the ECF and
ICF compartments (see Chapter 1, Fig. 1-11). Lipid-
soluble substances (e.g., oxygen [O
2
] and carbon diox-
ide [CO
2
]), which dissolve in the lipid layer of the
cell membrane, pass directly through the membrane.
Many ions (e.g., sodium [Na
+
] and potassium [K
+
])
rely on transport proteins located in the cell mem-
brane for movement across the membrane, accounting
for the compartmental difference in their concentra-
tions. Water crosses the cell membrane by osmosis
using special transmembrane protein channels called
aquaporins.
Dissociation of Electrolytes
Electrolytes are substances that dissociate in solution
to form charged particles, or
ions
. For example, the
sodium chloride (NaCl) molecule dissociates to form
a positively charged Na
+
and a negatively charged Cl
–
ion. Particles that do not dissociate into ions such as
glucose and urea are called
nonelectrolytes
. Positively
charged ions are called
cations
because they are
attracted to the cathode of a wet electric cell, and nega-
tively charged ions are called
anions
because they are
attracted to the anode. The ions found in body fluids
carry one charge (i.e., monovalent ion) or two charges
(i.e., divalent ion). Because of their attraction forces,
positively charged cations are always accompanied by
negatively charged anions. Thus, both the ICF and ECF
contain equal amounts of anions and cations. Cations
and anions may be exchanged for one another, provid-
ing they carry the same charge. For example, a posi-
tively charged hydrogen ion (H
+
) may be exchanged for
a positively charged K
+
, and a negatively charged bicar-
bonate ion (HCO
3
–
) may be exchanged for a negatively
charged chloride ion (Cl
–
).
The concentration of electrolytes in the ICF and ECF
can be expressed in several ways; for example, milli-
grams per deciliter (mg/dL), milliequivalents per liter
(mEq/L), or millimoles per liter (mmol/L) (Box 8-1).
Intracellular
water
Extracellular
(plasma) water
Extracellular
(interstitial) water
FIGURE 8-1.
Distribution of body water.The extracellular space
includes the vascular compartment and the interstitial spaces.
Acid–Base Balance
Acid–Base Chemistry
Acid and Base Production
Calculation of pH
Regulation of pH
Chemical Buffer Systems
Respiratory Control Mechanisms
Renal Control Mechanisms
Laboratory Tests
Disorders of Acid–Base Balance
Primary Versus Compensatory Changes in pH
Single Versus Mixed Acid–Base Disorders
Metabolic Acidosis
Metabolic Alkalosis
Respiratory Acidosis
Respiratory Alkalosis
