Essentials of Anatomy and Physiology

people, who may become severely dehydrated before
they begin to feel thirsty. An important behavioral
consideration is that elderly people who fear urinary
incontinence may decrease their intake of fluids.
Deficiencies of minerals in elderly people may be

the result of poor nutrition or a side effect of some

medications, especially those for hypertension that

increase urinary output. Disturbances in pH may be

caused by chronic pulmonary disease, diabetes, or kid-

ney disease.

Fluid–Electrolyte and Acid–Base Balance 451

STUDY OUTLINE

Fluid–Electrolyte Balance

1. Water makes up 55% to 75% of the total body
   weight.


2. Electrolytes are the ions found in body fluids; most
   are minerals.

Water Compartments (see Fig. 19–1)

1. Intracellular fluid (ICF)—water within cells; about
   two-thirds of total body water.


2. Extracellular fluid (ECF)—water outside cells;
   includes plasma, lymph, tissue fluid, and specialized
   fluids.


3. Water constantly moves from one compartment
   to another. Filtration: plasma becomes tissue fluid.
   Osmosis: tissue fluid becomes plasma, or lymph, or
   ICF.


4. Osmosis is regulated by the concentration of elec-
   trolytes in body fluids (osmolarity). Water will dif-
   fuse through membranes to areas of greater
   electrolyte concentration.

Water Intake (see Fig. 19–2)

1. Fluids, food, metabolic water—see Table 19–1.

Water Output (see Fig. 19–2)

1. Urine, sweat, exhaled air, feces—see Table 19–1.


2. Any variation in output must be compensated for
   by a change in input.

Regulation of Water Intake and Output

1. Hypothalamus contains osmoreceptors that detect
   changes in osmolarity of body fluids.


2. Dehydration stimulates the sensation of thirst, and
   fluids are consumed to relieve it.


3. ADH released from the posterior pituitary
   increases the reabsorption of water by the kidneys.


4. Aldosterone secreted by the adrenal cortex
   increases the reabsorption of Naions by the kid-
   neys; water is then reabsorbed by osmosis.
   5. If there is too much water in the body, secretion of
   ADH decreases, and urinary output increases.
   6. If blood volume increases, ANP promotes loss of
   Naions and water in urine.

Electrolytes

1. Chemicals that dissolve in water and dissociate into
   ions; most are inorganic.


2. Cations are positive ions such as Naand K.


3. Anions are negative ions such as Cland HCO 3 .


4. By creating osmotic pressure, electrolytes regulate
   the osmosis of water between compartments.

Electrolytes in Body Fluids (see Fig. 19–3 and

Table 19–2)

1. ICF—principal cation is K; principal anion is
   HPO 4 ^2 ; protein anions are also abundant.


2. Plasma—principal cation is Na; principal anion is
   Cl; protein anions are significant.


3. Tissue fluid—same as plasma except that protein
   anions are insignificant.

Intake, Output, and Regulation

1. Intake—electrolytes are part of food and beverages.


2. Output—urine, sweat, feces.


3. Hormones involved: aldosterone—Na and K;
   ANP—Na; PTH and calcitonin—Ca^2 and
   HPO 4 ^2.

Acid–Base Balance

1. Normal pH Ranges—blood: 7.35 to 7.45; ICF: 6.8
   to 7.0; tissue fluid: similar to blood.


2. Normal pH of body fluids is maintained by buffer
   systems, respirations, and the kidneys.

Buffer Systems

1. Each consists of a weak acid and a weak base; react
   with strong acids or bases to change them to sub-
   stances that do not greatly affect pH. React within