Chapter 19: Fluid–Electrolyte and Acid–Base Balance

Fluid–Electrolyte Balance

1. Water makes up 55% to 70% 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, lymph, or ICF.

4. Osmosis is regulated by the concentration of electrolytes in body fluids (osmolarity). Water will diffuse 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 Na+ ions by the kidneys; water is then reabsorbed by osmosis.

5. If there is too much water in the body (overhydration caused by overconsumption), secretion of ADH decreases and urinary output increases.

6. If blood volume increases, ANP promotes loss of Na+ ions 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 Na+ and K+.

3. Anions are negative ions such as Cl– and HCO3–.

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

5. Calcium, phosphorus, and magnesium are stored in bones; iron and copper are stored in the liver.

Electrolytes in Body Fluids (see Fig. 19-3 and Table 19-2)

1. ICF—principal cation is K+; principal anion is HPO4–2; 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 HPO4 –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 substances that do not greatly affect pH. React within a fraction of a second, but have the least capacity to prevent pH changes.

2. Bicarbonate buffer system—see text for reactions; important in both blood and tissue fluid; base to acid ratio is 20:1.

3. Phosphate buffer system—see Fig. 19-4 and text for reactions; important in ICF and in the kidneys.

4. Protein buffer system—amino acids may act as either acids or bases. See text for reactions; important in ICF.

Respiratory Mechanisms

1. The respiratory system affects pH because it regulates the amount of CO2 in body fluids.

2. May be the cause of a pH change or help compensate for a metabolic pH change—see Table 19-3.

3. Respiratory compensation is rapidly effective (within a few minutes), but limited in capacity if the pH imbalance is ongoing.

Renal Mechanisms

1. The kidneys have the greatest capacity to buffer pH changes, but they may take several hours to days to become effective (see Table 19-3).

2. Reactions: see Figs. 18–6 and 19-4.

3. Summary of reactions: in response to acidosis, the kidneys will excrete H+ ions and retain Na+ ions and HCO3 – ions; in response to alkalosis, the kidneys will retain H+ ions and excrete Na+ ions and HCO3 – ions (see also Fig. 19-5).

4. Only the kidneys are able to remove excess H+ ions from the body to counteract an ongoing acidosis, as well as to create new bicarbonate ions to retain or excrete as the pH situation requires.

Effects of pH Changes

1. Acidosis—depresses synaptic transmission in the CNS; result is confusion, coma, and death.

2. Alkalosis—increases synaptic transmission in the CNS and PNS; result is irritability, muscle spasms, and convulsions.