If the concentration of potassium ions increases, the kidneys conserve sodium ions and excrete potassium ions.

tion, muscle fiber contraction, and maintenance of cell membrane permeability. Potassium is especially important in maintaining the resting potential of nerve and cardiac muscle cells, and abnormal potassium levels may cause these cells to function abnormally.

Sodium ions account for nearly 90% of the positively charged ions in extracellular fluids. The kidneys and the hormone aldosterone provide the primary mechanism regulating these ions. Aldoste-rone, which the adrenal cortex secretes, increases sodium ion reabsorption in the distal convoluted tubules and collecting ducts of the nephrons. A decrease in sodium ion concentration in the extracellular fluid stimulates aldosterone secretion via the renin-angiotensin system, as described in chapter 20 (p. 834 and fig. 20.18).

Aldosterone also regulates potassium ions. An important stimulus for aldosterone secretion is a rising potassium ion concentration, which directly stimulates cells of the adrenal cortex. This hormone enhances the renal tubular reabsorption of sodium ions and, at the same time, stimulates renal tubular secretion of potassium ions (fig. 21.8).

Recall from chapter 13 (p. 524) that the calcium ion concentration dropping below normal directly stimulates the parathyroid glands to secrete parathyroid hormone. Parathyroid hormone increases activity in bone-resorbing cells (osteocytes and osteoclasts), which increases the concentrations of both calcium and phosphate ions in the extracellular fluids. Parathyroid hormone also indirectly stimulates calcium absorption from the intestine. Concurrently, this hormone causes the kid

If the concentration of calcium ions decreases, parathyroid hormone increases calcium ion concentration. Increased urinary phosphate excretion offsets bone resorption (which increases blood phosphate levels) to maintain a normal concentration of phosphate ions.

neys to conserve calcium ions (through increased tubular reabsorption) and increases the urinary excretion of phosphate ions. The increased phosphate excretion offsets the increased plasma phosphate. Thus, the net effect of the hormone is to return the calcium ion concentration of the extracellular fluid to normal levels but to maintain a normal phosphate ion concentration (fig. 21.9).

Abnormal increases in blood calcium (hypercalcemia) sometimes result from hyperparathyroidism, in which excess secretion of PTH increases bone resorption. Hypercalcemia may also be caused by cancers, particularly those originating in the bone marrow, breasts, lungs, or prostate gland. Usually the increase in calcium occurs when cancer causes bone tissue to release ions. In other cases, however, the blood calcium concentration increases when cancer cells produce bio-chemicals that have physiological effects similar to parathyroid hormone. This most often occurs in lung cancer. Symptoms of cancer-induced hypercalcemia include weakness and fatigue, impaired mental function, headache, nausea, increased urine volume (polyuria), and increased thirst (polydipsia).

Abnormal decreases in blood calcium (hypocalcemia) may result from reduced availability of PTH following removal of the parathyroid glands, or from vitamin D deficiency, which may result from decreased absorption following gastrointestinal surgery or excess excretion due to kidney disease. Hypocalcemia may be life-threatening because it may produce muscle spasms within the airways and cardiac arrhythmias. Administering calcium salts and high doses of vitamin D to promote calcium absorption can correct this condition.

Acid-Base Balance

As discussed in chapter 2 (p. 47), electrolytes that ionize in water and release hydrogen ions are called acids. Substances that combine with hydrogen ions are called bases. Thus, acid-base balance primarily concerns regulation of the hydrogen ion concentration of body fluids. Regulation of hydrogen ions is very important because slight changes in hydrogen ion concentrations can alter the rates of enzyme-controlled metabolic reactions, shift the distribution of other ions, or modify hormone actions. Recall that the internal environment is normally maintained between pH 7.35 and 7.45.

Sources of Hydrogen Ions

Most of the hydrogen ions in body fluids originate as byproducts of metabolic processes, although the digestive tract may directly absorb small quantities. The major metabolic sources of hydrogen ions include the following (All of these are reversible reactions but, for clarity, are presented as the net reaction only. Remember, it is the concentration of H+ at equilibrium that determines the pH.):

1. Aerobic respiration of glucose. This process produces carbon dioxide and water. Carbon dioxide diffuses out of the cells and reacts with water in the extracellular fluids to form carbonic acid:

Generally, the regulatory mechanisms that control positively charged ions secondarily control the concentrations of negatively charged ions. For example, chloride ions (Cl-), the most abundant negatively charged ions in the extracellular fluids, are passively reabsorbed from the renal tubules in response to the active reabsorption of sodium ions. That is, the negatively charged chloride ions are electrically attracted to the positively charged sodium ions and accompany them as they are reabsorbed.

Some negatively charged ions, such as phosphate ions (PO4-3) and sulfate ions (SO4-2), also are partially regulated by active transport mechanisms that have limited transport capacities. Thus, if the extracellular phosphate ion concentration is low, the phosphate ions in the renal tubules are conserved. On the other hand, if the renal plasma threshold is exceeded, the excess phosphate will be excreted in the urine. Clinical Application 21.2 discusses symptoms associated with sodium and potassium imbalances.

H How does aldosterone regulate sodium and potassium ion concentration?

^9 How is calcium regulated?

^9 What mechanism regulates the concentrations of most negatively charged ions?

The resulting carbonic acid then ionizes to release hydrogen ions and bicarbonate ions:

2. Anaerobic respiration of glucose. Glucose metabolized anaerobically produces lactic acid, which adds hydrogen ions to body fluids.

3. Incomplete oxidation of fatty acids. The incomplete oxidation of fatty acids produces acidic ketone bodies, which increase hydrogen ion concentration.

4. Oxidation of amino acids containing sulfur. The oxidation of sulfur-containing amino acids yields sulfuric acid (H2SO4), which ionizes to release hydrogen ions.

5. Breakdown (hydrolysis) of phosphoproteins and nucleic acids. Phosphoproteins and nucleic acids contain phosphorus. Their oxidation produces phosphoric acid (H3PO4), which ionizes to release hydrogen ions.

The acids resulting from metabolism vary in strength. Thus, their effects on the hydrogen ion concentration of body fluids vary (fig. 21.10).

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