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Bicarbonate buffer system

Phosphate buffer system

Protein buffer system

Respiratory mechanism (CO2 excretion)

Renal mechanism (H+ excretion)

Figure

Chemical buffers act rapidly, whereas physiological buffers may require several minutes to several days to begin resisting a change in pH.

hydrogen ions in the body fluids originate from carbonic acid produced when carbon dioxide reacts with water, the respiratory regulation of hydrogen ion concentration is important.

Renal Excretion of Hydrogen Ions Nephrons help regulate the hydrogen ion concentration of the body fluids by excreting hydrogen ions in the urine. Recall from chapter 20 (p. 838) that the epithelial cells lining the proximal and distal convoluted tubules and the collecting ducts secrete these ions into the tubular fluid. The tubular secretion of hydrogen ions is linked to tubular reabsorption of bicarbonate ions. In this way, the kidneys also regulate the concentration of bicarbonate ions in body fluids. These mechanisms also help balance the quantities of sulfuric acid, phosphoric acid, and various organic acids that appear in body fluids as byproducts of metabolic processes.

The metabolism of certain amino acids, for example, produces sulfuric and phosphoric acids. Consequently, a diet high in proteins may trigger excess acid formation. The kidneys compensate for such gains in acids by altering the rate of hydrogen ion secretion, thus resisting a shift in the pH of body fluids (fig. 21.12).

Once hydrogen ions are secreted, phosphates that were filtered into the fluid of the renal tubule buffer them. Ammonia aids in this buffering action.

Through deamination of certain amino acids, the cells of the renal tubules produce ammonia (NH3), which diffuses readily through cell membranes and enters the urine. Because ammonia is a weak base, it can accept hydrogen ions to form ammonium ions (NH4+):

Cell membranes are quite impermeable to ammonium ions, which are trapped in the urine as they form and are excreted with the urine. When increase in the hydrogen ion concentration of body fluids is prolonged, the renal tubules increase ammonia production. This mechanism helps to transport excess hydrogen ions to the outside and helps prevent the urine from becoming too acidic.

The various regulators of hydrogen ion concentration operate at different rates. Acid-base buffers function rapidly and can convert strong acids or bases into weak acids or bases almost immediately. For this reason, these chemical buffer systems are sometimes called the body's first line of defense against shifts in pH.

Physiological buffer systems, such as the respiratory and renal mechanisms, function more slowly and constitute secondary defenses. The respiratory mechanism may require several minutes to begin resisting a change in pH, and the renal mechanism may require one to three days to regulate a changing hydrogen ion concentration (fig. 21.13). Clinical Application 21.3 examines the effects of acid-base imbalances.

99 How does the respiratory system help regulate acid-base balance?

^9 How do the kidneys respond to excess hydrogen ions?

^9 How do the rates at which chemical and physiological buffer systems act differ?

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