Biological Chemistry - Berezov, T. T., Korovkin, B. F. 1998

Kidneys and Urine
The role of the kidneys in maintaining acid-base balance

The Kidneys exert a profound influence on the acid-base balance, though this effect takes significantly longer to manifest than that of the Blood Buffer Systems and the Lungs. The action of blood buffer systems becomes apparent within 30 s. While the lungs require roughly 1—3 min to compensate for an emerging shift in blood hydrogen ion concentration, the kidneys need about 10—20 h to restore disturbed acid-base equilibrium.

The primary mechanism for maintaining hydrogen ion concentration in the body, executed within the Cells of the renal tubules, involves sodium reabsorption and hydrogen ion secretion. This mechanism operates through several chemical processes. The first is sodium reabsorption coupled with The conversion of dibasic phosphates into monobasic phosphates. The renal filtrate formed in the glomeruli contains an adequate amount of salts, including phosphates. However, the concentration of dibasic phosphates progressively decreases as the primary urine moves along the renal tubules. For instance, in the blood, The ratio of monobasic to dibasic phosphate is 1:4, in the glomerular filtrate it is 9:1, and in the urine passing through the distal segment of the nephron it reaches 50:1. This is attributed to the selective reabsorption of sodium ions by the tubular cells. In exchange, hydrogen ions are secreted from the tubular cells into the lumen of the renal tubule. Thus, dibasic phosphate Na₂HPO₄ is converted into monobasic NaH₂PO₄ and excreted in the urine in this form. In the tubular cells, bicarbonate is formed from carbonic acid, thereby increasing the alkaline reserve of the blood.

The second chemical process that ensures sodium retention in the body and the elimination of excess hydrogen ions is the conversion of bicarbonates into carbonic acid within the tubular lumen. In the tubular cells, the interaction of Water and carbon dioxide, facilitated by Carbonic anhydrase, produces carbonic acid. The hydrogen ions of carbonic acid are secreted into the tubular lumen, where they combine with bicarbonate anions; an equivalent amount of sodium enters the renal tubular cells. The Н₂СО₃ formed in the lumen readily dissociates into СО₂ and Н₂О, leaving the body in this form.

The third process contributing to sodium conservation is the renal production of ammonia, which is utilized in place of other cations to neutralize and excrete acid equivalents in the urine. The main sources for this are the deamination of glutamine, as well as the Oxidative Deamination of Amino Acids, predominantly glutamic acid.

The breakdown of glutamine proceeds with the participation of the enzyme glutaminase, yielding glutamic acid and free ammonia (see Chapter 12). Glutaminase is found in various human Organs and Tissues, yet its highest activity is observed in renal tissues. Ultimately, the ratio of hydrogen ion concentration in urine to that in blood can reach 800:1—such is the remarkable capacity of the kidneys to eliminate hydrogen ions from the body. This process is intensified whenever There is a tendency toward the accumulation of hydrogen ions in the Organism.