Renal Control of Acid-Base Balance
The kidneys control acid-base balance by excreting either an acidic or a basic urine. Excreting an acidic urine reduces the amount of acid in extracellular fluid, whereas excreting a basic urine removes base from the extracellular fluid.
The overall mechanism by which the kidneys excrete acidic or basic urine is as follows: Large numbers of HCO3– are filtered continuously into the tubules, and if they are excreted into the urine, this removes base from the blood. Large numbers of H+ are also secreted into the tubular lumen by the tubular epithelial cells, thus removing acid from the blood. If more H+ is secreted than HCO3– is filtered, there will be a net loss of acid from the extracellular fluid. Con-versely, if more HCO3– is filtered than H+ is secreted, there will be a net loss of base.
As discussed previously, each day the body produces about 80 milliequivalents of nonvolatile acids, mainly from the metabolism of proteins. These acids are called nonvolatile because they are not H2CO3and, there-fore, cannot be excreted by the lungs. The primary mechanism for removal of these acids from the body is renal excretion. The kidneys must also prevent the loss of bicarbonate in the urine, a task that is quanti-tatively more important than the excretion of non-volatile acids. Each day the kidneys filter about 4320 milliequivalents of bicarbonate (180 L/day x 24 mEq/ L); under normal conditions, almost all this is reab-sorbed from the tubules, thereby conserving the primary buffer system of the extracellular fluid.
As discussed later, both the reabsorption of bicar-bonate and the excretion of H+ are accomplished through the process of H+ secretion by the tubules. Because the HCO3– must react with a secreted H+ to form H2CO3 before it can be reabsorbed, 4320 mil-liequivalents of H+ must be secreted each day just to reabsorb the filtered bicarbonate. Then an additional 80 milliequivalents of H+ must be secreted to rid the body of the nonvolatile acids produced each day, for a total of 4400 milliequivalents of H+ secreted into the tubular fluid each day.
When there is a reduction in the extracellular fluid H+ concentration (alkalosis), the kidneys fail to reab-sorb all the filtered bicarbonate, thereby increasing the excretion of bicarbonate. Because HCO3– normally buffers hydrogen in the extracellular fluid, this loss of bicarbonate is the same as adding an H+ to the extra-cellular fluid. Therefore, in alkalosis, the removal of HCO3– raises the extracellular fluid H+ concentration back toward normal.
In acidosis, the kidneys do not excrete bicarbonate into the urine but reabsorb all the filtered bicarbonate and produce new bicarbonate, which is added back to the extracellular fluid. This reduces the extracellular fluid H+ concentration back toward normal.
Thus, the kidneys regulate extracellular fluid H + con-centration through three fundamental mechanisms: (1) secretion of H+, (2) reabsorption of filtered HCO3-, and (3) production of new HCO3-. All these processes areaccomplished through the same basic mechanism.