Impairment of Lung Function Can Cause Respiratory Acidosis

We have discussed thus far the role of the normal respiratory mechanism as a means of buffering changes in H+ concentration. However, abnormalities of respiration can also cause changes in H+ concentration. For example, an impairment of lung function, such as severe emphysema, decreases the ability of the lungs to eliminate CO2; this causes a buildup of CO2 in the extracellular fluid and a tendency toward respiratory acidosis. Also, the ability to respond to metabolic aci-dosis is impaired because the compensatory reductions in PCO2 that would normally occur by means of increased ventilation are blunted. In these circumstances, the kidneys represent the sole remaining physiologic mechanism for returning pH toward normal after the initial chemical buffering in the extracellular fluid has occurred.

Aerosol Surface Tension
Figure 30-3

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. Conversely, 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 H2CO3 and, therefore, 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 quantitatively more important than the excretion of nonvolatile 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 reabsorbed from the tubules, thereby conserving the primary buffer system of the extracellular fluid.

As discussed later, both the reabsorption of bicarbonate 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 extracellular 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 + concentration through three fundamental mechanisms: (1) secretion of H+, (2) reabsorption of filtered HCOi, and (3) production of new HCO3. All these processes are accomplished through the same basic mechanism, as discussed in the next few sections.

Secretion of Hydrogen Ions and Reabsorption of Bicarbonate Ions by the Renal Tubules

Hydrogen ion secretion and bicarbonate reabsorption occur in virtually all parts of the tubules except the descending and ascending thin limbs of the loop of Henle. Figure 30-4 summarizes bicarbonate reabsorption along the tubule. Keep in mind that for each bicarbonate reabsorbed, an H+ must be secreted.

About 80 to 90 per cent of the bicarbonate reabsorption (and H+ secretion) occurs in the proximal tubule, so that only a small amount of bicarbonate flows into the distal tubules and collecting ducts. In the

Figure 30-4

Reabsorption of bicarbonate in different segments of the renal tubule. The percentages of the filtered load of bicarbonate absorbed by the various tubular segments are shown, as well as the number of milliequivalents reabsorbed per day under normal conditions.

Figure 30-4

Reabsorption of bicarbonate in different segments of the renal tubule. The percentages of the filtered load of bicarbonate absorbed by the various tubular segments are shown, as well as the number of milliequivalents reabsorbed per day under normal conditions.

thick ascending loop of Henle, another 10 per cent of the filtered bicarbonate is reabsorbed, and the remainder of the reabsorption takes place in the distal tubule and collecting duct.As discussed previously, the mechanism by which bicarbonate is reabsorbed also involves tubular secretion of H+, but different tubular segments accomplish this task differently.

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Responses

  • hagos
    What can cause lung impairment?
    6 years ago
  • Erno
    How does lung impairment cause acidosis?
    5 years ago

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