Not only does the alveolar ventilation rate influence H+ concentration by changing the Pco2 of the body fluids, but the H+ concentration affects the rate of alveolar ventilation. Thus, Figure 30-3 shows that the alveolar ventilation rate increases four to five times normal as the pH decreases from the normal value of 7.4 to the strongly acidic value of 7.0. Conversely, when plasma pH rises above 7.4, this causes a decrease in the ventilation rate. As one can see from the graph, the change in ventilation rate per unit pH change is much greater at reduced levels of pH (corresponding to elevated H+ concentration) compared with increased levels of pH. The reason for this is that as the alveolar ventilation rate decreases, owing to an increase in pH (decreased H+ concentration), the amount of oxygen added to the blood decreases and the partial pressure of oxygen (PO2) in the blood also decreases, which stimulates the ventilation rate. Therefore, the respiratory compensation for an increase in pH is not nearly as effective as the response to a marked reduction in pH.
Effect of blood pH on the rate of alveolar ventilation.
That is, whenever the H+ concentration increases above normal, the respiratory system is stimulated, and alveolar ventilation increases. This decreases the Pco2 in extracellular fluid and reduces H+ concentration back toward normal. Conversely, if H+ concentration falls below normal, the respiratory center becomes depressed, alveolar ventilation decreases, and H+ concentration increases back toward normal.
Efficiency of Respiratory Control of Hydrogen Ion Concentration. Respiratory control cannot return the H+ concentration all the way back to normal when a disturbance outside the respiratory system has altered pH. Ordinarily, the respiratory mechanism for controlling H+ concentration has an effectiveness between 50 and 75 per cent, corresponding to a feedback gain of 1 to 3. That is, if the H+ concentration is suddenly increased by adding acid to the extracellular fluid and pH falls from 7.4 to 7.0, the respiratory system can return the pH to a value of about 7.2 to 7.3. This response occurs within 3 to 12 minutes.
Buffering Power of the Respiratory System. Respiratory regulation of acid-base balance is a physiologic type of buffer system because it acts rapidly and keeps the H+ concentration from changing too much until the slowly responding kidneys can eliminate the imbalance. In general, the overall buffering power of the respiratory system is one to two times as great as the buffering power of all other chemical buffers in the extracellular fluid combined. That is, one to two times as much acid or base can normally be buffered by this mechanism as by the chemical buffers.
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