OBJECTIVE 3: TO DETERMINE THE FACTORS THAT ALTER THE
RATE OF PROTON SECRETION.
A. Buffers present in the tubular fluid remove secreted protons from solution, thereby maintaining a low concentration (high pH) in tubular fluid which promotes secretion by the proton transporters (fig. 9-3). Bicarbonate is the most abundant buffer in the glomerular filtrate and factors that alter its concentration in tubular fluid strongly affect the rate of proton secretion. A rise in the HCO3 concentration in the plasma increases the concentration in the filtrate.
Fig. 9-3. The factors controlling the rate of proton secretion.
B. An increase in blood PCO2 raises the proton concentration in the cytoplasm within proximal tubular cells and this stimulates the proton transporters as described above. An increase in blood PCO2 also stimulates the insertion of pump units into the apical membrane thereby stimulating proton secretion (fig. 9-3).
C. A rise in the rate of proton secretion increases the rate of HCO3 reabsorption and the rate of acid excretion (fig. 9-3).
QUESTIONS:
5. A major factor limiting the rate
of net hydrogen secretion is the effect of a rise in tubular fluid H+
concentration on the pump rate. How does the tubular fluid buffer
concentration affect this? What other factors affect the rate of H+
secretion?
6. How would a fall in the rate of proton secretion
affect the rates of bicarbonate excretion and acid excretion?
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