Chapter 39: Acidification of the Urine & Bicarbonate Excretion

OBJECTIVES

After reading this chapter, youshould be able to:

• Outline the processes involved in the secretion of H⁺ into the tubules and discuss the significance of these processes in the regulation of acid–base balance.

• Define acidosis and alkalosis, and give (in mEq/L and pH) the normal mean andthe range of H⁺ concentrations in blood that are compatible with health.

• List the principal buffers in blood, interstitial fluid, and intracellular fluid, and, using the Henderson-Hasselbalch equation, describe what is unique about the bicarbonate buffer system.

• Describe the changes in blood chemistry that occur during the development of metabolic acidosis and metabolic alkalosis, and the respiratory and renal compensations for these conditions.

• Describe the changes in blood chemistry that occur during the development of respiratory acidosis and respiratory alkalosis, and the renal compensation forthese conditions.

INTRODUCTION

The kidneys play a key role in the maintenance of acid–base balance and to do this they must excrete acid in the amount equivalent to the production of nonvolatile acids in the body. The production of nonvolatile acids will vary with diet, metabolism, and disease. The kidneys must also filter and reabsorb plasma bicarbonate, and thus prevent the loss of bicarbonate in the urine. Both processes are linked physiologically, due to the nephron’s ability to secrete H⁺ ions into the filtrate.

RENAL H⁺ SECRETION

The cells of the proximal and distal tubules, like the cells of the gastric glands (see Chapter 25), secrete hydrogen ions. Hydrogen secretion also occurs in the collecting ducts. The transporter that is responsible for H⁺ secretion in the proximal tubules is the Na–H exchanger (primarily NHE3) (Figure 39–1). This is an example of secondary active transport; Na⁺ is moved from the inside of the cell to the interstitium by Na, K ATPase on the basolateral membrane, which keeps intracellular Na⁺ low, thus establishing the drive for Na⁺ to enter the cell, via the Na–H exchanger, from the tubular lumen. The Na–H exchanger secretes H⁺ into the lumen in exchange for Na⁺.

The secreted H⁺ ion combines with filtered HCO₃^– to form H₂CO₃ and the presence of carbonic anhydrase on the apical membrane of the proximal tubule catalyzes the formation of H₂O and CO₂ from H₂CO₃. The apical membrane of epithelial cells lining the proximal tubule is permeable to CO₂ and H_...