The Controller and Acid-Base Physiology: An Introduction to a Complex Process

The Kidneys and Acid Elimination

It is beyond the scope of this book to detail the physiology of the kidneys and their role in acid-base homeostasis. However, a full understanding of the workings of the body requires one to integrate the physiology of multiple organ systems (you will see this again in Chapter 9, in which we introduce concepts of cardiovascular physiology in our exploration of exercise). Therefore, we will highlight a few of the major physiologic processes by which the kidneys contribute to acid-base balance.

The process of normal living requires the generation of energy and the metabolism of a mixture of carbohydrates, fats, and proteins. As already discussed, consumption of carbohydrates and fats leads primarily to the production of carbon dioxide, which is in equilibrium with carbonic acid. In a typical adult diet, we also metabolize protein, which results in the production of an additional 50 to 100 meq of noncarbonic acid every day. These hydrogen ions must be excreted in the urine.

The kidneys are designed to remove chemicals from the blood via filtration in the glomerulus and secretion in the tubule and to reabsorb some of these filtered molecules and ions at other portions of the tubule to maintain chemical, water, and acid-base balance. Bicarbonate is filtered in the glomerulus and must be reabsorbed to avoid lowering the serum bicarbonate level. Approximately 90% of bicarbonate reabsorption occurs in the proximal tubule, and the remainder occurs more distally in the nephron. To remove the acid load resulting from the metabolism of protein, the kidneys must secrete hydrogen ion in the tubule. After the hydrogen ions are secreted into the lumen, they must bind with urinary buffers, primarily phosphate (HPO₄^2-) and ammonia (NH₃). Thus, most metabolic acid is eliminated as H₂PO₄^- and NH₄⁺; there is relatively little free hydrogen ion eliminated via the urine. Disorders of the glomerulus or the tubule can disrupt these processes and impair the ability of the kidneys to eliminate acid.