Chapter 24: Acid–Base Homeostasis & pH Regulation

Acid–Base Homeostasis & pH Regulation summary elucidates the physiological mechanisms of acid-base homeostasis, focusing on the strict regulation of hydrogen ion concentration to maintain arterial pH within a specific survival range. It distinguishes between volatile acids, such as carbonic acid derived from carbon dioxide, and nonvolatile or fixed acids produced through the incomplete metabolism of carbohydrates and fats or the oxidation of dietary proteins. The text explains fundamental chemical principles, including the acid dissociation constant and the Henderson-Hasselbalch equation, which establishes the mathematical relationship between pH, the pKa, and the ratio of conjugate base to acid. A significant portion of the chapter is dedicated to the three primary lines of defense against pH fluctuations: chemical buffering in extracellular and intracellular fluids, respiratory regulation of carbon dioxide tension, and renal regulation of bicarbonate and hydrogen ion excretion. Detailed analysis is provided on the bicarbonate-carbon dioxide buffer system, described as the most important extracellular buffer due to its open-system nature where components can be independently regulated by the lungs and kidneys. The summary describes how the respiratory system provides rapid compensation by altering alveolar ventilation to adjust the partial pressure of carbon dioxide in response to pH changes. It deeply explores renal physiology, specifically how the kidneys reabsorb filtered bicarbonate in the proximal tubule and generate new bicarbonate via the excretion of titratable acid and ammonium to neutralize the daily net acid load. Mechanisms of tubular transport are detailed, including the role of the sodium-hydrogen exchanger, the proton ATPase, and the specific functions of alpha-intercalated and beta-intercalated cells in the collecting duct. The chapter categorizes the four simple acid-base disturbances—respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis—and explains their primary causes, such as hypoventilation, hyperventilation, renal failure, or vomiting. Finally, it outlines clinical diagnostic tools, including the use of the plasma anion gap to differentiate causes of metabolic acidosis and the application of the pH-bicarbonate diagram to interpret complex acid-base data.