Chapter 39: Transport of Acids and Bases

Potassium handling begins at the glomerulus where the electrolyte is freely filtered, then undergoes selective reabsorption in the proximal tubule and loop of Henle. The distal nephron provides precise regulation through aldosterone-mediated secretion by principal cells in the collecting duct, thereby linking potassium excretion to sodium balance and blood volume control. Multiple physiological factors modulate renal potassium handling, including acid-base status, which influences both renal excretion and cellular uptake, dietary potassium intake, and tubular fluid flow rate through the distal and collecting segments. The chapter then addresses calcium homeostasis through a detailed examination of filtration and reabsorption along the nephron. Although calcium is freely filtered at the glomerulus, most filtered calcium is reclaimed in the proximal tubule and thick ascending limb through paracellular pathways driven by electrochemical gradients. Fine-tuning of calcium reabsorption occurs in the distal tubule and early collecting duct, where parathyroid hormone stimulates active transcellular reabsorption while calcitriol enhances intestinal calcium absorption and renal reabsorption. Calcitonin provides additional regulatory input during periods of elevated blood calcium. The chapter integrates these mechanisms with phosphate handling and bone mineral metabolism, illustrating how renal regulation coordinates with endocrine control to maintain serum calcium within narrow limits. Clinical correlations throughout the chapter connect dysregulation of potassium and calcium to pathophysiological states including arrhythmias from electrolyte imbalances, nephrolithiasis from excessive urinary calcium, and the iatrogenic effects of diuretics on electrolyte homeostasis. By synthesizing renal transport mechanisms with hormonal regulation and disease presentations, this chapter demonstrates how integrated kidney function preserves the electrolyte balance essential for systemic physiological stability.