Chapter 35: Transport of Sodium and Chloride

The discussion covers primary active transport via the sodium-potassium pump, which hydrolyzes adenosine triphosphate to establish and maintain concentration gradients that drive secondary transport processes. Secondary active transport mechanisms including cotransport and countertransport are explored, demonstrating how electrochemical gradients generated by the primary pump enable the movement of sodium and chloride with other solutes across various epithelial tissues. The chapter details paracellular and transcellular pathways for ion movement, explaining how tight junction properties and channel selectivity regulate ion permeability in different organ systems. Specific transport mechanisms in major organs are analyzed, including intestinal absorption of sodium chloride, renal reabsorption and secretion in different nephron segments, and sweat gland function. The role of transporters such as the sodium-chloride cotransporter, potassium-chloride cotransporter, and sodium-hydrogen exchanger are examined within their physiological contexts. Regulation of these transport systems through hormonal signals including aldosterone and natriuretic peptides is discussed, along with how transport defects contribute to pathological states including cystic fibrosis, Bartter syndrome, and Gitelman syndrome. Clinical applications address how diuretics target specific transport mechanisms to manage hypertension and fluid retention, making this chapter essential for understanding how ion transport underpins both normal physiology and disease processes.