collecting tubule
Summary
The collecting tubule (collecting duct) is the final segment of the nephron responsible for fine-tuning urine concentration, volume, and electrolyte balance under hormonal control. It is regulated primarily by ADH (vasopressin) and aldosterone, making it critical for water and sodium/potassium homeostasis. It also plays a key role in acid-base balance via intercalated cells.
Detail
The collecting duct is composed of principal cells and intercalated cells (Type A and Type B). Principal cells reabsorb Na+ (via ENaC channels, aldosterone-regulated) and secrete K+; they also express aquaporin-2 channels regulated by ADH for water reabsorption, critical for urine concentration in the medullary collecting duct. Type A intercalated cells secrete H+ via H+-ATPase (important in metabolic acidosis) and reabsorb HCO3-/K+; Type B intercalated cells do the opposite (secrete HCO3-, reabsorb H+), important in metabolic alkalosis. Aldosterone acts on principal cells to increase Na+ reabsorption and K+/H+ secretion (via upregulation of ENaC and Na+/K+-ATPase), relevant in conditions like hyperaldosteronism (Conn syndrome) causing hypokalemia and metabolic alkalosis, and in Addison disease causing hyperkalemia. ADH increases water permeability by inserting aquaporin-2 channels, critical in diabetes insipidus (central or nephrogenic) where this mechanism fails, leading to polyuria and dilute urine. Diuretics acting here include K+-sparing diuretics (amiloride, triamterene - block ENaC; spironolactone/eplerenone - aldosterone receptor antagonists). Clinically, disorders like Liddle syndrome (constitutively active ENaC) cause hypertension and hypokalemia mimicking hyperaldosteronism but with low aldosterone. Renal tubular acidosis (RTA) Type 1 (distal) involves defective H+ secretion by intercalated cells, leading to inability to acidify urine and metabolic acidosis with hypokalemia.
Sources
- Guyton and Hall Textbook of Medical Physiology
- BRS Physiology
- First Aid for the USMLE Step 1
- Costanzo Physiology
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