proximal convoluted tubule
Summary
The proximal convoluted tubule (PCT) is the first segment of the nephron tubule where approximately 65-70% of filtered sodium, water, and other solutes are reabsorbed. It's the primary site for reabsorption of glucose, amino acids, and bicarbonate, and is commonly affected in acute tubular necrosis.
Detail
The proximal convoluted tubule extends from Bowman's capsule and is lined with cuboidal epithelial cells containing extensive microvilli (brush border) that increase surface area for reabsorption. The PCT performs both bulk reabsorption and secretion through various transport mechanisms. Sodium reabsorption occurs via Na+/K+-ATPase pumps on the basolateral membrane, creating gradients for secondary active transport of glucose (SGLT2), amino acids, and phosphate. Water follows sodium passively through aquaporin-1 channels and paracellular pathways. The PCT also reabsorbs virtually all filtered glucose and amino acids under normal conditions, and reclaims bicarbonate via carbonic anhydrase. Additionally, it secretes organic acids (PAH, creatinine), bases, and drugs. PCT cells are particularly vulnerable to ischemic and toxic injury due to high metabolic demands, making this the primary site of acute tubular necrosis. The PCT is also the target of several diuretics and the site where many nephrotoxic drugs exert their effects.
Sources
- Guyton and Hall Textbook of Medical Physiology
- Boron & Boulpaep Medical Physiology
- First Aid for the USMLE Step 1
- Brenner and Rector's The Kidney
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