Which segment of the nephron is responsible for the selective reabsorption of glucose, amino acids, and other useful substances via active transport?

Explanation

## Proximal Convoluted Tubule (PCT) — Structure and Function **Key Point:** The proximal convoluted tubule is the primary site of selective reabsorption of filtered nutrients and ions via active transport mechanisms. ### Structural Features - Single layer of **cuboidal epithelium** with abundant **microvilli** (brush border) - Numerous **mitochondria** — reflects high metabolic activity for active transport - Extensive **basolateral infoldings** — increase surface area for Na⁺-K⁺-ATPase activity ### Functional Reabsorption in PCT | Substance | Mechanism | Transporter | | --- | --- | --- | | Glucose | Active transport (secondary) | SGLT1, GLUT2 | | Amino acids | Active transport (secondary) | Amino acid cotransporters | | Na⁺ | Active transport (primary) | Na⁺-K⁺-ATPase | | Water | Osmosis | Aquaporin-1 | | Bicarbonate | Reabsorbed via H⁺ secretion | Na⁺-H⁺ exchanger | | Phosphate | Active transport (secondary) | Na⁺-phosphate cotransporter | **High-Yield:** Approximately **65% of filtered water, Na⁺, and other electrolytes** are reabsorbed in the PCT. This is the **most metabolically active segment** of the nephron. **Clinical Pearl:** Damage to the proximal tubule (as in acute tubular necrosis) results in loss of glucose, amino acids, and proteins in urine despite normal glomerular filtration. ### Why Other Segments Don't Fit - **Loop of Henle:** Primarily involved in countercurrent multiplication; minimal active reabsorption of glucose/amino acids - **Distal convoluted tubule:** Selective reabsorption of Na⁺ and K⁺ regulation; not a major site for glucose reabsorption - **Collecting duct:** Water reabsorption (ADH-regulated) and final urine concentration; minimal glucose reabsorption