Which segment of the nephron is responsible for the reabsorption of 65% of filtered sodium, chloride, and water, and contains cells with abundant mitochondria and basolateral Na⁺-K⁺-ATPase pumps?

Explanation

## Proximal Convoluted Tubule (PCT) — Structure and Function **Key Point:** The proximal convoluted tubule is the primary site of isosmotic reabsorption, recovering ~65% of filtered Na⁺, Cl⁻, and H₂O in the first pass through the nephron. ### Structural Features - **Epithelium:** Simple cuboidal to columnar with extensive brush border (microvilli) on the apical surface - **Mitochondrial density:** Very high — reflects massive ATP demand for active transport - **Basolateral membrane:** Abundant Na⁺-K⁺-ATPase pumps (up to 40% of basolateral membrane protein) - **Tight junctions:** Relatively leaky (high paracellular permeability) ### Functional Characteristics | Feature | PCT | Loop of Henle | DCT | Collecting Duct | |---------|-----|---------------|-----|----------| | **Primary reabsorbed solute** | Na⁺, Cl⁻, glucose, amino acids, water | Na⁺, Cl⁻ (ascending); water (descending) | Na⁺, Cl⁻ (via NCC) | Na⁺ (via ENaC); water (ADH-regulated) | | **Mitochondrial density** | Very high | Moderate | Moderate | Low to moderate | | **Water permeability** | High (constitutive aquaporin-1) | High (descending); low (ascending) | Low | Variable (ADH-dependent) | | **Osmolarity change** | Isosmotic reabsorption | Hypertonic (ascending) | Hypotonic fluid | Variable | **High-Yield:** The PCT reabsorbs ~65% of Na⁺, Cl⁻, K⁺, HCO₃⁻, glucose, amino acids, and water — essentially everything the body needs to retain, leaving waste products (urea, creatinine) in the tubular fluid. **Clinical Pearl:** Damage to the PCT (e.g., in acute tubular necrosis or heavy metal poisoning) results in loss of glucose, amino acids, and electrolytes in the urine — a hallmark of "tubular proteinuria." ### Why PCT Fits the Stem 1. **Abundant mitochondria** → ATP-dependent Na⁺-K⁺-ATPase activity 2. **High density of basolateral Na⁺-K⁺-ATPase** → drives secondary active transport of glucose, amino acids, and ions 3. **Brush border** → increases surface area for apical uptake via cotransporters (SGLT1, SMCT1) 4. **Isosmotic reabsorption** → water follows solutes via osmosis through aquaporin-1 [cite:Guyton and Hall Textbook of Medical Physiology Ch 26]