## Proximal Convoluted Tubule: Structure and Function ### Correct Statements (Options 0, 1, 2) **Key Point:** The proximal convoluted tubule is the workhorse of selective reabsorption and secretion. | Feature | Detail | |---------|--------| | **Epithelium** | Simple cuboidal with tall brush border (microvilli) | | **Mitochondria** | Abundant — provides ATP for active transport | | **Reabsorption** | Glucose, amino acids, Na⁺, K⁺, Cl⁻, HCO₃⁻ via Na⁺-coupled cotransporters | | **Secretion** | H⁺, organic acids (penicillin, probenecid), ammonia, uric acid | | **Water permeability** | **HIGH** — water follows osmotic gradient (aquaporin-1) | ### The Incorrect Statement (Option 3) **High-Yield:** The proximal tubule **IS highly permeable to water** — it reabsorbs ~65% of filtered water along with solutes, maintaining isotonicity of the tubular fluid. **Clinical Pearl:** The primary site of water reabsorption is the **collecting duct** (under ADH control), not the proximal tubule. The proximal tubule reabsorbs water passively because the tubular fluid becomes hypertonic due to active solute reabsorption. **Warning:** Do not confuse the proximal tubule (water-permeable, isotonic reabsorption) with the **thick ascending limb** (impermeable to water, creates osmotic gradient). ### Summary Table: Nephron Segment Permeability | Segment | Water Permeability | Na⁺ Reabsorption | Urine Osmolality | |---------|-------------------|------------------|------------------| | **PCT** | High | Active (65%) | Isotonic | | **Thick Ascending Limb** | **Impermeable** | Active | Hypotonic | | **Collecting Duct** | Variable (ADH-dependent) | Minimal | Hypertonic (with ADH) | [cite:Guyton & Hall Textbook of Medical Physiology Ch 26]
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