## Distinguishing PCT from DCT ### Structural Comparison | Feature | Proximal Convoluted Tubule (PCT) | Distal Convoluted Tubule (DCT) | |---------|----------------------------------|--------------------------------| | **Brush Border** | Present (tall microvilli) | Absent or sparse | | **Mitochondrial Density** | Very high (40% of cell volume) | Moderate | | **Epithelial Cell Height** | Tall columnar | Cuboidal | | **Lumen Diameter** | Wider | Narrower | | **Primary Function** | Bulk reabsorption | Fine-tuning of reabsorption | ### Key Anatomical Distinction **Key Point:** The brush border (microvilli) and exceptionally high mitochondrial density in the PCT are the hallmark structural features that enable massive ATP-dependent reabsorption of glucose, amino acids, ions, and water. The DCT lacks these features because it performs selective, hormone-regulated reabsorption rather than bulk reclamation. **High-Yield:** The PCT reabsorbs ~65% of filtered Na⁺, K⁺, Cl⁻, HCO₃⁻, glucose, amino acids, and water. The DCT reabsorbs only ~5% of filtered Na⁺ and is the primary site of aldosterone action. **Clinical Pearl:** In acute tubular necrosis (ATN), the PCT is the most vulnerable segment due to its high metabolic demand and reliance on oxidative phosphorylation. Loss of brush border is a hallmark finding on electron microscopy. ### Why This Feature Matters The brush border dramatically increases the apical surface area (300–400 fold), allowing simultaneous operation of multiple cotransporters (SGLT1 for glucose, amino acid transporters, Na⁺/H⁺ exchanger). The mitochondria power these active transport processes via ATP hydrolysis. The DCT, by contrast, has a smaller surface area and fewer mitochondria because it performs selective reabsorption guided by hormones (aldosterone, ADH, PTH). [cite:Guyton & Hall Ch 26]
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.