Clinical meaning
The nephron is the functional unit of the kidney, with approximately 1 million per kidney. Each nephron consists of the glomerulus (filtration unit), proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct. The glomerulus filters approximately 180 L/day of plasma (GFR ~125 mL/min), with the filtration barrier providing size and charge selectivity through fenestrated endothelium, glomerular basement membrane (GBM), and podocyte foot processes with slit diaphragms. The PCT reabsorbs ~65% of filtered sodium, water, glucose (via SGLT2 and SGLT1), amino acids, bicarbonate, phosphorus, and uric acid — it is the workhorse of reabsorption. The descending loop of Henle is permeable to water but not solutes (concentrating tubular fluid), while the thick ascending limb actively reabsorbs NaCl via the Na-K-2Cl cotransporter (NKCC2 — furosemide's target) without water (diluting tubular fluid and creating the medullary osmotic gradient). The DCT reabsorbs NaCl via the Na-Cl cotransporter (NCC — thiazide target). The collecting duct is the final regulator: principal cells reabsorb sodium (via ENaC — aldosterone-regulated, amiloride-blocked) and secrete potassium (via ROMK), while intercalated cells secrete or reabsorb hydrogen/bicarbonate for acid-base regulation. ADH acts on the collecting duct, inserting aquaporin-2 channels to reabsorb free water. Understanding segmental physiology predicts drug effects and disease manifestations at each nephron level.