Glomerular filtration occurs across a trilaminar sieve: fenestrated capillary endothelial cells (pores 70-100 nm), the negatively charged glomerular basement membrane (rich in type IV collagen and heparan sulfate), and podocyte pedicels with slit diaphragms (filtration slits 25-30 nm). Small molecules like water, ions, glucose, and urea filter freely, whereas serum albumin and cellular elements are electrostatically and sterically repelled.
In the PCT, secondary active transport driven by basolateral Na+/K+ ATPase pumps reabsorbs 65% of Na+ and water, alongside 100% of glucose via SGLT2 cotransporters. The Loop of Henle functions as a countercurrent multiplier: the descending limb is water-permeable and solute-impermeable, allowing water to exit down the osmotic gradient; the thick ascending limb actively extrudes Na+, K+, and 2Cl- via NKCC2 symporters without water, establishing a profound corticomedullary hyperosmotic gradient reaching 1200 mOsm/L at the hairpin turn.
In the late distal tubule and collecting duct, hormonal regulation fine-tunes final urinary output. Aldosterone promotes sodium retention and potassium/proton excretion, while Antidiuretic Hormone (ADH / Vasopressin) stimulates the insertion of Aquaporin-2 water channels into luminal membranes, driving water conservation and hypertonic urine production in response to dehydration.