Flow: glomerulus to proximal convoluted tubule to loop of Henle to distal tubule to collecting duct. The PCT reclaims 65-70 percent of filtrate including all glucose and amino acids; the descending limb is water-permeable, the ascending limb pumps NaCl but excludes water, building the medullary gradient.
GFR is about 180 L/day yet urine is only about 1.5 L โ 99 percent of filtrate is reabsorbed.
Filtration and Clearance
Net filtration pressure: Pn Efferent arteriole constriction (angiotensin II) raises glomerular pressure and GFR; a ureteral stone raises Bowman's pressure and lowers it.
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Clearance: Cxโ=PxโUxโโ Vโ Inulin clearance equals GFR; clearance above GFR means net secretion, below means net reabsorption.
Hormones and Concentration
ADH inserts aquaporins in the collecting duct (its absence causes diabetes insipidus with dilute polyuria); aldosterone drives Na+ reabsorption and K+ secretion; ANP opposes both.
The countercurrent multiplier (NKCC pumping in the thick ascending limb) creates a gradient up to about 1200 mOsm; loop diuretics collapse it, which is why they are the most powerful class.