bowman's capsule

Meaning: The Bowman's capsule is the double-walled cup at the head of each nephron, in the renal cortex. The glomerulus, a tuft of capillaries, sits inside it; together they form the renal corpuscle. Blood pressure in the glomerulus is high, partly because the efferent arteriole is narrower than the afferent arteriole. This pressure forces fluid through the fenestrated capillary endothelium, the basement membrane and the filtration slits between podocytes. The filtrate enters Bowman's space and flows into the proximal convoluted tubule. Filtration is passive bulk flow.

Equation: Net filtration pressure = PGC − PBS − πGC, where PGC is glomerular capillary hydrostatic pressure, PBS is hydrostatic pressure in Bowman's space, and πGC is the oncotic pressure of capillary plasma proteins, all in Pa.

On the MCAT: Know the flow order: afferent arteriole → glomerulus → Bowman's capsule → proximal tubule → loop of Henle → distal tubule → collecting duct. Questions ask you to predict how filtration changes. Constricting the efferent arteriole raises glomerular pressure and filtration; constricting the afferent arteriole lowers it. Protein or red blood cells in urine point to a damaged filtration barrier. Early filtrate has about the same glucose and salt concentration as plasma.

Common confusion: The capsule is not the glomerulus. The glomerulus is the capillaries and the capsule is the cup around them. The capsule only filters; reabsorption and secretion happen later along the tubule, and the filtrate is not yet urine.