Where does filtration of blood occur in the nephron?

• A. Glomerulus/Bowman’s capsule
• B. Proximal tubule
• C. Distal tubule
• D. Collecting duct

Answer: (A)

Filtration happens where blood is first filtered into the nephron’s hollow space, the renal corpuscle. In the glomerulus, the blood in the glomerular capillaries is under high hydrostatic pressure, pushing water and small solutes across the filtration barrier into Bowman's capsule. This barrier—composed of fenestrated endothelium, a basement membrane, and podocyte filtration slits—lets through water, ions, glucose, and small molecules, but blocks large proteins and blood cells. The result is the glomerular filtrate, which then enters the proximal tubule for reabsorption and secretion. The other nephron segments listed mainly handle reabsorption and secretion, not the initial filtration step.