Where does filtration occur in the nephron?

• A. In the proximal convoluted tubule.
• B. In the loop of Henle.
• C. In the glomerulus within Bowman's capsule.
• D. In the collecting duct.

Answer: (C)

Filtration is the process where fluid moves from the blood into the nephron, driven by pressure differences across the filtration barrier. This happens at the glomerulus, a knot of capillaries surrounded by Bowman's capsule. The filtration barrier—composed of the endothelial pores, the basement membrane, and the podocyte filtration slits—allows water, ions, and small solutes to pass into Bowman's space while keeping blood cells and large proteins in the bloodstream. From there, the filtrate enters the proximal tubule for reabsorption and processing. The loop of Henle and collecting duct are involved in reabsorption and concentration of urine, not filtration, so the glomerulus within Bowman's capsule is the site where filtration occurs.