What is the network (tuft) of capillaries that performs the 1st step of filtering blood?

• A. Nephron
• B. Renal pyramid
• C. Renal capsule
• D. Glomerulus

Answer: (D)

Filtration of blood begins in the glomerulus, a tuft of capillaries that sits inside Bowman's capsule. Blood enters through the afferent arteriole and is forced under pressure across a filtration barrier (fenestrated endothelium, basement membrane, and slit diaphragms), allowing water and small solutes to pass into Bowman's space while keeping cells and large proteins out. The filtrate then moves on to the tubules for processing. The other options don’t form a capillary tuft: the nephron is the whole functional unit, the renal pyramid is a medullary structure, and the renal capsule refers to the kidney’s outer covering rather than a capillary network.