What is the glomerulus?

• A. A small sac lining the kidney pelvis
• B. The tubular system where urine is collected after filtration
• C. A collection of smooth muscle controlling urine flow
• D. The place where regulation of fluid balance starts; a group of capillaries that perform filtration

Answer: (D)

The glomerulus is the tuft of tiny blood vessels in the nephron where filtration begins. Blood from the afferent arteriole enters this capillary cluster and, under pressure, water and small solutes pass through a filtration barrier into Bowman's space, forming the filtrate. Larger molecules and cells stay in the bloodstream. This filtration step sets the stage for how the body regulates fluid balance, since the volume of filtrate produced (and what is subsequently reabsorbed or excreted) determines overall fluid and electrolyte handling. It’s not a sac in the kidney pelvis, not the tubular system that collects urine after filtration, and not a smooth-muscle structure responsible for urine flow.