What is the primary function of the kidney's glomerulus?

• A. Filtration of blood to form filtrate (primary urine) based on pressure differences
• B. Reabsorption of water in proximal tubule
• C. Secretion of renin
• D. Production of erythropoietin

Answer: (A)

The glomerulus is the filtering unit of the nephron, where blood is filtered to form a filtrate that enters Bowman's capsule. This filtration is driven by pressure differences across the filtration barrier: the hydrostatic pressure in the glomerular capillaries pushes fluid out, while pressures from plasma oncotic forces and Bowman's space oppose it. The barrier—composed of fenestrated endothelium, a basement membrane, and podocyte filtration slits—allows mainly water and small solutes to pass while restricting large proteins and cells. So the primary function is to generate a filtrate from blood based on these pressure-driven filtration dynamics. Downstream processes like reabsorption in the proximal tubule, secretion of renin by juxtaglomerular cells, and production of erythropoietin by interstitial cells occur outside the glomerulus and are not its main role.