Blood enters each kidney through the renal artery that ramifies (splits) into progressively smaller vessels that ultimately become thin hair-like afferent arterioles. These arterioles branch out as even thinner vessels, known as glomerular capillaries, whose function is to filtrate from the blood large amounts of fluid and solutes (but virtually no proteins). Bunches of glomerular capillaries form glomeruli in the kidney of vertebrates, which are enclosed in a structure termed Bowman's capsule. Each glomerulus filters water and dissolved substances (i.e., solutes) from the blood, through the endothelium of the capillaries, and through the epithelium of the capsule, delivering the filtrate into the peritubular capillaries that surround the renal tubules of the Loop of Henle. From the Loop of Henle, the fluid is ultimately transported through a series of tubular renal structures to the medullary collecting tubule and, from there, to the collecting duct.

Urine formation...