Filtration begins when blood arrives at the kidney. The kidney is supplied with blood at the hilum through the renal artery (one of two blood vessels that leads from the abdominal aorta and supplies blood to the kidneys), which then splits off into segmental arteries. The segmental arteries branch off into arcuate arteries, and these, in turn, branch off into interlobular arteries. The interlobular arteries run throughout the renal columns and renal pyramids and eventually lead, via the afferent arteriole, to the glomerulus, where filtration occurs. The afferent arteriole is the incoming blood vessel that supplies blood to the glomerulus of the nephron and helps to regulate blood pressure within the kidneys.
The diameter of the afferent arteriole is wider than that of the efferent arteriole. Therefore, blood that passes through the glomerulus is forced to exit through a narrower tube. Because of the increasingly small diameters of these tubes, the blood becomes highly pressurized. This high-pressure system results in plasma (the yellow fluid part of the blood that consists of water, proteins, glucose, and electrolytes) being separated out of the blood. The plasma contains blood solvents such as salts, water, amino acids, glucose, and urea (a by-product of metabolized protein). The separated blood plasma remaining within the glomerulus is called filtrate.
Following filtration in the glomerulus, blood leaves the glomerulus through the efferent arteriole. The efferent arteriole is an outgoing blood vessel that carries blood away from the glomerulus of the nephron. The blood then enters the individual vessels of a second capillary bed, each of which is called a peritubular capillary, a capillary that surrounds the proximal and distal convoluted tubules in the kidney, or to the vasa recta and then travels through a network of venules that converge into the cortical radiate veins and then the arcuate veins. The arcuate veins send blood to the interlobar veins, which converge to form the renal vein, the large vein that carries blood away from the kidney. The renal vein returns the blood to the heart. As blood is exiting the kidney, the filtrate travels out of the kidney through the ureters and on to the bladder to be stored until it will be excreted from the body.