The functions of the kidneys are to filter waste and drugs from the body and to regulate fluids in the body, reports the National Kidney Foundation. Kidneys are also responsible for the release of hormones that control blood pressure, and they take part in the creation of red blood cells and a type of vitamin D. They support the body's calcium levels as well, according to HowStuffWorks.
The kidneys work by filtering, reabsorbing and secreting substances, notes HowStuffWorks. They process 20 percent of the blood that comes from the heart. The blood comes in through the renal arteries, and the kidneys cleanse and then route it to the renal vein. Waste material collects in the urine, which leaves the kidneys through ureters that connect to the bladder. The urine and its waste material collect in the bladder for excretion later through the urethra.
The kidneys filter blood through a complicated process; basically, the blood travels from the renal artery into the arcuate arteries, into the cortical arteries to the afferent arterioles, explains HowStuffWorks and InnerBody. The blood then enters the glomerulus, which consists of tiny capillaries. This is where the kidneys carry out the filtration process.
The Bowman's capsule, which cups the glomerulus, is the beginning of a coiled tube called a nephron, notes HowStuffWorks. Some of the blood plasma filters through the capillaries in the glomerulus into the Bowman's capsule and flows down the nephron. The filtered blood left in the glomerular capillaries then enters an efferent arteriole, which divides into cortical capillaries, explains InnerBody. These capillaries pick up substances the body needs from the nephron. The substances include water, salts and nutrients. The nephrons then reject the waste products, which remain behind in the urine.