Each human has two kidneys which function as essential parts of the urinary system.
Each kidney is separated into three regions, the cortex (outer region), the medulla (inner region) and the pelvis (central region). The renal artery carries blood to each kidney and the renal vein carries blood away from each.
Essentially, blood enters the kidneys, wastes are removed, and blood is returned to circulation.
The ureters carry urine to the bladder, a bag with muscular walls which stores urine. The sphincter muscle surrounds the urethra and keeps the bladder closed. It relaxes to allow urine to pass out through the urethra.
The kidneys are made up of thousands of structures called nephrons or kidney tubules, which produce urine.
The nephrons start with the Bowman's capsule in the cortex which surround a bundle of capillaries (from the renal artery) called a glomerulus. The nephron itself has three sections: the first convoluted tube in the cortex, the loop of Henle in the medulla and the second convoluted tube in the cortex.
An arteriole leads from the renal artery to each glomerulus. The nephrons all have a network of blood capillaries surrounding them which lead to the renal vein. Nephrons join to collecting ducts, which lead through the medulla and into the pelvis.
This is where things get a little convoluted... tube (Get it?) Anyway, there are two processes responsible for the production of urine in the nephrons, ultra filtration and selective reabsorption.