Blood from the body enters the kidney through a renal artery.  This blood is full of wastes, such as urea and ammonium.  The blood is filtered through a series of nephrons, which are small, fan-shaped membranes, whose purpose is to remove the urea, ammonium, and other waste products, and combine them with a little water to make urine.  The urine leaves the kidney via a slender tube called a ureter, on its way to the bladder, where it will wait until the bladder is full.  When the bladder is full, the person gets the sensation they need to go to the restroom, where the urine is removed from the body in a process called urination. 

The blood, after filtration, is returned to the body via a renal vein, under low pressure.  Kidneys are also responsible for reabsorption of water, glucose, and amino acids.  Kidneys help maintain homeostasis of the body by monitoring acid/base balance.  Kidneys produce several hormones as well, all critical to the bodys preservation.