Excretion can be defined as the removal of toxic waste products of metabolism from the body. These wastes can be either sold, liquid or in the gaseous state. The liquid wastes are ammonia and urea, which exist in the blood along with the nutrients and other useful substances. So there is a need of complex organ that may separate or filter out the dissolved excretory wastes from blood while retaining the nutrients in the latter. Two kidneys in human beings are such organs that perform this task. There is a distinct advantage of the two kidneys in our body. If one kidney fails, the other can still deal with functions of excretion and regulation. 

The kidneys are solid; bean shaped, reddish brown-paired structure, which lie in the abdominal cavity one on either side of the vertebral column. The kidneys collect the excretory products and eliminate them in the form of urine, which then passes down the two tubular ureters into the collapsible urinary bladder, which is a muscular reservoir of urine. The urine is released periodically to the outside via the urethra. 

A section through a kidney shows a darker outer region, the cortex and a lighter inner zone, the medulla, where the ureter leaves the kidneys is a space called the pelvis. Each kidney is made of numerous (about 1 million) called tubules known as nephrons, which are the functional unit of the kidneys. About 180 litres of blood, which run through these nephrons, are reduced to urine by the process of filtration, reabsorbtion and secretion by the nephrons. The urine enters the pelvis of the kidney where it collects and continues down the ureters to the bladder. In the urinary bladder urine is temporarily stored and is finally eliminated from the body. Human being on an average excrete about 1 to 1.5 litres of urine per day.