Excretion can be defined as the removal of toxic waste products of metabolism from the body. These wastes can be either solid, 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. They are about 11 to 12 cms in length, 5 to 6 com in breadth and about 3 cms in thickness and weigh about 120 to 150 gms each. The kidney is approximately the size of your fist. They are well protected and lie in the retro-peritoneal section of the abdomen which means that they lie behind the stomach and other digestive organs and nearer to the back of the body.

A cross section of the kidney shows a darker outer zone called the cortex and a lighter inner zone called the medulla. A funnel shaped hollow pelvis meets the ureter and the junction is called the pelvi-ureteric junction. This junction can have a functional blockage giving rise to obstruction to flow of the kidney.

Each kidney is made up of about 1 million tubules that are the basic functioning unit of the kidney that can make urine and these are known as nephrons. About 180 litres of blood, which run through these nephrons daily produces just one to two liters of 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. The bladder has an average capacity of about 400 milliliters. The urinary bladder is a muscular reservoir for the urine and can expand without exerting any pressure within the bladder. It function is to store and evacuate urine. The urine is released periodically to the outside via the urethra.