The elimination of nitrogenous waste materials like urea from the body is called excretion. This is done by the kidney. Kidney is the chief excretory organ. In the human body there is a pair of kidneys located in the abdomen, one on either side of the vertebral column. The right kidney is slightly smaller than the left one. It is placed slightly lower in position than the left kidney. Per minute 100 to 120 ml of blood passes through the kidneys. Per day 170 litres of water is filtered by the kidneys. Out of this 168.5 litres of water is reabsorbed by the kidneys. Out of this 168.5 litres of water is reabsorbed by the kidneys. Only 1.5 litres of water is reabsorbed by the kidney. Only 1.5 litres of water is excreted as urine.

The kidneys are bean shaped organs. They are dark red in colour. The outer surface of each kidney is convex or bulged and the inner surface is concave. There is a depression on the concave side called as hilus. A tube called ureter passes out of the hilus. The ureter opens into the urinary bladder. To see the inner parts of the kidney, it has to be cut all along its length. This is called longitudinal section of kidney. In the longitudinal section of kidney, one can see an outer part called cortex. The inner part is medulla. The cortex is dark red in colour while the medulla is pale red.

Each kidney is made up of about a million functional units which are in the form of very small tubes. These are called nephrons. A nephron is made up of two parts. They are the Renal capsule and the tubule. The renal capsule is a double walled cup. This is called Bowmen's capsule. Inside the bowmen's capsule there is a network of very small blood vessels. This is called as glomerulus. The Bowmen's Capsule and the glomerulus are together called as Malpighian capsule. The Bowmen's capsule opens into funnel. This funnel opens into the tubule. The first part of the tubule is not straight but has several bends or convolutions. This opens into another tube which like a hair-pin. This is called as the loop of Henley. The part of the renal tubule after the loop of Henley. The part of the renal tubule after the loop of Henley is again convoluted and finally opens into collecting tubule. There are a large number of collecting tubules in the kidney. All the collecting tubules open into a funnel like structure called pelvis. The pelvis continues as ureter.

The kidney acts as a filter. When blood flows through it, waster materials present in the blood are filtered by kidney. Large molecules and cells are not filtered. The filtration occurs in the Bowmen's capsules. The filtered material is called urine. During the filtration of blood by the kidneys, large amount of water, substances like glucose and salts are also filtered from the blood. Man will die if all the filtered water is excreted through urine. These substances are useful to the body. The tubules of the kidneys reabsorb these useful substances and release them back into the blood.

The kidneys maintain the required level of salts in the blood. If the salts are more than the requirement, they are excreted. Kidneys also regulate the amount of water in the body. They remove the excess water present in the body. They help to maintain water balance system in the body. Urine is pale yellow in colour. It contains about 96% of water and 4% of organic and inorganic salts. It is acidic in reaction. The kidneys have very great reserve power. If one kidney is removed due to damage or disease the other kidney takes up the function of the two kidneys. So a human being can survive with one kidney. In case the both the kidneys are temporarily damaged artificial kidneys are used. Artificial kidney was first introduced by a Dutchman, William.J.Kolff in 1943.