Cells utilize the essential substances that are carried to them by the transport system of the body. As a result of cellular metabolism, some harmful and toxic substances are also produced. Living organisms have means of getting rid of these substances. The process of getting rid of metabolic wastes is known as excretion. In unicellular plants and animals this process takes place through diffusion.
Higher animals have a very well developed excretory system. Excretory organs include liver, lungs, kidney and skin. If it were not for the non-stop activity of these organs, the body would be quickly poisoned by its own metabolic wastes. These organs also help in getting rid of the excess body heat.
Blood is filtered in the kidneys to remove urea, water and salts which form urine. There are a pair of kidneys which are dark brown in colour and bean shaped. They are situated in the lower region of the abdominal cavity. The location of the kidneys and other parts of the urinary systems are as follows:
L. S of a Human Kidney
Structure of a Nephron
The kidney is a very complex organ and is made up of a large number of units called nephrons. Each nephron is able to perform complete filtration of blood to form urine. A nephron has a cup-like structure called the Bowman's cup followed by a highly coiled tube. The Bowman's cup encloses a bunch of capillaries called the glomerulus. The rest of the nephron is also supplied by capillaries. When the blood containing metabolic waste is brought to the glomerulus, water, salts and urea are filtered here. These wastes pass through the long tubule and are poured in the pelvis (the beginning of the ureter) in the form of a liquid, the urea. When the bladder is full, urine is passed out through the urethra.