Excretion

Excretion is the removal of toxic substances and metabolic waste products. Every organism, from the smallest protist to the largest mammal, must cleanse itself of the potentially toxic by-products of its own vital activities. This process in living things is named elimination, which may be considered to include all of the different mechanisms and processes by which life forms dispose of or gets rid of waste products, harmful substances, and perished portions of the organism.In vertebrate animals, this process is mainly carried out by the lungs, skin and the kidneys, with the urinary system being the main excretory system. Excretion through the skin occurs through sweating, which results in the removal of water and small amounts of urea and salts. The kidneys filter about 180 litres of blood every day and removes urea, toxins, medications, and excess ions through the formation of urine.

An example of such is cell excretion process. The cell clears its waste products by bringing the waste products close to the cell membrane and then closing the cell membrane around the waste products, isolating it from the rest of the cell. Then the cell could open the cell membrane on the outside of the cell, letting the products out without losing any cytoplasm or letting anything else in.

Many unicellular organisms such as, Amoeba excrete their waste products by diffusion from their body surface as they are a small in size. Multicellular organisms such as Hydra carry out water excretion by creating a break in its body wall which is a result of strong contraction when its gut is distended with fluid.

Excretory System in Man:

Our excretory system consists of kidneys, blood vessels that join them, ureters, urinary bladder and urethra. They help produce and excrete urine.

There are two bean-shaped kidneys that lie in the abdominal cavity, one on either side of the vertebral column. The kidneys are reddish brown. Each of them is about 10 cm long and weighs about 150 g. Although they weigh less, they receive a lot of blood for filtration.

A volume of blood nearly equivalent to that in the whole body passes through the kidneys every four or five minutes. The kidneys produce urine to filter out the waste products, like urea and uric acid, from the blood.

(A) Excretory Organs of Man
(B) Internal Structure of a Kidney

Urine leaves each kidney through a tube called ureters. The ureters from both the kidneys are corrected to the urinary bladder that collects and stores urine. Ureters carry urine from the kidneys into the urinary bladder. The urethra is a canal that carries urine from the bladder and expels it outside the body.