Living cells are constantly active. Even if a cell is neather moving nor growing, it is continously building up and breaking down chemical compounds. Aside from carbon dioxide, cell activities produce water and nitrogenous wastes from the breakdown of protein. This waste products are likewise poisonous to cells and must be excreted, just like carbon dioxide.
It is excretory sysytem which functions to move or waste and maintain the internal stability of the cell. Excretion may occur by diffusion through the cell membrane or by means of specialized structures and systems.
Animals have evolved some adaptations for expelling waste products through excretion. This process enables cells to maintain a balance with the external environment.
Sponge ans hydra excrete waste products from each cell directly to the body surface. Larger organisms have special organs for excretion. This organs only remove from water that constantly diffuses into the cells.
The major nitrogen containing compounds excreted by various animals are ammonia, urea, and uric acid. All these nitrogenous wastes are toxic to cells. Ammonia is very poisonous to cells. Ammonia is excreted mostly by very small animals. Humans excrete urea along with a large volume of water as urine.
In humans the kidneys remove wastes from the blood while maintaining a balance of ater, salts and organic molecules. Each kidney consist of about 1 million microscopic structures called nephrons.
Each nephrons consistof along long tube. This enlarged at one end. Capillaries sorround each nephron.
The importance of the kidney in removing waste substances from the blood that flows through it. About 1/4 of the blood pump out of the heart with every beat goes through the kidney. In the kidneys the blood is filtered by the nephrons. Blood flowing from the arteries into the kidney is under high pressure. As blood flows through a tuft of capillaries called the glomerulus some plasma is forced into the brownmans capsule. The fluid in the capsule is similar to blood plasma. However, it does not contain protein and blood cells.
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