HOMEOSTATIS (Regulation of internal environment)
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All living organisms must be able to maintain steady internal environment.
Homeostasis
It is the maintenance of a fairly constant internal environment in an organism.
Body fluid such as; blood, lymph, and tissue fluids is made up of the internal environment of the organism. The internal environment must be kept fairly constant for the healthy growth and efficient functioning of body cells. A living organism must be able to adjust to any change in the physical and chemical conditions of it's body fluids. Some of the conditions are; body temperature, pH(hydrogen ion concentration), osmotic pressure, concentration of dissolved substances in the body such as; (carbon dioxide, oxygen, urea etc).
Osmoregulation in Man
Osmoregulation is the process whereby an animal regulates the balance between water and salts in it's body fluid.
In man the kidneys carry out osmoregulation. When the osmotic concentration of blood is higher than that of the cell content by means of sugar, mineral salts, amino-acid and other plasma solute, the kidneys extract these substances from the blood or body cells and absorb more water. When these substances are present in small amount in blood, It's create higher osmotic pressure in the body cells thus decreasing the osmotic concentration of blood and the kidneys extract more water than usual. By the action of Anti-Diurectic Hormone (ADH), more concentrated urine is produced thus keeping the osmotic concentration of blood and cells fairly constant.
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Homeostasis or osmoregulation in Unicellular organisms
Unicellular organisms such as Amoeba and paramecium possess contractile vacuole which enables them to carry out osmoregulation. The cell content of these animals is hypertonic to the external medium hence water enters them by osmosis into the cytoplasm and then into the contractile vacuole. In order to prevent the cell from being over turgid and bursting, the contractile vacuole discharges it's content from time to time thereby maintaining water balance within the organism.