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RESPIRATORY SYSTEM

External and Internal Respiration: Most of the animals respire aerobically using oxygen for the oxidation of nutrients and producing energy and carbon dioxide. The exchange of oxygen and carbon dioxide between the surrounding medium is known as external respiration whereas utilization of oxygen carrying out of the oxidation and the elimination of CO2 by the body cells is known as internal respiration.

In the lower organisms: The exchange of respiratory gases is performed by the body surface (Hydra). As the evolution proceeded this function was taken over by the skin also called as cutaneous respiration (earthworms, leeches, toads, frog etc). But in insects this function is taken over by the air tubes, trachea.

The development of gill facilitated the exchange of gases through aqueous medium and the blood. In higher vertebrates, the lungs are the main respiratory organs. These communicate the atmospheric air through external nostril, nasal chamber, internal nostril, pharynx, larynx, trachea, bronchi and bronchioles. Exchange of respiratory gases takes place between the alveoli and the blood.

Exchange of gases between lungs and environment: The lungs are present in the closed pleural cavities. The contraction of external intercostal muscles and the diaphragm increases the volume of thomcic cavity. This causes the lowering of the internal pressure resulting the air to enter the lungs from the atmosphere. This is inspiration. The relaxation of external inter costal muscles and the daphragm decreases the volume of thoracic cavity. Due to this the pressure increases over the lungs. This results in the expulsion of air from lungs This is expiration.

This exchange is called breathing. In man, the lungs act as a suction pump In normal or quiet breathing, diaphragm plays about 75% role and the muscles that move ribs and sternum play an accessory 25% role.

The quiet breathing is often termed as abdominal breathing. Breathing by effort is called deep breathing because in deep breathing volume of chest increases by about 15-20% and its movement becomes apparent that is why it is also called as throacic breathing.

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