Breathing is a mechanical process consisting of two phases:
- Inspiration or inhalation or intake of fresh air in to the lungs.
- Expiration or exhalation or out push of foul and consumed air out of the lungs.
Breathing is associated with the existence of life itself. The lungs themselves can neither draw in air nor push it out. Therefore, man breath by suction pump mechanism. The chest-wall and the diaphragm acts as a large pump in moving air into and out of the lungs. In man during ordinary breathing, 500ml of air moves in and out of the lungs with each breath. Mechanism of breathing: We know that breathing consists of two phases namely breathing in or inspiration and breathing out or expiration. Many factors are involved in the mechanism of breathing, e.g.
- Contraction and relaxation of the intercostal muscles in between the ribs bring changes in the position of ribs and sternum. When these muscles contract, the ribs are elevated and move upward and forward and when these muscles relax, the ribs settle down and move inward and backward. The sternum also moves inward and backward.
- Contraction and relaxation of the muscles of diaphragm bring changes in its shape. When these muscles contract, the diaphragm becomes dome like or convex
- The spongy and elastic nature of the lungs causes expansion and contraction. They expand during breathing in and contract during breathing out.
Breathing in or inspiration: In man, inspiration is active process. During inspiration, the intercostal muscles between the ribs contract and pull the ribs forward and outward, pushing the sternum farther away from the vertebral column. By the contraction of the intercostal muscles and of the diaphragm, the size of the thorax as a whole is increased and the pleural cavities within it are therefore enlarged. Since the pleural cavities are closed therefore their enlargement tends to create partial vacuum within them. The lungs are elastic and communicate with the atmosphere through the air passages (trachea, bronchi). As soon as the pressure around the lungs is lowered, the air rushes into them through the trachea and bronchi. In this way the lungs expand to fill the pleural cavities and the pressure on the inside and outside of the thorax are equalized. Thus the mechanism of human breathing is a suction-pump mechanism. The lungs are made to expand and contract by movements of the ribs and diaphragm.
Breathing out or expiration: Expiration in man is normally a passive process. In severe muscular exercise however, the expiration also becomes active. During expiration, the intercostal muscles of the ribs relax, the ribs move downward and inward. Thus the size of the chest cavity is reduced from side to side. The sternum comes to its original position, decreasing the size of the chest cavity from front to back. At the same time muscles of the diaphragm relax and so the diaphragm assumes its dome shaped position. Thus with the relaxation of the muscles of diaphragm and of the intercostal muscles, the size of the thorax as a whole is decreased. This reduction in the size of the thorax exerts pressure on the lungs. The lungs themselves are very elastic and tend to return to their original size. When the lungs are pressed, the foul air inside them is expelled or expiration occurs.