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Preksha Dhyana :
alveoli covering a total surface area of more than 90 sq. mtrs enough to carpet a tennis court. Each alveolus is a roughly globular structure about 100 micrones in diameter. It has an extremely thin wall and surrounded by a network of equally thin-walled capillaries. An exchange of the two gases occurs in the lungs. Oxygen passes out through the thin walls of the alveoli and in through those of the capillaries that surround them. At the same time there is a net movement of carbon dioxide in the opposite direction. Lungs themselves are devoid of muscular action but the system includes a bellow's arrangement-the rib cage-operated by muscle and controlled by nerves.
To a physiologist breathing is divided into :
(i) external respiration i.e. passage of oxygen from the lungs into the blood and the passage of carbon dioxide from the blood to the alveoli, and
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(ii) internal respiration in which the body cells exchange carbon dioxide with fresh oxygen carried by the blood. The final aspect of respiration is cellular respiration i.e. chemical reactions of oxidatian within the cells.
Three sets of muscles participate in the process of breathing :-(i) diaphragm, (a dome-shaped muscle between the chest and the abdomen) (ii) intercostal muscle (attached to the ribs) and (iii) clavicular muscles (attached to the collar bone).
The average adult, at rest and not emotionally excited, breathes about 14 to 20 times a minute. Emotional stimulation, pain, temperature, carbon dioxide level and age cause variations from this basic level. Like the heart-beat, the respiration rate tends to decrease from the birth to adulthood and increases in old age.
4. Digestive System
Energy is essential to maintain such vital functions and processes as breathing, blood circulation and brain function. It is derived from foodstuffs but food cannot be utilised by the tissues until they have been broken down to smaller simpler components through the process of digestion.
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