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Human Body
that of oxygen. Most of it is carried in the plasma in the form of bicarbonate ions (HCO,), some is carried in the form of carbamino haemoglobin and a small amount dissolves in the plasma. Nitrogen which constitutes nearly -four-fifths of the volume of air is generally ignored by the body.
Internal Respiration
The major function of the circulatory system is to deliver oxygen and carry off carbon dioxide. The gas exchange between the blood and tissues is very similar to that in the lungs except that the gases go in the opposite direction. Carbon dioxide from the tissues goes into the blood and oxygen from the blood goes into the tissue fiuid and from there into the cells.
Control of Respiration
Normally breathing is an unconscious act that goes on continually throughout one's daily activities and even when one is asleep at night. Breathing can also be controlled (consciously) voluntarily to some extent. One can breathe rapidly or slowly, deeply or shallowly at will. One can even stop breathing entirely for a time. But most of the time respiration is under automatic control by special centres in the central nervous system. They are located in the medulla and pons. The medullary respiratory centres set the basic rhythms of respiration. The hypothalamus and cerebral cortex also participate in respiratory control.
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 age1.
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In addition to the normal actions of respiration, the respiratory passages participate in a number of reflex mechanisms and other modified movements. Both cough
1. A new born baby breathes rapidly from forty to seventy times a minute. By the end of the first year, the rate is 35 to 40. By five years, about 25 and at ten years, 20 times per minute. By the late teen years, the normal adult level of 14 to 20 is reached.
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