THE RESPIRATORY SYSTEM

THE RESPIRATORY SYSTEM

We breathe air in and out in order to take oxygen into the lungs, and to expel the waste gas carbon dioxide, a by product of respiration. Breathing is not quite the same as respiration, which is the process whereby oxygen and carbon dioxide are exchanged in the lungs, and in the cells of the body. The process of gas exchange within the body tissues is described in the Circulatory System. When we breathe in, air is drawn in at the nose and mouth, and then passes down the main airway to the lungs, Within the lungs, it travels along a broadening network of air passages that finally open into tiny air sacs (alveoli). Here, the oxygen is taken up by the blood. Carbon dioxide-bearing air is then expelled as we breathe out, enabling fresh oxygen-bearing air to be drawn in with the next breath.

HOW WE BREATHE

Breathing consists of three phases: breathing in (inspiration), breathing out (expiration, and a pause. When we breathe in, muscles in the chest work to expand its volume, drawing air into the lungs, When we breathe out, the elastic chest wall regains its resting position, and the air is pushed out, Some air is always left in the lungs so that oxygen is constantly available to the blood. 

What controls breathing?

Breathing is controlled by the respiratory centre in the brain. No conscious effort is required to breathe, though in normal circumstances we can change the depth and rate of breathing voluntarily. An adult normally breathes about 16 times per minute; children breathe 20 to 30 times per minute. The rate may be altered (usually increased) by the respiratory centre as a response to stress, exercise, injury, or illness.

The composition of air

Air is a mixture of gases, of which 80 per cent is nitrogen and 20 per cent is oxygen. Only some of this oxygen is used up by respiration, so the air we breathe out still contains 16 per cent oxygen, in addition to a small amount of carbon dioxide. The oxygen level in exhaled air is thus adequate to resuscitate another person.