Air constantly enters and leaves the lungs, enabling the tissues of the body to receive an adequate supply of oxygen and to dispose of their waste product, carbon dioxide. Breathing is controlled by the respiratory centre in a part of the brain known as the medulla. The respiratory centre stimulates the intercostal muscles around the chest cavity to contract and relax so that we breathe in and out.
During breathing, air moves from areas of high pressure to areas of low pressure. When the pressure in the lungs is lower than the pressure in the atmosphere, air enters the airways. If the pressure in the lungs increases, air moves out of the lungs and is then exhaled.
Even while we sleep, our basic breathing rhythm is controlled by a collection of nerve cells in the brain called the respiratory centre. From there, messages travel down nerves to the diaphragm and rib muscles and stimulate them so that we continue breathing. As activity changes, so does the amount of carbon dioxide in the blood. Receptors in some of the large arteries detect the changes and send instructions to the brain.
The exchange of oxygen and carbon dioxide gases occurs constantly throughout the body. In the lungs, oxygen crosses the delicate walls of the alveoli (air sacs) and enters tiny blood vessels (capillaries), where it binds to the molecule haemoglobin in the red blood cells. At the same time, carbon dioxide is released from the blood into the alveoli and exhaled through the mouth or nose. In tissue cells, blood from the lungs exchanges its supply of oxygen for carbon dioxide.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.