Every living cell in the body needs a constant supply of oxygen to survive. Cells must also dispose of their primary waste product, carbon dioxide. The role of the respiratory system, together with the circulatory system, is to deliver oxygen from the lungs to the cells and to remove carbon dioxide and return it to the lungs to be exhaled. This exchange of oxygen and carbon dioxide is very effective because of the combination of the vast surface area of the lungs, which is about 40 times greater than that of the body, and the fact that all of the blood passes through the lungs every minute.
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The exchange of oxygen and carbon dioxide between the air, blood, and body tissues is known as respiration. Air enters and leaves the body by the respiratory system, which is made up of the mouth, nose, throat, branching airways, and lungs. The circulatory system, which consists of the heart and blood vessels, ensures that oxygen is delivered to the cells and carbon dioxide is carried away.
Healthy lungs take in about 500 ml (1 pint) of air 12–15 times each minute. Breathing is an automatic process that is controlled by the respiratory centre in the brain; we only become aware of breathing when it is difficult, such as during exercise. When we inhale, air enters the body through the mouth or nose and is warmed and moistened.
Air passes down the pharynx (throat) and larynx (voice box) and into the trachea (windpipe). A small flap of cartilage called the epiglottis covers the trachea during swallowing to prevent food from entering the airways.
In the lungs, oxygen from the air passes into the surrounding blood vessels. There, it binds to the molecule haemoglobin in the red blood cells. The oxygen-rich blood then travels from the lungs to the heart and the body tissues, where oxygen is released. At the same time, carbon dioxide waste from the tissues passes into the blood and dissolves in the plasma, the fluid part of blood. This carbon dioxide is returned to the lungs and exhaled.
Sometimes, infectious organisms or foreign particles enter the body when we inhale. The respiratory system has several ways of protecting the lungs from damage. Situated in the walls of the pharynx are collections of tissue called the tonsils and the adenoids. These help to destroy some infectious organisms as they travel towards the lungs. Airborne particles may also be trapped by mucus in the airways and then directed away from the lungs by tiny hairs called cilia. Finally, foreign particles or excess mucus that irritate the airways or lungs can be expelled by a sneeze or a cough.
The respiratory system also contains the vocal cords, the organs that are responsible for speech production. Sounds are formed when the vocal cords are partially closed and air from the lungs causes them to vibrate. Using these vibrations, we can produce a remarkable range of sounds.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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