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Deep in the lungs.

Like all movements in the body, those of breathing rely on muscle power (see page 38). There are two main sets of breathing muscles: the intercostal muscles and the diaphragm. The intercostal muscles are shaped like straps and sit between each pair of ribs. As they contract, they pull the ribs closer together. This makes the whole ribcage swing upward and outward. Breathe in deeply and watch your ribs rise and your chest expand. The diaphragm is a curved sheet of muscle at the base of the chest, forming a roof over the liver, stomach' the intestines, and other parts in the abdomen below it. As the diaphragm contracts, it changes from an upwardly curved, dome shape (like an upturned bowl) to a flatter plate shape. This has the effect of pulling down the curved bases of the lungs, which are situated just above the diaphragm, and increasing them in size.

How you breathe

Together, these muscles make the chest bigger and stretch the spongy lungs inside. As the lungs enlarge, they suck in air down the windpipe. This is how you breathe in. Then the muscles relax. The ribs fall back down and the diaphragm resumes its domed shape as the spongy, elastic lungs spring back to their smaller size. The lungs blow some of their air up the windpipe. This is how you breathe out.

The movements of breathing are controlled by the rain. Its "automatic pilot" (see page 102) sends out signals to make the muscles contract. The signals pass along nerves to the intercostal and diaphragm muscles, making them contract. This happens every few seconds throughout your life, even when you are asleep. If you are exercising heavily and need lots more oxygen, other muscles help with breathing, such as those in the shoulders, back, and abdomen.

Air flowing down the windpipe passes into the main tube, or bronchus, to each lung. This tube splits into smaller ones, which divide again into even smaller ones. Deep inside the lungs, the air tubes are as thin as hairs, and are called terminal bronchioles. They end at groups of microscopic "air bubbles" known as alveoli, like hollow grapes on a stalk. Each alveolus has a network of tiny blood vessels -- capillaries -- wrapped around it. Oxygen from the air passes through the alveolus lining into the blood in the capillaries, which carries it around the body.

Two-way traffic

Breathing is not a one-way process of taking in oxygen. Cellular respiration makes a waste product -- carbon dioxide. If it built up in cells, it would poison them. So carbon dioxide is collected around the body and taken away by the blood that has just deposited its oxygen. As this blood flows back through the lungs, carbon dioxide passes through the alveolus lining to the air on the other side. This means that gases are moving from the air into the blood and vice versa. The effect is that in the lungs, oxygen moves from air to blood, and carbon dioxide moves from blood to air. Then you breathe out, getting rid of stale lower-oxygen air, ready tO rake in fresh air with the next breath.
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Title Annotation:excerpt from 'The Human Body, An Amazing Inside Look at You' describes lung function and physiology
Author:Parker, Steve
Publication:Children's Digest
Date:Oct 1, 1997
Words:525
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