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				LUNGS AND DIAPHRAGM
				LUNGS

Network of airways

Your lungs are a pair of large sponge-like organs that almost fill your chest cavity. Your left lung is slightly smaller than your right lung, to make space for your heart.

When you breathe in, you suck air in through your nose and mouth and down a tube called the trachea. Your trachea divides into two tubes called the primary bronchi. One enters each lung. From there, the bronchi progressively branch into smaller airways, which eventually lead to tiny air sacs called alveoli. This intricate network of airways looks like an upside-down tree.

Exchanging gases

Your alveoli are surrounded by minute blood vessels, as this is where gases diffuse from your lungs into your blood and from your blood into your lungs. Oxygen passes from your alveoli into your blood and carbon dioxide, which is produced when your cells break down nutrients, passes from your blood into your alveoli.

The total surface area of your alveoli is about the size of a tennis court. However, if you're not doing vigorous exercise, you only use about one-twentieth of your lungs' gas-exchanging surface.

Breathing in and out

You normally breathe in and out about 500ml of air 15 times a minute. Your nervous system automatically increases the rate and depth of your breathing if your body needs more oxygen, for example when you're doing exercise.

Air is forced in and out of your lungs by movements of your diaphragm and other breathing muscles. When you breathe in, your breathing muscles contract, pulling your ribs up and out. The space within your chest increases and reduces the air pressure inside your lungs. As a result, air flows into your lungs. When you breathe out, your muscles relax and your ribs move down and in. The space within your chest decreases again, the pressure inside your lungs increases, and air flows out.

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