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Passage of air into the lungs

1. Air enters the body and is warmed as it travels through the mouth and nose.
2. It then enters the trachea.
3. The trachea divides into two bronchi. One bronchus enters each lung.
4. Each bronchus branches out into smaller tubes called bronchioles. Air travels through these bronchioles.
5. At the end of the bronchioles, the air enters one of the many millions of alveoli where gaseous exchange takes place.

Gaseous exchange

Gas exchange occurs at the alveoli in the lungs and takes place by diffusion. The alveoli are surrounded by capillaries so oxygen and carbon dioxide diffuse between the air in the alveoli and the blood in the capillaries.

Diffusion is the movement of gas from an area of high concentration to an area of low concentration.

There is a high concentration of oxygen in the alveoli and a low concentration of oxygen in the blood, so oxygen diffuses from the alveoli into the blood.

There is a high concentration of carbon dioxide in the blood and a low concentration in the alveoli, so carbon dioxide diffuses from the blood into the alveoli.

Both oxygen and carbon dioxide are capable of combining with an iron-rich protein in the blood called haemoglobin. Haemoglobin carries oxygen to be exchanged at the working muscle and carbon dioxide to be exchanged at the lung.

At the muscles, the opposite occurs as carbon dioxide enters the blood from the muscle while oxygen enters the muscle from the blood.

Capillaries surround the alveoli in the lungs. Both the capillaries and alveoli walls are very thin - just one cell thick. They are made of semi-permeable membranes which allow oxygen and carbon dioxide to pass through them.