1. Air enters the body through the mouth or nasal cavity.
2. It travels down the trachea (windpipe).
3. It then moves into the bronchus and into the bronchioles.
4. It ends in the alveoli where gasesous exchange takes place.

Gaseous exchange occurs in the alveoli in the lungs and takes place by diffusion.

In the alveoli, there is a high concentration of oxygen and in the bloodstream, there is a high concentration of carbon dioxide.

Oxygen diffuses into the blood from the alveoli and carbon dioxide diffuses into the alveoli from the blood.

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

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.

Vital capacity is the maximum amount of air that can be breathed out after breathing in as much air as possible. Taking part in regular aerobic exercise has been shown to increase a person’s vital capacity.

Breathing rate (frequency) is the number of breathes in a minute. The average breathing rate is 12 breaths/minute.

Tidal volume is the amount of air breathed in with each normal breath. The average tidal volume is 0.5 litres (500 ml).

Minute ventilation (VE) is the total volume of air entering the lungs in a minute. The average minute ventilation is 6 litres/minute.

During exercise, tidal volume increases as the depth of breathing increases and the rate of breathing increases too. This has the effect of taking more oxygen into the body and removing more carbon dioxide.