The gas exchange surface of a mammal is the alveolus.

There are numerous alveoli - air sacs, supplied with gases via a system of tubes (trachea, splitting into two bronchi - one for each lung - and numerous bronchioles) connected to the outside by the mouth and nose.

These alveoli provide a massive surface area through which gases can diffuse. These gases diffuse a very short distance between the alveolus and the blood because the lining of the lung and the capillary are both only one cell thick.

The blood supply is extensive, which means that oxygen is carried away to the cells as soon as it has diffused into the blood. Ventilation movements also maintain the concentration gradients because air is regularly moving in and out of the lungs.

This breathing in (inspiration) and breathing out (expiration) is controlled via nervous impulses from the respiratory centre in the medulla of the brain.

Both the intercostal muscles (in between the ribs) and the diaphragm receive impulses from the respiratory centre. Stretch receptors in the lungs send impulses to the respiratory centre in the brain giving information about the state of the lungs.

1. external intercostal muscles contract

2. ribs and sternum move up and out

3. width of thorax increases front to back and side to side

4. diaphragm contracts

5. diaphragm moves down, flattening

6. depth of thorax increases top to bottom so the...

   • volume of thorax increases.
   • pressure between the pleural surfaces decreases.
   • lungs expand to fill thoracic cavity.
   • air pressure in alveoli is less than atmospheric pressure.
   • air is forced in by the higher external atmospheric pressure.

As the lungs fill with air the stretch receptors send impulses to the expiratory part of the respiration centre to end breathing in.

1. External intercostal muscles relax

2. ribs and sternum move down and in

3. width of thorax decreases front to back and side to side

4. diaphragm relaxes

5. diaphragm moves up

6. depth of thorax decreases top to bottom. So the ...

   • volume of thorax decreases.
   • pressure between the pleural surfaces increases.
   • lung tissue recoils from sides of thoracic cavity
   • air pressure in alveoli is more than atmospheric pressure.
   • air is forced out.

As the air leaves, the stretch receptors are no longer stimulated. The inhibition of breathing in (via the expiratory part of the centre) stops so breathing in can start again.

There are also chemoreceptors in the medulla and certain blood vessels that are sensitive to changes in carbon dioxide levels in the blood.

If the level is too high (the pH would drop, enzyme action would be affected with serious results), impulses are sent from these cells to the inspiratory part of the centre so that breathing rate increases.

This means that carbon dioxide is got out of the body as quickly as possible and more oxygen comes in.