Gas Exchange in Humans

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…