What is external respiration?

Oxygen diffuses from within your alveoli across the wall of the alveoli and the wall of the capillary into the blood (image).

This is a very short distance, just 0.3 micrometres (a micrometer is a millionth of a meter) in places. The total area of all the walls of all your alveoli is very large (typically around 50 to 100 sq. metres in an adult). This combination of a short distance and a large surface area is ideal for the maximum rate of diffusion of oxygen from the aleoli into the blood.

Your blood consists of around 45% blood cells - chiefly red blood cells - and around 55% plasma, which is mostly water with electrolytes, proteins and other substances dissolved in it. A very small amount of oxygen is dissolved in the plasma. Most oxygen in the blood is carried by haemoglobin which is contained inside the Red blood cells.

Oxygen diffusing from the alveoli into the blood, and carbon dioxide diffusing from the blood into the alveoli are together known as 'external respiration'.