Steps of Gas Exchange

1. Pulmonary arteries bring deoxygenated blood to the capillaries surrounding the alveoli and oxygen is breathed into the lungs.

2. Carbon dioxide diffuses across the capillary and alveolus wall and evaporates as gas in the alveoli.

3. Oxygen dissolves in the moisture on the inside of the alveolus and diffuses across the alveolus and capillary wall and binds with haemoglobin in red blood cells.

4. Oxygenated blood leaves the capillaries of the alveoli and is taken to the heart via the pulmonary veins and carbon dioxide is beathed out of the lungs.

   
   

Importance of the Concentration Gradient in Gas Exchange

For the diffusion of gases into and out of the blood, there needs to be a concentration gradient. This concentration gradient is produced by the difference in oxygen and carbon dioxide concentrations and thus allows the gases to passively diffuse in and out of the bloodstream. The concentration gradient is maintained by two factors:

• The constant flow of deoxygenated blood into the lungs and of oxygenated blood out of the lungs

  • Blood that is lacking oxygen is brought to the lungs by the pulmonary arteries. This blood has just returned from delivering oxygen to the body and therefore has a lower concentration of oxygen than the air in the alveoli. The oxygen in the alveolus therefore travels along the concentration gradient to leave the alveoli and enter the blood.

  • Also, blood entering the capillaries of the alveoli has a higher concentration of carbon dioxide than the air in the alveolus. This is because it has collected the carbon dioxide from the cells in the body. Due to the difference in concentration, the carbon dioxide diffuses out of the blood and into the alveolus.

• The constant flow of oxygen into the lungs during inspiration and of carbon dioxide out of the lungs during expiration

  • In the same way that blood is constantly flowing through the pulmonary capillaries, air is constantly flowing in and out of the alveoli, oxygen is being pulled into the lungs by the mechanism of inspiration and carbon dioxide is being pushed out of the lungs during the mechanism of expiration. This maintains the concentration gradient that allows gas exchange to occur.

Structure and Function Relationship

The structure of the alveoli is very important for their function in gas exchange:

• The wall of the alveolus and the capillary is just one cell thick each. This means gases only must travel through/between two cells.

• The inside of the alveolus is lined with a film of moisture. This moisture allows gases entering the lungs to dissolve, allowing for easier diffusion across the walls.

• The grape-like structure of the alveoli means they have a large surface area. The larger the surface area, the faster gases can travel into and out of the blood. This allows gas exchange to be more efficient.