Gas Exchange at the Alveoli (AQA GCSE Physical Education (PE))

Gaseous Exchange

• The term gas exchange refers to the exchange of gases between the air and the blood; this occurs in the alveoli of the lungs

  • Oxygen enters the blood from the air

  • Carbon dioxide leaves the blood and enters the air

• Gas exchange occurs by the process of diffusion:

The movement of particles from an area of high concentration to an area of low concentration

• The features of the alveoli assist with gas exchange:

  • Large surface area

    • There are many alveoli within the lungs, resulting in a large surface area across which diffusion can occur

  • Short diffusion distance

    • Alveoli walls and capillary walls are one cell thick, reducing the diffusion pathway for gases

    • The capillary vessels are located directly next to the alveoli to minimise the diffusion pathway

  • Many capillaries

    • This means that the alveoli have a good blood supply, maintaining oxygen and carbon dioxide concentration gradients

    • The blood arriving at the alveoli is high in carbon dioxide in comparison to the alveoli, so carbon dioxide diffuses out of the blood and into the alveoli

    • The blood is low in oxygen in comparison to the alveoli, so oxygen diffuses into the blood from the alveoli

  • High oxygen concentration

    • The process of ventilation ensures that the air in the alveoli always contains a higher concentration of oxygen than the blood; this means that oxygen diffuses into the blood

  • Layer of moisture

    • Gases in the air dissolve in this layer on the surface of the alveoli, aiding diffusion

Gas exchange in the alveoli diagram

Alveoli are adapted to maximise gas exchange

The role of haemoglobin

• After diffusing into the blood oxygen binds to haemoglobin within red blood cells to form oxyhaemoglobin:

oxygen + haemoglobin → oxyhaemoglobin

• Haemoglobin can also bind to carbon dioxide, aiding the transport of carbon dioxide to the lungs