The human gas exchange system

The human lungs provide an exchange surface adapted for:

  • absorbing oxygen – needed for respiration – into the blood from the air
  • transferring carbon dioxide – produced by respiration – from the blood into the lungs then the air

The lungs are organs enclosed within the chest or thorax. Air needs to be breathed in to be brought into contact with the exchange surfaces within the lungs. This process is called ventilation.

The structure of the respiratory system

The human respiratory system is adapted to allow air to pass in and out of the body, and for efficient gas exchange to happen.

The lungs are enclosed in the thorax, surrounded and protected by 12 pairs of ribs. The ribs are moved by two sets of intercostal muscles. There is a muscular diaphragm below the lungs. The lungs are sealed within two airtight pleural membranes. These wrap around the lungs and line the rib cage.

Diagram showing the process of breathing and the components used

The trachea, or windpipe, branches into two bronchi – one bronchus to each lung. Rings of cartilage in the walls of the trachea help to keep it open as air is drawn in.

The bronchi split into smaller branches and then into smaller tubes called bronchioles. Each bronchiole ends in a cluster of microscopic air sacs called alveoli.

Gaseous exchange

The exchange of gases occurs between the alveoli and blood in the capillaries that supply the lungs. Capillaries cover 70% of the outside of alveoli, providing a large surface area for gases to diffuse across.

Diagram illustrating the air sacs called alveoli which are attached to bronchioles

In cross section:

The alveoli are adapted to provide a very large surface area for gaseous exchange:

The alveoli are adapted to provide a very large surface area for gaseous exchange:

  • small size - each alveolus is a small sphere about 300 μm in diameter, giving it a larger surface area to volume ratio than larger structures
  • number - there are around 700 million alveoli – ie 350 million per lung

The total surface area of the alveoli is around 70 square metres.

There is also a short diffusion path - the walls of blood capillaries and alveoli are just one cell thick. The alveoli are also lined with a thin film of moisture. Gases dissolve in this water, making the diffusion path even smaller.

The ventilation of the lungs and the blood flow through the surrounding capillaries mean gases are being removed continually, and steep concentration gradients are set up for gases to diffuse.