absorbing oxygen - needed for respiration - into the blood from the air in the lungs
transferring carbon dioxide - produced by respiration - from the blood into the lungs (which then goes into 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.
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.
The exchange of gases occurs between the alveoli and blood in the capillaries that supply the lungs.
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 m2 - capillaries cover 70 per cent of the surface of the alveoli
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.