The human lungs provide an exchange surface adapted for:
The transfer of these two gases between organisms and their environment is called gas exchange.
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 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.
In cross section:
The alveoli are adapted to provide a very large surface area for gaseous exchange:
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.