The cells in the body need oxygen to release energy from food efficiently by carrying out aerobic respiration. A waste product of aerobic respiration is carbon dioxide.
Carbon dioxide must be removed from the body or it makes the blood dangerously acidic. Oxygen and carbon dioxide enter and leave the blood by diffusion through the lining of the .
The lungs
Air gets from the mouth and nose to the lungs through the windpipe (trachea). The trachea branches into two tubes called bronchi (one to each lung). The bronchi split into smaller and smaller tubes called bronchioles, which then end in microscopic (air sacs).
The alveoli are lined with and are surrounded by a network of blood capillaries. They have very thin walls for gases to be absorbed through. An individual air sac is called an alveolus.
The layer of moisture in the alveoli allows gases to dissolve so that they can diffuse quickly.
The alveoli have a very large total surface area and a very good blood supply, provided by the dense network of capillaries that surround them. There is an exchange of gases between the alveoli and their surrounding capillary blood vessels. Oxygen diffuses from the alveoli into the blood. Carbon dioxide diffuses from the blood into the alveoli.
These three features are particularly important to our lungs for efficient gas exchange. Most of them also apply to other exchange surfaces such as the small intestine, body tissues and placenta.
- Thin lining: the lining of the alveoli is very thin so that gases can quickly diffuse through it.
- Large surface area: human lungs contain about 500 million alveoli, which creates a surface area around half the size of a tennis court. This speeds up diffusion because gases have more area over which to diffuse.
- Good blood supply: the alveoli have a dense capillary network so that large volumes of gases can be exchanged.
