Gas exchange

Gas exchange is the diffusion of oxygen and carbon dioxide across body surfaces, both between the environment and the body's fluids and between blood and cells. Every gas-exchange surface, from the gills of a fish to the human lung, is thin, moist and large in area, because diffusion is fast only over very short distances. In humans it happens at two sites: in the lungs, called external respiration, and between blood and body cells, called internal respiration.

In the lungs, air reaches millions of tiny sacs called alveoli, each wrapped in capillaries. The alveolar and capillary walls together form the respiratory membrane, less than a micrometer thick. Each gas diffuses down its own partial pressure gradient, from high to low: oxygen passes from alveolar air into the blood, where most of it binds hemoglobin in red blood cells, and carbon dioxide diffuses out of the blood into the alveoli to be breathed out. In the tissues the gradients are reversed, so oxygen leaves the blood and carbon dioxide enters it. Efficient exchange also needs airflow and blood flow to be matched.

Diseases that thicken the membrane, as in pulmonary fibrosis, destroy alveoli, as in emphysema, or fill them with fluid, as in pneumonia, impair gas exchange and can lower blood oxygen.