Gas exchange is a process that occurs as part of respiration, allowing an organism to exchange waste gases for gases it needs to function. In the case of humans and other mammals, this allows the expression of carbon dioxide and the ingestion of oxygen. Gas exchange takes place along the respiratory membrane, like the lining of the lungs in animals, and the body's circulatory system allows gases to move throughout the body. In single-celled animals, gas exchange allows gases to move freely directly in and out of the cell.
In plants, the leaves act as a respiratory membrane, with small capillaries in the leaves that act to transport gases and nutrients throughout the tree. Animals like amphibians can depend on their skins for gas exchange, while gill animals like fish use the large surface area of the gills to extract dissolved oxygen from the water and carry it into their bodies.
Every time a person breathes, air rich in oxygen and other gases enters the lungs. Air moves from an area of high pressure, the full lung, to an area of low pressure, one of the alveoli, the small sacs that line the lungs. From there, the oxygen passes through a thin membrane into the capillaries that supply the lung, where it binds with hemoglobin for transport throughout the body. As the blood circulates, it also carries waste such as carbon dioxide, carrying it to the lungs for gas exchange so that it can be expelled from the alveoli and exhaled when the person exhales.
People who have trouble breathing can begin to develop hypoxia, a state in which not enough oxygen reaches the body. This can occur when fluid in the lungs inhibits gas exchange, when a lung cannot inflate properly, or when the airways are blocked. A patient who cannot expel carbon dioxide and get oxygen in return will begin to develop soiling of the limbs and may gasp for air.
In an environment rich in another gas, such as nitrogen, that gas can displace oxygen, making it impossible for oxygen to reach the lungs. A patient can breathe normally, but because gas exchange with oxygen does not take place, the person will slowly starve to death. Depending on the gas that is breathed in, patients can develop symptoms such as vertigo, confusion, and drowsiness before succumbing to coma and eventual death due to lack of oxygen.