Gas exchange is a process that occurs as part of respiration, allowing an organism to exchange waste gases for gases it needs in order to function. In the case of humans and other mammals, this allows for the expression of carbon dioxide and the intake of oxygen. Gas exchange takes place along the respiratory membrane, such as the lining of the lungs in animals, and the circulatory system of the organism allows the gases to move throughout the body. In unicellular 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 capillary vessels in the leaves acting to transport gases and nutrients throughout the tree. Animals like amphibians may rely on their skins for gas exchange, while gilled animals like fish use the large surface area of the gills to pull dissolved oxygen out of the water and into their bodies.
Every time a person takes a breath, air rich in oxygen and other gases enters the lungs. The air moves from an area of high pressure, the filled lung, to an area of low pressure, one of the alveoli, the tiny sacs lining the lungs. From there, the oxygen passes through a thin membrane into the capillaries supplying the lung, where it binds to hemoglobin for transport throughout the body. As the blood circulates, it also carries away wastes like carbon dioxide, delivering them to the lungs for gas exchange so they can be forced out of the alveoli and exhaled when the person breathes out.
People who have difficulty breathing can start to develop hypoxia, a state where not enough oxygen is reaching the body. This can occur when gas exchange is inhibited by fluid in the lungs, when a lung can't inflate properly, or when the airway is obstructed. A patient who cannot expel carbon dioxide and get oxygen in exchange will start to develop bluing of the extremities and can gasp for air.
In an environment rich with another gas, like nitrogen, that gas can displace oxygen, making it impossible for oxygen to reach the lungs. A patient may be able to breathe normally, but because gas exchange with oxygen is not taking place, the person will become slowly oxygen-starved. Depending on the gas being breathed, patients can develop symptoms like giddiness, confusion, and sleepiness before succumbing to coma and eventual death due to lack of oxygen.