Respiratory gaseous exchange occurs between the interior of an organism and the environment. In all animals with a vascular system, in which the blood transports respiratory gases, there is a twofold gaseous exchange within actively respiring tissues and the respiratory organs. In humans, the respiratory organs are the lungs, and there gaseous exchange takes place in the alveoli.

The quantity of gas diffusing through the alveolar tissue depends on pressure differences. There are pressure gradients between the air in the alveoli and the blood. The steeper the gradient, the more rapid the gaseous exchange. The steepness of the pressure gradient corresponds to the difference between the partial pressure of the respiratory gas in the alveolar air and its partial pressure in the blood (the pressure exerted by a particular component of a mixture of gases is called the partial pressure of that gas). Inspired air contains...