It is possible to have multiple independent variables and/or multiple dependent variables. For instance, in multivariable calculus, one often encounters functions of the form, where z is a dependent variable and x and y are independent variables.
Gas exchange between the air within the alveoli and the pulmonary capillaries occurs by diffusion. The oxygen must first dissolve before passing through the respiratory epithelium. Gas moves from a region of high partial pressure to a region of low partial pressure, down a partial pressure gradient.
Once in the alveoli, the carbon dioxide in the blood is expelled by the alveoli up through the bronchi of the lungs and the trachea then expelled from the body through exhalation. At the same time, oxygen enters the alveoli, and the alveoli sends oxygen to the capillaries.