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.
Where does the exchange of gases between the lungs and the blood take place? An alveolus (plural: alveoli, from Latin alveolus, little cavity) is an anatomical structure that has the form of a hollow cavity.