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.
Pulmonary ventilation (breathing). Inspiration and expiration-inhale oxygen rich air, exhale carbon dioxide rich air. External respiration. Gas exchange between alveolar capillaries and the alveoli (oxygen movement from the alveoli into the blood, carbon dioxide movement from the blood into the alveoli). Transport of respiratory gases. Oxygen from lungs to tissues, carbon dioxide from tissues to lungs.