Metabolically produced CO2 diffuses into tissue capillaries and flows convectively via the bloodstream into alveolar capillaries. There it diffuses across the alveolocapillary membrane into the alveoli, from where it is transported via convective alveolar ventilation into the atmosphere. O2 flows in the reverse direction. The flow of gases differs significantly in various parts of the lungs, depending on the anatomical location, the body position and level of physical activity. The exchange of gases within a particular subdivision of the lung parenchyma is determined by the local ratio of alveolar ventilation versus perfusion, while efficiency of gas exchange at the organ level is determined by the distribution of the alveolar ventilation/perfusion ratio across the lungs. This paper presents an original, illustrative and quantitative approach to dealing with the demanding topic of how ventilation and perfusion influence pulmonary gas exchange in order to provide the reader with a rationale for the diagnosis and treatment of the most common conditions affecting respiratory function.