- The lungs are designed to continually exchange gas with the external environment in order to maintain low concentrations of carbon dioxide and high concentrations of oxygen in their gas exchange areas. To do so, the lungs must intake and efflux a large volume of air with each breath. "Ventilation" refers to the volume of air that the lungs exchange each minute and defines an important variable of an individual's pulmonary physiology.
- The total volume of air which the lung exchanges with the external world is known as the Total Lung Ventilation; however, because of the lung's architecture, some of the air volume inhaled with each breath never reaches the gas exchange areas and thus is said to exist within the lung's "Dead Space". In contrast, the ventilatory volume that renews the gas exchange areas contributes to the so called Alveolar Ventilation rate and displays some regional variation within the upright lung as described in Pulmonary Ventilation Distribution.