Illustration of the 3 lung zones defined by alveolar pressure. -Zone 1: P alveolar > P arterial > P veinous so flow is nil, negligeable zone in a healthy individual, corresponds to alveolar dead space. -Zone 2: P arterial > P alveolar > P veinous so flow is proportional to P arterial - P alveolar. -Zone 3: P arterial > P veinous > P alveolar so flow is proportional to P arterial - P veinous, the zone that is totally independent from alveolar pressure corresponds to the veinous admission zone.