Blood Flow Distribution in the Lung

Blood flow is not uniform across the lung. Because the pulmonary circuit operates at low pressure, the local balance among arterial, alveolar, and venous pressures—together with the lung's vascular architecture—produces marked regional differences in perfusion, classically described as a gradient from the top to the bottom of the upright lung.

Definition

Blood flow distribution in the lung is the regional pattern of pulmonary perfusion, determined chiefly by the relationships among pulmonary arterial, alveolar, and pulmonary venous pressures and by the branching geometry of the pulmonary vascular tree.

Scope

The entry covers how regional pulmonary blood flow is distributed, the pressure-based zonal model that explains the gravitational gradient, the contribution of vascular structure to non-gravitational heterogeneity, and how distribution changes with posture and exercise. It is a reference physiology topic; it explains mechanisms rather than offering clinical guidance.

Core questions

• Why is perfusion greater at the base than at the apex of the upright lung?
• How do arterial, alveolar, and venous pressures define zones of flow?
• How much of the heterogeneity is gravitational and how much is structural?
• How does distribution change with posture and with exercise?

Key concepts

• Zone 1 (alveolar > arterial pressure)
• Zone 2 (arterial > alveolar > venous pressure)
• Zone 3 (arterial > venous > alveolar pressure)
• Gravitational perfusion gradient
• Fractal vascular branching
• Structural (non-gravitational) heterogeneity
• Postural and exercise redistribution

Mechanisms

In the classic model, the relationship among alveolar, pulmonary arterial, and pulmonary venous pressures defines three zones from the top to the bottom of the upright lung: where alveolar pressure exceeds arterial pressure, capillaries are compressed and flow may cease (zone 1); lower down, arterial pressure exceeds alveolar pressure, which in turn exceeds venous pressure, so flow depends on the arterial-alveolar difference (zone 2); and at the base, both arterial and venous pressures exceed alveolar pressure, so flow is governed by the usual arterial-venous gradient (zone 3) (West, Dollery & Naimark, 1964). Gravity establishes these pressure differences down the lung. Later work showed that gravity is not the whole story: the fractal branching geometry of the pulmonary vasculature imposes substantial heterogeneity that persists independent of posture, so distribution reflects both gravitational and structural determinants (Glenny & Robertson, 2011; Suresh & Shimoda, 2016). With exercise and the accompanying rise in pressure and flow, recruitment makes the distribution more uniform.

Clinical relevance

The regional distribution of perfusion is part of how the lung matches blood flow to ventilation and underlies the interpretation of regional gas exchange and perfusion imaging. This entry describes normal physiology and how it is studied; it is educational and not a basis for clinical decisions about any individual.

Evidence & guidelines

The zonal model derives from West and colleagues' isolated-lung experiments relating flow to vascular and alveolar pressures (West et al., 1964). Subsequent high-resolution measurements refined the picture, showing a large structural component to flow heterogeneity, as synthesized in dedicated reviews (Glenny & Robertson, 2011; Suresh & Shimoda, 2016).

History

The pressure-based explanation of the lung's perfusion gradient was established by West, Dollery, and Naimark's 1964 isolated-lung experiments, which related regional flow to the interplay of vascular and alveolar pressures and gave rise to the zonal model taught ever since. From the late twentieth century, microsphere and imaging studies revealed that branching structure contributes heterogeneity beyond gravity, reframing distribution as the product of both forces (Glenny & Robertson, 2011).

Debates

Is the lung's perfusion gradient primarily gravitational?

The classic zonal model attributes regional differences chiefly to gravity, but high-resolution studies show that vascular branching structure produces substantial heterogeneity independent of posture, so the relative weight of gravitational versus structural determinants is debated.

Key figures

• John B. West
• Robert W. Glenny
• H. Thomas Robertson

Seminal works

• west-1964
• glenny-2011

Frequently asked questions

Why does the base of the lung receive more blood flow than the apex when upright?

Gravity raises vascular pressures toward the base, so the arterial pressure more fully exceeds alveolar pressure there; capillaries are better distended and recruited, giving greater flow than at the apex.

What are West's zones?

They are three regions of the upright lung defined by the relationship among alveolar, arterial, and venous pressures: zone 1 (little or no flow, alveolar pressure highest), zone 2 (flow set by the arterial-alveolar difference), and zone 3 (flow set by the usual arterial-venous gradient).