Pulmonary Circulation pressure exceeds capillary pressure. This tends to collapse the capillary and reduce flow. If flow actually stops, then pressure in the capillary rises toward the arterial level until the capillary is reopened and flow resumes. In zone 2, the relevant pressure gradient driving blood flow is the arterial-alveolar difference and venous pressure is not important in determining zone 2 flow. Systems with flow determined by upstream and outside (instead of downstream) pressures are called Starling resistors. Figure 13 shows flow progressively increasing down zone 2 because the hydrostatic column increases arterial pressure while alveolar pressure is constant. Both capillary recruitment and distention can contribute to increased flow in zone 2.
Zone 3 occurs near the bottom of the lung, where venous pressure is increased sufficiently by the hydrostatic column to exceed alveolar pressure. Therefore, the arterial-venous pressure difference determines blood flow in zone 3. Figure 13 shows flow increasing down zone 3 because the hydrostatic column distends the capillaries. Some data suggest a zone 4, with decreased flows at the very bottom of the lung. It was hypothesized that high intravascular pressure leads to edema and vascular compression by the interstitium. However, zone 4 can be measured even after animals are inverted, suggesting that factors other than gravity may be involved.
Other methods of measuring the distribution of pulmonary blood flow suggest that factors other than gravity are important also. Radioactive microaggregates of albumin or plastic microspheres can be injected to the pulmonary circulation and they will lodge in capillaries in proportion to local blood flow. Gradients in blood flow have been measured between the center and the periphery of the lung at a given height up the lung. Local stresses and the anatomic details of vascular branching may contribute to such intraregional heterogeneity of blood flow. Intraregional heterogeneity may explain up to half the total heterogeneity of blood flow in the lungs.
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