Distribution of Blood Flow

The distribution of pulmonary blood flow throughout the pulmonary vascular tree and to different parts of the lung is not uniform. This was first shown in humans with a technique measuring pulmonary blood flow at different heights in the erect human lung using an insoluble radioactive gas. A saline solution containing radioactive xenon was infused in a vein, so the gas would enter the lung in proportion to blood flow (similar to CO2 elimination from the blood). Radioactive counters were placed at different heights outside the chest to determine relative blood flow rates at different heights in the lung. Relative blood flow increased progressively from top to bottom of the lung.

The effect of gravity on pulmonary vascular pressures is a major factor determining the regional differences in blood flow in the upright human lung. The pulmonary vasculature can be considered a continuous hydrostatic column that is about 30 cm tall in the upright human lung. This means there is a hydrostatic pressure difference of 30 cm H2O (or 23 mm Hg) between vessels at the top and bottom of the lung. This pressure difference is nearly as large as the pulmonary artery pressure, so it has profound effects on regional distribution of blood flow. Evidence for a gravitational mechanism includes a reduction in the gradient of blood flow in erect persons during exercise when pulmonary arterial pressure increases, and a reduction in the gradient of blood flow in the supine posture. A dorsal-ventral gradient can be measured in people lying supine and the vertical gradient is reversed in persons suspended upside down.

Figure 13 illustrates these effects using the zone model for pulmonary blood flow. This model conceptually divides the lung into three zones to explain how gravity affects blood flow through alveolar vessels at different heights up the lung. Zone 1 would occur at the top of the lung, where the pulmonary arterial pressure may not be sufficient to pump blood to the top of the lung. In this case, pulmonary arterial pressure is less than the hydrostatic pressure column between the heart and the top of the lung. Alveolar pressure, even if 0, is greater than arterial pressure so the capillaries collapse. Normally, zone 1 does not occur because the normal pulmonary arterial pressure (30 cm H2O) is greater than the height of a water column between the heart and top of the lung (about 15 cm).

Zone 2 occurs near the middle of the lung, where pulmonary arterial pressure is increased by the hydrostatic column, and blood flow occurs. However, venous pressure is less than alveolar pressure because these veins may be below the level of the heart. Intravascular pressure decreases from the arterial to venous level along the capillary, and at some point the alveolar

FIGURE 13 West's zone model of pulmonary blood flow predicts increasing blood flow down the lung because of the effects of gravity on pressures, as explained in the text. Pa, arterial pressure; Pa, alveolar pressure; Pv, venous pressure. (After West, Chap. 58 in Fenn and Rahn, eds., Handbook of physiology, Section 3, Respiration. Bethesda, MD: American Physiological Society, 1965.)

FIGURE 13 West's zone model of pulmonary blood flow predicts increasing blood flow down the lung because of the effects of gravity on pressures, as explained in the text. Pa, arterial pressure; Pa, alveolar pressure; Pv, venous pressure. (After West, Chap. 58 in Fenn and Rahn, eds., Handbook of physiology, Section 3, Respiration. Bethesda, MD: American Physiological Society, 1965.)

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