The role of the capillary blood vessels of the lung in HPH is to reduce the resistance to flow in pulmonary microcirculation. The reason that these vessels can do it is because of their geometry. When the pulmonary capillary blood vessels of cat were photographed on face, they appeared as shown in Figure 1 A. When these capillary vessels were photographed in cross section, they appear as shown in Figure IB. From these photographs one sees that the capillaries form sheets of blood space that are about 5 to 10 (am thick, 200 to many 1000 (im wide. In the on-face view, the blood space is interrupted by more or less uniformly spaced roundish obstructions which are called posts (Fig. 1B). Fung and Sobin (12-14) called the geometry of pulmonary capillaries a sheet model, which consists of two thin membranes separated by posts which are about 3 Jim in diameters. The thickness of the blood sheet space is a function of the capillary blood pressure. Experimental results of the sheet thickness versus the transmural pressure of the cat lung are shown in Figure 1C. The transmural pressure Ap is the difference of capillary blood pressure minus the alveolar gas pressure. When Ap is negative, the thickness is zero. When Ap is slightly positive the thickness jumps to 4.5 (am. The thickness increases linearly with increasing positive Ap until Ap reaches about 30 cm H20. For larger Ap, the rate of increase of sheet thickness decreases.
With the geometry and mechanical properties shown in Figure 1, the flow in the pulmonary capillaries as seen by a red blood cell might be said to be like a car moving in an underground parking garage with many posts. When the blood pressure increases, the ceiling height increases, and the resistance to flow decreases. The resistance of the pulmonary microcirculation is transferred to the pulmonary arteries and veins.
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