The hydraulic analogy of Ohm's law can be used to define the relationship between pulmonary vascular pressure, flow, and resistance:
AP = Q • PVR, where AP is the pressure gradient between the inlet and outlet of a vessel (in mm Hg or cm H2O), Q is blood flow (in liters per minutes), and PVR is pulmonary vascular resistance. PVR is by definition the resistance for both lungs and is about 1.7 (mm Hg • min)/L for a normal cardiac output of 6 L/min with an average pressure drop of 10 mm Hg from the pulmonary artery to left atrium.
The resistance to flow through a vessel obviously depends on its dimensions. The dimensions of pulmonary vessels are strongly influenced by several external forces, which is different from the situation for rigid pipes in a plumbing system, or even systemic arteries. The fundamental geometry of the pulmonary capillary network is also different from pipes or systemic capillaries, as illustrated in Fig. 11. The numerous capillaries in the alveolar wall constitute an almost continuous sheet for blood flow between two flat membranes held together by numerous posts. This is called sheet flow, and the resistance to sheet flow can be less than the resistance to flow through a network of tubes. Therefore, Poiseuille's law (Chapter 19) cannot be used to calculate pulmonary capillary resistance from capillary dimensions. Still, PVR increases with the length and decreases by a power function with the internal size of pulmonary capillaries.
The primary determinant of vessel size is the transmural pressure, which depends on the pressure difference between the inside and outside of the vessel:
= Pi inside
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