Distensibility of the Pulmonary Capillaries

Abstract

A sheet flow has been employed as a morphometric idealization of the vascular space of the pulmonary capillaries (Sobin and Fung, 1972 and Fung, 1980). In this model, blood flows in between two membranes and around the posts holding the membranes apart at a sheet thickness h. Because of the distensibility of the pulmonary capillaries, the thickness increases when the transmural pressure (P_tm), the capillary blood pressure minus the alveolar gas pressure, is increased. As one increases the transpulmonary pressure (P_tp), the alveolar gas pressure minus the pleural pressure, to inflate the lung to a larger volume, the surface area of all alveolar sheets becomes larger. Since the blood volume of the pulmonary capillaries is related to the product of the sheet thickness and the surface area in the sheet flow model, we have the following incremental relation to describe the distensibility of the pulmonary capillaries

$$ \Delta V_c /V_c = \Delta P_{tp} /E_1 + \Delta P_{tm} /E_2 $$

(1)

where E₁ and E₂ are elastic moduli and Δ represents the increment of the quantity that follows.