PULMONARY VASCULAR TRANSPORT IN SHEEP - MATHEMATICAL-MODEL

A mathematical description of passive transvascular fluid and protein exchange was developed. The transport model includes vascular membrane characteristics, i.e., equivalent pore sizes and numbers and membrane thickness, as well as the physiological variables of microvascular hydrostatic pressure (Pmv), perimicrovascular hydrostatic pressure and plasma protein molecular sizes and concentrations. The model is being used to investigate the sensitivity of each of the membrane characteristics and physiological variables upon the pulmonary transvascular exchange in the steady state. In the sheep lung Pmv was measured relative to the left atrium, assuming a linear pressure distribution over the measured height of the lung for constant density and vascular membrane characteristics. Theoretical results for the total lymph/plasma concentration ratios of albumin and globulin as well as the lymph flow fractions relative to baseline lymph flow rate were obtained from the model as a function of the referenced Pmv over the range 10-40 mm Hg. Results were obtained for various models of membrane characteristics. The theoretical results of lymph/plasma ratios and lymph flow fractions were compared with the experimentally measured values from unanesthetized sheep. A multiple-pore model was found that included a small pore system which completely sieved the smallest molecule investigated (albumin, pore radius = 34 .ANG.), an intermediate-sized pore of 125 .ANG. radius and a few large pores of 1000 .ANG. radius. The theoretical results from this model were found to compare with the experimental results from several sheep for the Pmv range investigated.