A quantitative estimation of regulation and transport limitations in the human cardiopulmonary system
Richard L. Engen
The object of this dissertation is to quantitatively describe the regulation of some of the exchange processes within the human body. Conceptually this dissertation is divided into two sections. In the first section a macroscopic view was adopted to describe the overall regulation of the cardiovascular and respiratory systems. These overall system models were used as heuristic tools to gain an understanding of physiological behavior in micro-gravity. In the second section, a microscopic view was used to estimate the role played by the surfactant system of the lung in regulating the transfer of fluid across the pulmonary-capillary wall;The basis of the cardiovascular system model is the maintenance of arterial blood pressure homeostasis. Sub-models constituting the overall model are: the pressure-flow model, the heart action model, the controller model which describes short term-control, and the renal model which describes long term control and the regulation of total body water content. Model predictions show that incorporating the fluid shift from the lower to the upper part of the body in micro-gravity is sufficient to account for the cardiovascular changes occurring in micro-gravity;The respiratory model is concerned with the maintenance of a constant carbon dioxide level in the tissue and body fluids. The sub-models constituting the overall respiratory model are: the gas-exchange model, the mechanics model, and the controller model which determines the ventilation and cardiac output on the basis of arterial blood gas tensions. Simulation results show that pleural pressure homogeneity, increased lung diffusing capacity and decreased lung volume are sufficient to describe respiratory changes in micro-gravity;In the penultimate section the lung mechanics model is coupled with a model of fluid exchange across the pulmonary-capillary wall. The lung mechanics model estimates the influence of the surfactant system of the lung in controlling the interstitial space hydrostatic pressure while the fluid exchange model determines the influence of the interstitial space hydrostatic pressure in regulating fluid movement across the pulmonary-capillary wall. This model quantitatively estimates the influence of the surfactant alone in regulating fluid movement across the pulmonary-capillary wall.