Analysis of Factors Affecting Partial Pressures of Oxygen and Carbon Dioxide in Gas and Blood of Lungs: Theory

Abstract

The major physiological factors affecting the partial pressures of O2 and CO2 in the gas and blood in the lungs can be classified in 4 broad categories: ventilation, circulation, -distribution and diffusion. To be representative of the lung as a whole, the net effects of these physiol. factors must be consistent with the gas exchange of the lung as a whole. They can be indicated graphically by partial pressure gradients laid off between or along the gas and blood R lines for the lung as a whole. The entire gradient between inspired gas and mixed venous blood can be subdivided into 5 major gradients corresponding to the net effects of ventilation, circulation, distribution (2 gradients) and diffusion. The size of the various gradients varies at different levels of oxygenation, and each physiol. factor is therefore characterized by a function of its gradient which remains relatively constant at different levels of oxygenation. The following gradients and functions are used: PIO2-PEO2, ventilation in 1/min./sq.m. body surface; PEO2-P[image]O2, dead space ventilation in percentage of total ventilation; P[image]O2-PcO2, diffusing e capacity for oxygen; P[image]O2-P[image]O2, venous admixture in percentage of cardiac output; PaO2-P[image]O2, cardiac output in l/min./sq.m. body surface. The distribution factor is represented by both the dead space and venous admixture gradients. Various hypothetical but precisely definable values for alveolar gas and alveolar capillary blood are discussed. A method is presented for determining the mean oxygen partial pressure gradient between alveolar gas and alveolar capillary blood.