Gas exchange

Abstract

The main function of the respiratory system is to replenish oxygen and remove carbon dioxide from the systemic venous blood. Composition of blood gases depends on atmospheric and alveolar pressures of individual gases, alveolar ventilation (VA) and perfusion (Q), V/Q matching, diffusion of gases across the alveolar-capillary barrier, binding of O2 with hemoglobin, cardiac output, tissue metabolism and CO2 transport. Alveolar gas composition is determined by PO2 in the inspired gas, PCO2 in the alveoli, and the respiratory quotient. Alveolar ventilation is the predominant determinant of PaCO2. The diffusion of gases from the alveoli into the pulmonary capillary blood is predominantly influenced by their solubility and to a lesser extent by their molecular weight. The processes of alveolar ventilation and pulmonary perfusion are not uniform throughout the lungs even in normal persons. This discrepancy in V:Q relationship is exaggerated in various disease states resulting in characteristic changes in blood gas composition. Arterial blood gas abnormalities can be classified in 4 categories: (1) Alveolar hypoventilation, (2) V:Q mismatch, (3) Diffusion impairment and, (4) Right to left shunting. Oxygen delivery to the tissues is a product of arterial O2 content (CaO2) and cardiac output. Mixed venous O2 represents the balance of O2 remaining in the blood after being consumed by the tissues from the amount delivered. If CaO2 and O2 consumption remain constant, mixed venous O2 is an indicator of cardiac output.