Gravity and Human Respiration: Biophysical Limitations in Mass Transport and Exchange in Spaceflight Environments

A major requirement for humans is a breathable atmosphere for adequate respiratory CO2/O2 gas exchange. In microgravity, despite environmental life support systems regulating air exchange, astronauts complain about air quality, with elevated CO2-levels resulting in detrimental health and performance effects. Using high-fidelity computational fluid dynamics, we create a model of human respiratory ventilation to show how gravity biophysically shapes and drives respiratory exchange on Earth and in microgravity. On Earth, gravity influences gas exchange through buoyancy and biothermal convection, generating a ‘human thermal body plume’ that drives airflow around the human body and so facilitates effective gas exchange. We show that the absence of biothermal convection (in microgravity or high temperatures) reduces this fresh air exchange around the human body by creating environmental hypercapnia immediately in front of the face, and significant CO2-rebreathing with direct implications for astronaut health and countermeasures. The scientific model for the HTBP was used to estimate the engineering requirements for directed external airflow equivalents, as a countermeasure for IBD induced hypercapnia in spaceflight. Our model also links terrestrial hyperthermia and heat stress to hypercapnic induced cardiovascular respiratory events. Climate associated increase in ambient air temperatures can also reduce efficient respiratory exchange with implications for respiratory health on Earth, and synergistic stress effects in microgravity.