Fig. 5: Biophysical CO₂ exchange as a function of buoyancy-driven convection.

Biophysical simulations of respirometric efficiency of human metabolic gas exchange can account for temperature and gravity as physical factors in human respiration. The graph (A) shows net exhaled CO₂ under constant 22 °C temperature and variable gravity (blue plot, closed circles), as well as net exhaled CO₂ under 1 g terrestrial conditions combined with variable temperatures (red plot, closed squares). The cumulative CO₂ produced and released was calculated using our biophysical BTC-HTBP approach (as laid out in Fig. 2). We also include comparative data based on our CFD simulations, related to indirect biophysical diffusion (indirect biophysical diffusion solid lines vs. RANS dashed lines; see “Methods”). All results were compared to theoretical maximal levels with no physical inefficiencies (maximum exhaled CO₂). HTBP Schlieren-type simulation images depicting varying thermal conditions (22 °C to 37 °C) at constant 1 g simulation are shown (B). Temperature profiles of the HTBP (B) and y-velocity (C) visualize the decrease in the integrity of the HTBP at gravitational minimums for BTC. The plume structure is deflected towards the face as increasing air temperatures decrease the buoyancy forces associated with the temperature gradient. The standing human profile trends warmer from the feet and legs (30–33 °C) up to maximal body surface temperatures (35–37 °C) around the head, neck and chest (see CFD/indirect biophysical diffusion in “Methods”). Based on the predictions of the model (A), combinations of low gravity and high temperatures are particularly problematic, leading to potentially catastrophic conditions in spaceflight. The clip art images of the Moon and Mars are adapted from the part of Fig. 1 that was originally designed by D.M. Porterfield (2026) using BioRender.