Fig. 3: The BTC-HTBP system is altered under microgravity exposure, leading to diminished CO₂ gas exchange and respiratory efficiency.

The full simulations of [CO₂] dynamics are available to view as video files (Movie S2). Under 1 g conditions (A and insert), when candle flame morphology is nominal, airflow is following the HTBP during numerous cycles and various stages (i, ii, iii, iv) of ventilatory breathing. In contrast, the microgravity environment (0 g, B) is associated with inhibition of buoyancy-driven flow, resulting in a diffusion-only environment that limits combustion and mass transport, and that physically constrains and deforms the morphology of a burning candle. (B, insert). The flow patterns of exhaled breath in microgravity resemble the morphology of the microgravity candle flame, acting like a “trapped bubble” at the source (B). The graph (C) shows the cumulative CO₂ released, normalized by the potential maximum exhaled, during a normal gravitation-exposed respiratory cycle (solid red line plot) and CO₂ release under microgravity conditions (solid blue line plot) as a result of CO₂-rebreathing in microgravity. Also shown is the tracking of the concentration of CO₂ at the mouth during the inhalation phase of the ventilatory cycle in microgravity (blue triangle, dashed plot), which is significantly higher than under 1 g conditions (red circle, dashed plot). This difference can be appreciated from the beginning of the respiratory cycle (i and ii) where these plotted profiles show that fairly low CO₂ exposure levels are favored in the 1 g environment as compared to the microgravity simulations, especially during peak levels of CO₂-rebreathing. At times the measured CO₂ concentrations are roughly equivalent (iii) but overall trends reveal both average and peak (iv) CO₂ are functionally elevated in microgravity. The plots of net exhaled CO₂ release (C) indicate that microgravity exposure (blue line plot) is associated with biophysical CO₂-rebreathing in spaceflight when compared to 1 g conditions (red line plot). The inset candle flame images were modified from²⁵.