Fig. 4: Biophysically relevant patterns of respiratory airflow and the HTBP in microgravity.

HTBP flow patterns are driven by buoyancy-driven convection and are gravity dependent, just like a candle flame (A). Time points of maximal inspiratory and expiratory velocity (B) are depicted in (C–F). Note how peak inhale/exhale patterns of velocity magnitude (C and D) and projected air velocity (E, F) change due to gravitational forces. The impact of biothermal convective airflow and the HTBP is significant in terms of limiting and restricting buoyant forces. Our results show how the HTBP perfectly barters the efficient exchange of respiratory O₂/CO₂, which is associated with mitochondrial metabolism. On Earth, being exposed to normal gravity (C and E), a prominent white-banded envelope of airflow (E) is formed at the base of the chin and directed upwards and away from the face at approximately a 45° angle. Under 1 g conditions, this white-banded envelope separates two high velocity volumes of moving air, as indicated by the dark blue and red masses (E). In contrast, in microgravity (F) the prominent white-banded envelope is missing and the projected air velocities are too low to facilitate normal respiratory exchange (D). The inset candle flame images were modified from²⁵.