Overview of the model used to investigate the effects of a multistep O₂ supply chain.

(A) The model tracks the concentrations of O₂ as well as unbound and bound O₂ transporting proteins (“Hx,” “HxO”) in 4 compartments representing external and internal volumes of water or body fluid, which are connected through a linear O₂ supply chain with external ventilation, diffusion, and internal circulation. (B) A model run at a single temperature resembles a closed system respirometry experiment. The saturation of O₂ (solid colored) and proportion of HxO (dashed colored) decline in all compartments until the O₂ level in the metabolizing tissue (dark red) reaches a critical limit near zero, at which point metabolic consumption slows down and P_crit (dashed black) can be determined from the rate of environmental O₂ depletion. (C) Effects of increasing the concentration, half-saturation pressure (P₅₀), or temperature sensitivity (ΔH) of O₂ transport protein on the P_crit curve. (D) Effects of increasing the rate coefficients of biophysical supply and demand processes. A higher metabolism elevates the curve, while increasing the rate of any supply process lowers it. No experimental data underlie this figure. Code underlying this figure can be found in https://doi.org/10.6084/m9.figshare.24547255.