Figure 7.

Schematic diagram of a four-region, axially distributed, blood–tissue exchange (BTEX) model for advection, diffusion, permeation, reaction, and binding of oxygen (O₂), carbon dioxide (CO₂), bicarbonate (HCO₃⁻), and hydrogen ions (H⁺) in the heart. Endothelial cells region in the capillary membrane were ignored. Detailed kinetics of the binding of O₂ and CO₂ to hemoglobin (Hb) in blood (nonlinear O₂-CO₂ interactions) and the binding of O₂ to myoglobin (Mb) in the parenchymal cells were considered. For the four species, there are four four-region models in parallel. Consumption of O₂ and production of CO₂ occurred only in the parenchymal cells, ignoring O₂ utilization elsewhere. Detailed cellular metabolism leading to consumption of O₂ and production of CO₂ was ignored. The rates of CO₂ production and O₂ consumption were phenomenologically related to respiratory quotient (RQ) which was set 1.0 for glucose metabolism, 0.8 for fat metabolism, and 0.7 for protein metabolism, averaging about 0.8 in normal physiological conditions. F denotes blood flow, V denotes anatomical volumes, V′ denotes the effective water spaces (including binding spaces), PS denotes permeability surface area product, D denotes the effective axial diffusion (dispersion), and G denotes the rate of O₂ consumption and CO₂ production; rbc: red blood cells, pl: plasma, isf: interstitial fluid, pc: parenchymal cells. (The figure is taken from Dash and Bassingthwaighte⁴⁴ with permission.)