Cardiovascular regulation depends on pressure regulation (via baroceptors) and on chemoregulation (via chemoceptors). Chemoregulation ensures adequate blood concentrations of oxygen (O2) and of carbon dioxide (CO2). The regulation of CO2 is very important for acid-base equilibrium in blood as well as tissue.
In the model we chose a single waste product called 'CO2' as simulated vector for chemoregulation. CO2 is produced in the tissues, transported by the blood to the lungs, where it is removed. For this, the model does a bookkeeping of the amount of CO2-molecules per volume in all the simulated capacitances. The concentration is graphically illustrated by changing the color of the model capacitances and resistances. In brief:
CO2-production
In the model CO2 is produced by the brain hemispheres as well as by the rest of the body. There are sliders to vary the metabolic rate expressed as the amount of CO2 molecules produced per unit of time.
The formula is: amount (mol) = slider metabolic rate (mol/s) x timestep (s)
The calculated amount is added to the molecules already present (inflow concentration) to calculate the outflow concentration.
CO2-transport
After calculating the blood flow in l/s from one capacitance to the next the amount of CO2 molecules transferred between both capacitances can be calculated from:
amount (mol) = flow (l/s) x concentration (mol/l) x timestep (s)
The volume of the receiving capacitance is increased with flow (l/s) x timestep (s) and the concentration can be calculated by adding the amount of molecules transferred to the amount of molecules already present in the receiving capacitance. Than the concentration is calculated by dividing the total amount by the total volume.
CO2-removal
The amount of CO2 removed from the blood in the lungs can be set by the slider for respiration rate. Increasing this slider will cause a more rapid variation in heart rate (see Cardiac arrhythmia) but also the refresh CO2 fraction: this is the fraction of all CO2 molecules in the inflow that is removed. In formula
amount (mol) = refresh CO2 fraction x inflow concentration (mol/l) x inflow (l/s) x timestep (s)
Then the outflow concentration can be calculated by subtracting the amount removed from the amount already present divided by the outflow.
CO2 color scale
The CO2 color scale is based upon the CO2 concentration in mol/l. It becomes bright red when the CO2 concentration is zero and bright blue when it is larger than 1.66 mmol/l (or in CO2 pressure ≥ 7.2 kPa).
