The reaction of hemoglobin (Hb) with oxygen (O2) is mostly considered in an equilibrium state, where it is expressed as saturation S. A step further is calculation of the rate at which oxygen is released from the molecule. The reaction of O2 with Hb is quite complicated but can be formally expressed as Hb+O2⇄O2Hb with forward rate k' and backward rate k. So, in equilibrium k[O2Hb]=k'*[O2][Hb], in saturation terms kS=k'PO2(1−S) (1). The k turns out to be approximately constant so that k' is easily calculated from oxygen pressure PO2 and saturation S:
| k' = k | S |
| PO2(1−S) |
However, in capillary exchange this rate is insignificant. There, the problem is to get the O2 from inside the red blood cell to just outside the capillary. For the surrounding tissue, this looks as if the PO2 in the blood is lower than it actually is, and the difference is called Extraction Pressure (EP). It can be several kPa and is not easily calculated; for more information consult the literature.
Both reaction rates k, k' can be evaluated from the MWC model if that model is extended with reaction rates for the individual states T and R – see this document for explanation and for the complete set of formulas. E.g., for k – a suffix T or R is for T- or R-state:
| k = | L4kT{1 + 1/(KTP)}3 + kR{1 + 1/(KRP)}3 |
| L4{1 + (1/KTP)}3 + {1 + (1/KRP)}3 |