– Mb binds O2 much stronger than Hb
– Mb concentrations are much lower than Hb concentrations.
The NIRS method is mostly applied to obtain information about the amount
and oxygenation
of hemoglobin (Hb). The
oxygenated form is here indicated as O2Hb, the de(s)oxygenated form as
HHb. The other substances are not oxygen sensitive so NIRS has ways to disregard
these. However, that is not true for red muscle where the oxygen-binding molecule
myoglobin (Mb) is present, with infrared properties very similar to Hb,
too similar to be discerned.
Yet, often Mb is neglected because:
– Mb binds O2 much stronger than Hb
– Mb concentrations are much lower than Hb concentrations.
But both arguments have failures. Because of the different binding curves, Mb saturation is even lower than for Hb at high oxygen level PO2. Then, muscle tissue PO2 can be very low, so Mb significantly deoxygenated. And Hb is only present in red blood cells, a small fraction of all tissue. The figure shows calculations of overall concentrations c for human skeletal muscle at various levels of work. O2Mb is oxymyoglobin, HMb de(s)oxymyoglobin and clearly these cannot be neglected.