What measures O2C (oxygen to see)?
During venous congestion
blood volume increases (especially in the venous system) and due to the decreased
perfusion pressure difference the blood flow decreases and with it the oxygen
Flow and hemoglobin volume decreases, so that oxygen extraction increases with constant metabolism and oxygen saturation decreases.
Oxygen extraction increases. Therefore oxygen saturation decreases. With strongly increased metabolism additionally flow is increased to serve increased O2-demand.