Why does venous PO2 decrease in anemic hypoxia? Shouldn't it be constant as PO2 is a measure of the dissolved oxygen?
Assuming oxygen consumption remains constant, getting the same amount of O2 from less haemoglobin drops the venous spO2 even further than usual. O2 attached to Hb and dissolved in water are in equilibrium, so lower spO2 = lower pO2.
This principle is sometimes used in critical illness, where mixed venous spO2 from the pulmonary artery is compared to arterial spO2 to give a measure of O2 extraction.