I am getting confused on what really triggers erythropoietin release from kidney, hypoxemia (decrease of partial pressure of O2)or hypoxia (decrease of O2 delivery to tissue)? In the setting of anemia, the delivery of O2 to tissue is decreased due to decreased hemoglobin but PO2 is normal, while in hypoxemia, it causes a decrease in both PO2 and delivery of O2 to tissue even though hemoglobin is normal or even higher as in TOF. It seems to me that both conditions lead to defective O2 supply to tissue so they both cause an increased release of erythropoietin from kidney.

You ARE Correct ... ultimately both conditions result in decreased O2 delivery to kidney causing increased Epo secretions. Thats why we get secondary polycythemia in cyanotic heart disease, high altitude living, heavy smoking etc...

Just try to understand the concept that hypoxia is a broad sense term, with the hypoxemia coming under it a sub class. i would recommend you to listen to the goljan respiratory part..i think its in the start of that respiratory lecture..it ll clear all your concepts regarding the hypoxia and the hypoxemia..try it

The answer should be Hypoxemia and not hypoxia because hypoxia is not a definitive term in general(Read definition/meaning of hypoxia from Taber's if u need.). Think something like if you get gangrene in a toe(ultimate cause is hypoxia if u dig enough) due to some arteritis, it will never stimulate kidney to release epo! But hypoxemia is a more definitive term, and it may cause or result from hypoxia(How? please think yourself)

The only stimulus to release epo is decreased O2 carrying capacity of blood. Causes are:
In general, anything that causes left shift(of what? I hope u know)
Definitive classic causes:
1) Hypoxemia
2) Severe anaemia(I am a old school british type person)
3) High altitude