The answer to your question is,
if the oxygen poor blood mixes back with oxygen rich blood, that means that oxygen poor blood is not going back to the lungs to swap CO2 (carbon dioxide) with O2 (oxygen), and this swapping is vital to tissue survival, since oxygen is what keeps your tissues, muscles, and organs alive. so if this blood mixed, ultimately your organs are not receiving enough oxygen and will become starved of it, this can kill you and lots of people every year get heart surgery because their atrium or ventricles in their heart rupture
-Mabel <3