What factors influence diffusion of oxygen?

Oxygen has been utilised in aerobic respiration as venous blood that returns to the heart has a PO2 of 40mmHg. A blood passes through the capillary bed, partial pressure of oxygen substantially increases highlighting a reoxygenation of the blood, due to the partial pressure gradient. Eventually there is an equilibrium and the oxygenate arterial blood returns to the left atrium and is ejected by the left ventricle.