During exercise, the oxygen diffusion capacity increases dramatically because transit time through the pulmonary capillaries is much shorter reduced venous return leads to decreased pulmonary capillary flow rate lower venous O2 content increases the PO2 gradient oxygen diffusion capacity is no longer calculated using capillary mean PO2

Lower venous O₂ content increases the PO₂ gradient.

Explanation:

When we perform rigorous activities like exercise, more energy is utilized by the muscles. For the production of this extra energy, our cells need more oxygen to perform aerobic respiration and generate more number of ATP to fulfill the demand of body cells.

As a result the oxygen that is coming in the arteries are taken up rapidly by the cells and thus the oxygen content in the venous blood decreases. Due to the low amount of oxygen in venous blood the PO₂ of oxygen decreases in the venous blood and PO₂ gradient gets increased there.