How much difference is there in the ability of hemoglobin to hold oxygen when comparing slightly alkaline blood to slightly acidic blood at a Po2 of 40 mm Hg?

The partial pressure of oxygen (pO2) in alveolar air is higher (~13 kPa or 100 mmHg) than that of the venous blood (~5.3 kPa or 40 mmHg) flowing on the other side of the membrane, so oxygen diffuses from the alveoli to blood.