Q. You are conducting a physiology experiment wherein you have individuals run on a treadmill. A closed-loop spirometry is attached to a face-mask to measure the participant's oxygen consumption rate. Arterial and venous blood oxygen content assessment is done just after the start of exercise and after 45 minutes -- the treadmill speed is increased during the exercise prior to the evaluation at 45 minutes. Which of the following factor(s) is/are responsible for maintaining an adequate ratio between the rate of oxygen consumption and the blood oxygen content at the assessment 45 minutes into the experiment?