Final answer:
During strenuous exercise, venous O₂ content decreases and the A-VO₂ difference increases due to higher oxygen utilization by the muscles and increased CO₂ production.
Step-by-step explanation:
At the onset of strenuous exercise, you would expect to see a decrease in venous O₂ content as muscles use up more oxygen. Concurrently, the arteriovenous oxygen difference (A-VO₂ difference) would increase because the tissues extract more oxygen from the blood to meet the higher energy demands.
During exercise, as oxygen is utilized by the tissues, the difference in partial pressures drives oxygen out of the capillaries and into the tissue cells, therefore the venous blood returning to the lungs has a lower Po₂ (partial pressure of oxygen), typically around 40 mm Hg compared to arterial Po₂ which is around 100 mm Hg. This results in a larger A-VO₂ difference. The Pco₂ (partial pressure of carbon dioxide), conversely, increases as tissues produce more carbon dioxide as a byproduct of metabolism.