One element of the circulatory system is the

Heart.

Explanation:

The circulatory system is the path of your blood, and your blood goes through your heart.

It is false that during exercise, the oxygen concentration in the blood increases to twice that of resting levels. As a matter of fact, the oxygen either stays the same or decreases, or it may increase slightly than it used to be when the person was resting, but it definitely does not double while exercising. Hope this helps. Let me know if you need additional help!