Final answer:
Oxygen levels are higher when they leave the heart and lower when they return to the heart in the circulatory system.
Step-by-step explanation:
In the circulatory system, oxygen levels are higher when blood leaves the heart and lower when it returns to the heart. This is because oxygen is carried by red blood cells and is delivered to the tissues, where it is used in cellular respiration. As a result, the blood that returns to the heart has less oxygen and appears more burgundy in color.