The heart pumps blood throughout the body. The blood serves as the medium to transport oxygen, carbon dioxide, food, and other substances to the various parts of the body.
The human circulatory system (and also that of many animals) is a “double circulatory system”. One circuit goes to the lungs, where the blood is able to get oxygen and release carbon dioxide. The second circuit goes to the other parts of the body – where the blood will release and supply oxygen to these parts.
Questions typically show a diagram similar to leftmost diagram, although a more accurate diagram would be the one on the right.
That is, blood from the lungs (A) go through the heart to the body (B), gets returned to the heart (D) which is then sent back to the lungs (C).
A and B would have the highest level of oxygen and the lowest level of carbon dioxide, since the blood flow comes straight from the lungs, while C and D will have the lowest level of oxygen and the highest level of carbon dioxide.