Best Answer: Blood is circulated in the body on a pressure system. The Blood leaving the heart is at the highest pressure the systolic pressure (120 mmHg). This blood is oxygen rich blood and is sent to the body through arteries. As it circulates throughout the body it loses pressure, and when it reaches the capillaries, the oxygen is taken up by the various organs and is returned to the heart by the veins (oxygen poor blood is in veins).