1. the cardiovascular system is an intricate organ system that allows blood to flow to and from all parts of the body carrying, among other substances, oxygenand nutrients. the system includes the heart and blood vessels: arteries, arterioles, veins, venules, and capillaries. the heart is a strong muscle about the size of your fist and contains four chambers. the two upper chambers, called atria, receive blood that was collected from the body or lungs, whereas the ventricles, the two lower chambers, pump the blood from the heart out to the lungs or the body. most arteries carry oxygenated blood from the heart to the rest of the body. the exceptions are the pulmonary arteries that carry deoxygenated blood from the heart to the lungs, where the blood becomes oxygenated. the four chambers beat in an orchestrated manner due to an electrical impulse that, when traveling across the heart muscle, gives a signal that causes the orderly contraction of the chambers. this signal starts in the , which functions as a natural pacemaker for the heart