Key Parts

       The control of the circulatory system is the heart. It is the organ that pumps blood around the body. It is mainly made of five parts. They are the aorta, the atria, the ventricles, the superior/inferior vena cava, and the valves. There is a left and right atrium(singular). It simply pumps blood from the body into the heart. The left ventricle is slightly larger than the right ventricle, because the left has to pump blood out from your heart to your whole body . While the right gets ventricle gets the easy job, and only has to pump blood to the heart. The valves prevent the blood from backflowing.

Human heart
          Blood travels through blood vessels from arteries to capillaries to veins. The heart pumps the blood under pressure, but the strong blood vessels are all capable of withstanding this. The arteries' job is to carry red, oxygenated blood away from the heart. The largest artery is called the aorta. It is an artery that has a diameter of 25 mm. The arteries eventually get smaller and turn into arterioles.
     This is where the blood goes into capillaries. Capillaries are like trading stores. Oxygen, nutrients, and glucose diffuse into the walls. What also happens here, is the carbon dioxide and other wastes diffuse into the cells.
     At the end, the blood becomes deoxygenated and goes into the venules, and then the veins. Finally, through the veins, blood is pumped back to the heart.