In its most basic form, the circulatory system is a simple loop which starts, and ends, at the heart. It is also a closed system in the sense that blood does not enter or leave the system during its journey from the heart, to the body, and back again. In such a system, a continuous flow of the same liquid can be pumped through the loop again and again.
The circulatory system is comprised of five main parts:
Circulation Begins in the Heart:
By convention, the circulatory system can be thought of as beginning in the right atrium. In the model of the heart discussed above, this is the upper right-hand chamber of the square. As blood moves through the heart, it passes through each of the four chambers (upper right, lower right, upper left, lower left), takes a quick detour to the lungs (to get rid of carbon dioxide and pick up oxygen) and ends up in the lower left-hand chamber, called the left ventricle.
In the context of pushing blood out to the body and through the circulatory system, the left ventricle is the most important chamber in the heart. It is the largest of the four chambers, and is responsible for generating the force necessary to propel the blood out into the aorta, which is first artery that blood enters as it leaves the heart.
Blood travels from the aorta through a series of smaller and smaller blood vessels until it reaches the capillaries. Before reaching the capillaries, however, blood must travel through the arterioles, where its speed and pressure are constantly adjusted as different segments of the arterioles change diameter in response to pressure and chemical sensors positioned nearby. These sensors adjust blood flow via the arterioles in response to changing conditions in the body.
Because of arteriole action, by the time blood reaches the capillaries it is no longer traveling in a pulsing fashion - blood actually flows continuously through the capillaries, it does not "squirt" and "pause" along with the beating of the heart. This continuous flow is necessary because there is a constant exchange of oxygen and nutrients happening through the walls of the capillaries. No cell in the body is very far away from a capillary.
As blood travels through the capillaries, its supply of oxygen is reduced and it acquires waste products. From the capillaries, blood enters the venules and then veins, and travels back to the heart to be refreshed and sent out once again.