All of the veins connect to the vena cava, which is the entry for deoxygenated blood into the heart. For our purposes, we will treat this as the closed system under consideration. This means that we are going to ignore the pulmonary subsystem, which pumps deoxygenated blood from the heart to the lungs, and re-oxygenated blood back to the heart for entry into the system we described above. Since we think of pressure as an energy density, this is really just a way of saying that the energy of the blood is equal to the work done against dissipation. All arteries connect to the aorta at one end and to arterioles on the other.
All arterioles connect to an artery on one end and to capillaries on the other.
All venules connect to capillaries on one end and a vein on the other. And all veins connect to the vena cava. This is a gross simplification of the actual system, but it is a necessary one if we are to be able to treat the system at all quantitatively. There is another pressure variation within the body which occurs when one part of the body is at a different elevation than another. We will usually ignore the hydrostatic pressure, by assuming that our patients are lying down.