From an arteriole, the blood enters one or more capillaries. We will ignore this detail. For our purposes, the only distinguishing characteristic of a venule is its relative size. It is larger than a capillary, and smaller than a vein. In reality, the subsystem of arterioles, capillaries and venules forms a complicated mesh of interconnections, but we will ignore those details.
Many venules feed into one vein.
The veins have one-way valves, which prevent the flow of blood away from the heart, and the movement of the body s muscles help to pump the blood toward the heart. We will of course ignore this detail. So again, for us the only distinguishing characteristic of a vein is its size. 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.