The muscular system is important for blood and the entire circulatory system. While the heart is usually included as an organ of the circulatory system, it is important to remember that the heart is a muscle. It is made of special muscle tissue called cardiac muscle, and without the heart's actions, blood would not flow through the body.
Muscle tissue is also a key component of veins and arteries. Arteries carry blood away from the heart, so they must be able to withstand the high pressures that are created when the heart contracts. A major reason why arteries can withstand those pressures is because they have a layer of relatively thick muscle tissue. The muscle is also important because it can constrict the arteries to limit blood flow to various parts of the body.
Veins also have a layer of muscle, but the layer is not as thick as the layer in the arteries. This is because the veins do not have to withstand such high pressures. As powerful as the heart is, it is not powerful enough to pump blood all the way back to itself; therefore, the veins need help getting blood back to the heart. The veins contain one-way valves that prevent blood from flowing backward, and the surrounding muscles naturally squeeze the veins from normal bodily movements. The result is that the blood is constantly being squeezed back toward the heart.