The closed circulatory system consists of a heart that serves as the pump for circulation, as well as a network of arteries and veins of varied diameters that form an enormous web throughout the body. Nutrients and oxygen are delivered to individual cells through the blood vessel walls, while cellular wastes are taken up and removed from the body. The heart delivers its pulse twice per minute, providing the only force necessary for circulation. Blood flows through the vascular system in order to supply organs with nutrients and remove waste products. The main type of movement involved in blood flow is called "blood flow". Heart muscle cells are able to expand and contract which causes blood vessels to constrict or dilate. This process is known as "vasoconstriction" and "vasodilation". Constricted blood vessels increase the pressure inside them and cause blood to flow toward them. Dilated blood vessels allow more blood to flow through them. A "closed" cardiovascular system prevents contamination of the venous blood by bacteria from the intestinal tract. 2 by the presence of valves in the great veins that prevent backflow of bacteria-laden blood into those vessels. The lungs also play a role by filtering bacteria out of the blood stream.