The study of the flow of blood through the circulatory system (see circulation). Blood flow is influenced by numerous factors, depending on the type of system, for example whether it is open or closed, or single or double. In a closed system, such as that of mammals, flow velocity is determined primarily by the pumping action of the heart and the total cross-sectional area of the vessels through which the blood is being pumped. Hence, blood flows fastest through the aorta and pulmonary arteries, points in the circulation with relatively small overall cross-sectional areas, and slowest through the capillaries, which have the greatest combined cross-sectional area. Other factors affecting blood flow include blood volume and viscosity and the elasticity of blood vessels.