The study of the properties of the blood flow is called hemorheology. The circulatory system is often seen to comprise two separate systems: the cardiovascular system, which distributes blood, and the lymphatic system, which circulates lymph. The passage of lymph for example takes much longer than that of blood. Blood is a fluid consisting of plasma, red blood cells, white blood cells, and platelets that is circulated by the heart through the vertebrate vascular system, carrying oxygen and nutrients to and waste materials away from all body tissues.