The study of blood vessels, i.e., veins and arteries, and of lymphatic vessels, as well as the related physiological regulatory mechanisms, is known as angiology. The network of blood vessels constitutes a closed system of ducts that transports blood to the body tissues and from tissues back to the heart. This vascular system consists of both the arterial and venous systems. The blood flows out of the heart through the arterial system and returns to the heart through the venous system. Part of the interstitial or extra cellular fluid also penetrates the tiny capillary veins, and is thus carried to the vascular system.

The main cause of blood circulation is the cardiac systole followed by the contraction of the arterial walls, known as diastolic recoil, as well as the compression of the veins by the skeletal muscles during body movement and the negative intra-thoracic pressure caused by...