Veins are vessels designed to collect and return blood, including deoxygenated hemoglobin, from tissues to the heart. Veins and the venous vascular system can be divided in to three separate systems depending on anatomical relationships and function. Initially, veins can be divided into systemic and pulmonary systems. The veins that drain the heart, comprising the coronary venous system, may be described as an independent venous system, or be considered a subset of the systemic vascular system. The systemic veins transport venous blood--deoxygenated when compared with arterial blood--from the body to the heart. The pulmonary veins return freshly oxygenated blood from the lungs to the heart so that it may be pumped into the systemic arterial system.

Veins can also be described by their anatomical position. Deep veins run in organs or connective tissues that support organs, muscle, or bone. Superficial veins are those that drain the outer skin...