Grossly, the venous system is composed of venules and small and great veins, which serve to return blood from tissues to the heart (see the image below). The systemic venous system brings deoxygenated blood from tissues and organs back to the right atrium of the heart, whereas the pulmonary venous system brings oxygenated blood from the pulmonary circulation back to the left atrium of the heart.
Venous drainage, anterior view.
The systemic venous channels are further classified as superficial veins, deep veins, or venous sinuses. The superficial, or cutaneous, veins reside just beneath the surface of the skin. They channel blood from cutaneous tissues to deep veins via perforations in the deep fascia. Most of the deep veins share routes with the arteries, and many are enclosed in the same sheaths. Venous sinuses are present only within the skull.