There is a network of tubes in the human body to circulate blood. This network of tubes is called blood vessels. Blood vessels are thin tubes that are attached to the heart. They are the roads on which the blood as a vehicle flows. The three major types of vessels are arteries, veins and capillaries. They all have different functions in the blood circulation process. Arteries transfer blood away from the heart, veins transfer blood towards the heart and capillaries form a capillary lake by uniting the arteries and the veins.