Blood vessels in muscles. Illustration of the arteries (red) and veins (blue) bringing blood to and from muscles in the human body. Within the muscles, smaller blood vessels known as capillaries bring blood to the tissues where it is used. The main role of blood in the muscles is to supply oxygen for cellular respiration, and to remove the waste product carbon dioxide. Muscles require a plentiful supply of oxygenated blood to replace the energy expended during exercise, otherwise an oxygen debt is created. For this illustration without labels, see image C046/9167.