Due to the lower pressure in the venous system and the fact that most blood is trying to return back to the heart against the force of gravity, veins have valves that prevent blood from moving backwards. Veins do not contain thick, muscular walls like arteries do. If venous walls were thick like arterial walls, there would be more resistance for blood flowing into veins. Since blood in the venous side is already under a lower pressure, this would create a problem for blood to return to the heart. Arteries have thick muscular walls due to the higher pressure of blood flow in the arterial side of circulation. Veins carry blood that is leaving tissue after it has unloaded oxygen, therefore they carry oxygen-depleted blood, along with metabolic waste back towards the heart. Venous blood carries oxygen-deficient blood back to the heart. Arteries carry oxygen-rich blood from the heart to tissues where oxygen can be delivered and used for cellular respiration. Due to the higher pressure in which arterial blood must flow, arteries have thicker walls and smaller lumen, or diameter, as compared to veins. Veins are often considered to be capacitance vessels as they carry a larger volume of blood, and have thinner walls, and greater wall compliance to accommodate the higher volume of blood.