This is due to the fact that the left ventricle works the hardest. It must force blood throughout the body. When the left ventricle contracts, blood is forced into the aorta. The aorta is the main artery leaving the heart. The oxygenated blood is now forced throughout the body through a series of arteries that gradually become smaller and smaller.
Blood flows from arteries into arterioles.
From arterioles into capillaries. At this point the blood is able to make close contact with individual cells. Here is were waste products are picked up and oxygen is delivered. The blood now starts its return trip to the heart. From the capillaries the blood flows into venules.