The blood vessels

There are three types of blood vessels; arteries, veins and capillaries. Arteries are blood vessels which carry blood away from the heart. Arteries deliver oxygenated blood except the pulmonary artery. The walls of arteries are much thicker and stronger than those of veins because they have to withstand the high pressure of blood coming from the heart. Veins are blood vessels which carry blood back to the heart. Veins deliver deoxygenated blood except pulmonary veins. Veins are thinner walls than arteries because blood pressure in the veins is much lower than in the arteries. Since the blood in the veins may be moving against gravity, they contain valve to prevent the blood from flowing back. Capillaries are tiny blood vessels which connect arteries and veins. They have walls which are only one cell thick. Oxygen and nutrients move from the blood through the capillary walls into the body cells. At the same time, carbon dioxide and other waste products move from the cells into the blood.

Source: http://academic.kellogg.edu/herbrandsonc/bio201_mckinley/f23-1_walls_of_an_arter_c.jpg