A blood vessel is a structure that is part of the circulatory system. Blood vessels carry blood to and from all of the parts of the body bringing nutrients and oxygen and removing waste and carbon dioxide. There are generally considered to be three types of blood vessels, arteries, veins, and capillaries.
Arteries carry blood away from the heart. The blood in arteries is higher pressure than the blood in veins. Arteries usually carry red, oxygenated blood although there is one exception. The pulmonary arteries carry deoxygenated blood from the heart to the lungs.
Veins return blood towards the heart. The blood in veins is low pressure. Veins usually carry deoxygenated blood. The pulmonary veins which carry blood from the lungs back to the heart are an exception.
Capillaries are the tiny blood vessels that transfer oxygen and nutrients to the cells of the body. Here the walls of the blood vessels can be as thin as one cell thickness.