Human blood vessels. Historical artwork of the blood vessels in a human body. The outlines of the arms and legs are clearly seen, and the main body artery (the aorta) runs down centre. The heart is not shown. The blood vessels spread out at top is the extensive network that supplies blood to the head and the brain. The mass of blood vessels in the abdomen show the numerous blood vessels needed to supply blood to organs like the liver, kidneys, stomach and intestines. The thickness of the blood vessels varies from the centimetres-thick aorta to the microscopic capillaries. The entire volume of blood in an adult human is approximately 5 litres.