There are three types of blood vessel in the human body
arteries, veins and capillaries
Arteries carry blood away from the heart. The largest is the aorta. The circulation to all the body except the lungs is called the systemic circulation and carries oxygenated blood. The circulation to the lungs is called the pulmonary circulation and carries deoxygenated blood.
Veins are large blood vessels which carry blood back to the heart. Systemic veins carry deoxygenated blood. The largest veins are the superior and inferior vena cava, which return blood to the heart from the upper body and lower body respectively. Veins have a system of valves to prevent back-flow.
Capillaries are the smallest blood vessels. The average diameter is eight micrometers with a very thin wall of 0.2 micrometers. The body has approximately 100,000 km. of capillaries. They are the site for exchange of gases, nutrients and waste between circulation and body tissues.
