Its main parts all have separate entries in this encyclopedia. General description Blood is carried throughout the body in a closed system of tubes known collectively as blood vessels. Vessels carrying blood away from the heart are called arteries . The smallest arteries are called arterioles.
Arterioles merge into very tiny tubes with thin walls called capillaries .
The walls are so thin that oxygen and nutrients pass through to reach the cells, while carbon dioxide and wastes from the cells pass through the walls into the blood. The waste-carrying blood then passes into veins , vessels that lead back to the heart. An important side trip is made when blood is pumped from the heart to the lungs to pick up fresh oxygen and to release carbon dioxide. This part of the circulatory system forms a second loop for the blood to travel through. It is not until it has made the trip to the lungs and back that blood is directed into the first loop that passes through the rest of the body.