Blood Vessels - Circulation
The blood vessels have elastic walls and a layer of smooth muscles in their walls which enable them to change their diameter. There are three main types of blood vessels - arteries, capillaries and veins with characteristic differences among them. Arteries have relatively thick walls that consist of heavy, strong layers of elastic fibres and smooth muscles. As the arteries branch, their diameter becomes smaller and the relative amount of muscle tissue increases in proportion to the elastic tissue.
Capillaries are the smallest units, they are single cell thick. Almost all exchange between tissue and blood takes place through the walls of the capillaries. The veins also contain smooth muscles and fibres.