noun, plural: blood vessels
The blood vessels are one of the major components of the cardiovascular system. There are three major types of blood vessels: (1) arteries, (2) veins, and (3) capillaries. Other types include the venules (smaller veins) and the arterioles (smaller arteries).
The arteries and arterioles carry blood away from the heart whereas the veins and venules carry blood toward the heart. Both arteries and veins have three layers: (1) tunica intima, (2) tunica media, and (3) tunica adventitia. The tunica intima is the innermost layer of the arteries and veins. It is also the thinnest layer. The tunica media is the middle layer and the tunica adventitia is the outer layer. The arteries have thicker tunica media than the veins. The tunica media has vascular smooth muscle that contracts and relaxes thus changing the volume of the vessels as well as the local blood pressure.
The capillaries are the smallest blood vessels. The capillary wall is a single layer thick and acts as a site where chemicals (e.g. carbon dioxide, oxygen, nutrients, water, etc.) are exchanged between blood and the interstitial fluid.