Arteries, arterioles, capillaries, venules and veins are the five major varieties of blood vessels. Arteries convey blood away from the heart to other organs.
- While they exit the heart, large, elastic arteries divide into medium-sized muscular arteries that spread out into the several zones of the body. Afterwards medium-sized arteries divide into small arteries, which further divide into even smaller arteries called arterioles.
- When the arterioles travel inside a tissue, they divide into numerous small vessels referred to as capillaries. The thin walls of capillaries enable the exchange of substances in between the blood and body tissues
- Groups of capillaries within a tissue reunite to form small veins called venules. These in turn merge to form progressively larger blood vessels called veins. Veins are the blood vessels that communicate blood from the tissues back to the heart.
Arteries are usually further subdivided into three classes, according to the variable amounts of smooth muscle and elastic fibers contributing to the density of the tunica media, the total size of the vessel and its function:
- Large elastic arteries include substantial amounts of elastic fibers in the tunica media, allowing growth and recoil throughout the typical cardiac cycle. This assists in maintain a constant circulation of blood during diastole. Examples of large elastic arteries are the aorta, the brachiocephalic trunk, the left common carotid artery, the left subclavian artery, and the pulmonary trunk.
- Medium muscular arteries are composed of a tunica media which contains mainly smooth muscle fibers. This characteristic permits these vessels to control their diameter and control the flow of blood to different parts of the body. Examples of medium muscular arteries are most of the named arteries, consisting of the femoral, axillary, and radial arteries.
- Small arteries and arterioles control the filling of the capillaries and directly contribute to the arterial pressure in the vascular system.
The wall of an artery consists of three layers. The innermost layer, the tunica intima a.k.a. tunica interna, is simple squamous epithelium surrounded by a connective tissue basement membrane having elastic fibers. The middle layer, the tunica media, is mainly smooth muscle and is generally the thickest layer. It not only provides support for the vessel but also alters vessel diameter to control blood circulation and blood pressure. The outermost layer, which connects the vessel to the surrounding tissue, is the tunica externa or tunica adventitia. This layer is connective tissue with varying amounts of elastic and collagenous fibers. The connective tissue in this particular layer is quite thick where it joins the tunic media; however it changes to loose connective tissue around the border of the vessel.