Blood vessels, along with the heart and blood, make up part of the circulatory system. At the center of this system is the heart, which pumps blood through the body. The blood carries nutrients, oxygen, and waste products (such as carbon dioxide) to and from cells. Blood vessels can be thought of as a delivery system used by the blood and heart. Blood vessels are generally categorized into arteries, capillaries, or veins.
Arteries
Arteries are large, elastic vessels that carry blood away from the heart. Arteries have three tunics (coats) or layers. The innermost layer is called the tunica intima and is made up of a single layer of endothelial cells. The middle layer is called the tunica media, which contains some elastic fibers, but it mostly consists of smooth muscle. The thick middle layer in arteries is particularly important because when the heart contracts and blood is ejected into the arteries they are then able to expand and accept large volumes of blood. The outermost layer is called the tunic adventitia, which is an outer connective tissue sheath with small amounts of elastic tissue. Arteries work under higher blood pressure when compared to other vessels which helps drive blood around the body. An example of an artery in the human body is the femoral artery which is the primary artery of the thigh.
Capillaries
Arteries divide into smaller arteries and eventually become smaller vessels called capillaries. Capillaries are only a single layer of endothelial cells (tunica intima) thick which facilitates the exchange of oxygen, nutrients, and waste products. Capillaries form capillary beds and connect the arterial and venous side of the system.
Veins
Veins are large vessels that return blood to the heart. Groups of capillaries come together to form small veins that eventually become larger vessels called veins. Veins have the same three tunics as arteries. In veins, however, the layers are thinner and contain more fibrous tissue and less elastic tissue and smooth muscle. Veins still have a large capacity to expand and their diameters are commonly larger than corresponding arteries. A unique characteristic of veins is that they contain one-way valves that prevent the backflow of blood. These valves along with the muscular contractions in the limbs keep low pressure venous blood flowing toward the heart. The second largest vein in the human body is the superior vena cava, which returns low-oxygen blood to the heart from the upper half of the body.
References
- Clinically Oriented Anatomy (6th ed.)
- Handbook of Anatomy and Physiology
