A common analogy when talking about the blood vessels is a system on pipe where blood circulates. Understanding more deeply these structures, one can infer that is a dynamic structure that’s able to proliferate, constrict, relax and pulsate.
There are three major types of blood vessels, namely the arteries, capillaries and veins. When the heart contracts, it pushes the blood out through the arteries that leaves the ventricle. The blood traverses the vessel system, reaching the smallest arteries called arterioles. The arterioles meet with the capillary beds of the tissues and organs, delivering the much needed nutrients and oxygen. The deoxygenated blood then drains back to the capillaries and into the venules or the smallest veins. The blood travels from the venules to the larger veins that empty into the heart. It is mind blowing to think that when the blood vessels in our body are stretched, it will cover 100,000 km.
Structure of Blood Vessel Walls
Except for the smallest vessels, all of the blood vessels in the body have three tunics or coverings. The tunic covers the vessel’s lumen, which is the central blood-containing space.
The innermost layer or tunic is always in intimate contact with the blood, hence the name tunica intima. This layer contains the endothelium, a simple squamous epithelium that is present in all the vessel’s lumen. The endothelium found in the layers of the blood vessels is a continuation of the endocardial lining of the heart. There is less friction within the tunica intima since the flat cells are fitted closely together, forming a slick surface.
The tunica media is the middle layer of the vessel walls, made up mostly of circular smooth muscles and a sheet of elastin. The tunica media is the one who enacts the signals from the autonomic nervous system, which orders it to either vasoconstrict or vasodilate depending on the needs of the body. During vasoconstriction, the smooth muscle contract, reducing the diameter of the lumen. On the other hand, as the smooth muscle relaxes, the diameter of the lumen increases. The changes that happen in the tunica media greatly influences blood flow and blood pressure. With this function on hand, the tunica media is the bulkiest layer in the arteries, which bears the primary responsibility for maintaining blood pressure and ensuring continuous blood circulation.
The tunica externa is the outermost layer of the blood vessel and was formerly called the tunica adventitia. This layer is composed of collagen fibers. This loosely woven collagen fibers function to reinforce and protect the vessel walls. At the same time, it also anchors the vessel to its surrounding structures. The tunica externa is infiltrated with lymphatic vessels and nerve fibers and in some really large veins, elastin fibers can be found.
In really large blood vessels, the vasa vasorum or a system of tiny blood vessels exists. They serve to deliver nutrients to the more external tissues of the vessel walls.