Video: Microanatomy of Blood Vessels

Did you know that if you took all the blood vessels in our body and laid them out end to end, they would measure about one hundred thousand kilometers? That's enough to wrap around the earth two-and-a-half times! But what exactly does it take to make a blood vessel? Let's find out now as we explore the microanatomy of blood vessels.

In this tutorial, we'll learn all about the microanatomy of the two major types of blood vessels - arteries, and veins. When talking about these vessels, we'll pay particular attention to their wall structure and thickness. We will see that both arteries and veins share a common structural arrangement formed by three layers - the tunica intima, the tunica media, and the tunica externa. We'll investigate how their structure is related to their function. We will also, of course, compare the composition of these layers in both types of vessels. Once we've covered all of that, we'll conclude as always with a clinical note.

But firstly, what is the circulatory system? Before we jump right into the deep end, however, let's have a little recap on the circulatory system. The circulatory system is your very own biological delivery service. It functions to deliver oxygen, nutrients, and hormones to cells across the entire body whilst simultaneously removing any unwanted waste products to other parts of the body for disposal.

Through a network of arteries and veins of varying sizes which are interconnected by capillaries, blood is carried around to all the tissues in the body. Arteries are defined as vessels bringing blood away from the heart. In the systemic circuit, arteries carry oxygenated nutrient-rich blood towards various organs and tissues of the body. The exception to this is the pulmonary circulation where arteries carry deoxygenated blood from the heart to the lungs. There are three types of arteries in the body - elastic arteries, muscular arteries, and arterioles.

Veins, in contrast, carry blood away from the rest of the body and towards the heart. One exception is the portal vein which carries blood from the gastrointestinal tract to the liver. In the systemic circuit, veins carry deoxygenated blood while in the pulmonary circulation, pulmonary veins carry oxygenated blood from the lungs to the heart. Their structure is not as varied as that of arteries and they are generally just categorized by size into small, medium, and large veins.

Except for their smallest examples, all arteries and veins consist of three concentric layers called tunicae. From innermost to outermost, they are the tunica intima, the tunica media, and finally, the tunica externa which you can see in these illustrations.

Now that we've covered the basics, let's look at the first layer of blood vessels - the tunica intima.

The tunica intima is the innermost and thinnest layer of a blood vessel and is in direct contact with the blood circulating in the lumen of the vessel. Despite being the thinnest layer, the tunica intima is perhaps the most complex of the three tunicae and actually consists of four components - an epithelial layer specific to blood vessels known as endothelium, a basement membrane, a subendothelial connective tissue layer, and finally, the internal elastic membrane.

Let's talk about them in a little more detail. We're starting with the endothelium, which is a layer of simple squamous epithelium that extends continuously over the luminal surface of all blood vessels. The cells are oriented so that their long axes are parallel to the direction of the blood flow which is optimal for the hemodynamics within the vessel lumen. The endothelium is responsible for a variety of blood vessel functions. It helps to regulate the diffusion of substances through the vessel wall, meaning it helps to control what enters and leaves the blood. The endothelium also plays an important role in the control of blood flow by secretion of vasoactive molecules, and in certain locations, can detect changes in blood pressure. It also plays a role in blood clotting in the event of an injury. The endothelium rests upon the basement membrane which functions as a foundation for the overlying epithelial cells and binds the endothelium to the underlying subendothelial connective tissue layer.

The subendothelial layer is a region of loose connective tissue which is usually very thin except in the case of the largest vessels. It consists of a fibrocollagenous extracellular matrix, some fibroblasts, and varying amounts of elastic and smooth muscle fibers depending on the type of vessel. For example, the density of elastic and smooth muscle fibers in elastic arteries is higher compared to smaller vessels. The subendothelial layer allows the tunica intima as a whole to move independently from other tunicae as larger arteries expand with the increase in systolic blood pressure. The subendothelium of arteries and sometimes larger veins is separated from the tunica media by a fenestrated layer known as the internal elastic membrane. The fenestrations allow diffusion of substances from the bloodstream into the deeper layers of vessels while the elastic fibers provide structure to arteries and help them recoil from distension.

The images of the artery and vein we have here are simplified representations of large arteries and veins. While they do not represent any one specific type of vessel, they still represent the main differences between arteries and veins. Let's take a look at the differences in the tunica intima.

Composition-wise, the two are very similar; however, the tunica intima is somewhat thicker in arteries and appears wavy in cross-section while the tunica intima of a vein is thinner and smoother. If you look at our illustrations, you'll also be able to identify a major difference between the tunica intima of arteries and veins, and that is the presence of valves. These are most commonly found in veins of the lower limbs and are formed by a pair of cusps which are extensions of the tunica intima of the vein. They are lined with endothelial cells, similar to that on the vessel wall and present a luminal surface which faces the opposite cusp and a parietal surface which faces the vessel wall. In arteries, blood pressure is relatively high which ensures a unidirectional flow. However, in veins, the pressure is much lower and these valves prevent the backflow of blood.

There are some differences in the internal elastic membrane, too. This layer varies in thickness. It is not easily identified in elastic arteries because they already contain similar elastic sheets in the tunica media. It is easily distinguished in muscular and medium arteries but becomes thinner in small arteries then often absent in arterioles. It is absent in capillaries and is very thin in veins.

Alright, now that we've covered the tunica intima, let's move on to the tunica media. The tunica media is the middle layer which makes up the wall of the blood vessels. Let's talk a little bit now about its composition. The main and the most obvious element in the tunica media is the smooth muscle. If we look closer, you can see that the smooth muscle fibers are generally arranged into circular layers. Contraction and relaxation of the tunica media decreases and increases the diameter of the vessel lumen, respectively. In the case of arteries, vasoconstriction causes the lumen to narrow decreasing the blood flow which in turn increases blood pressure. This function is important for restricting blood flow to specific areas such as in injuries to reduce blood loss or to reduce heat loss through skin. Vasodilation, on the other hand, allows the lumen to widen, blood flow to increase, and blood pressure to drop. Contraction of the muscle fibers of the tunica media is controlled by what's known as nervi vasorum which literally means nerves of the vessel. These run within the walls of the vessels and are mostly sympathetic fibers which cause vasodilation or vasoconstriction in specific circumstances.

When compared, the arterial tunica media is generally the thickest layer of the arterial wall and consists of many smooth muscle layers. In veins, it is much thinner with a less smooth muscle and contains a high proportion of collagenous fibers. Veins don't constrict to the same extent which is why they don't need as much smooth muscle. In elastic arteries, these layers are separated by fenestrated sheets of elastic connective tissue which function as a sort of elastic reservoir that propels blood forward and maintains organ perfusion during ventricular relaxation. This is known as elastic recoil or the Windkessel effect. In larger arteries, the tunica media is separated from our next layer, the tunica externa, by the external elastic lamina, also known as the external elastic membrane. It is not usually seen in smaller arteries nor is it found in veins.

Alright, let's move on to our outermost vessel layer - the tunica externa. The tunica externa, also known as the tunica adventitia, is similar in both arteries and veins and is composed of connective tissue which functions to provide protection for the vessel. The longitudinally arranged collagen fibers of the tunica externa also help to prevent the vessel from overdistension as well as helping to anchor the vessel to surrounding tissues.

Also associated with the tunica externa is the vasa vasorum. The vasa vasorum are tiny blood vessels which provide nourishment and remove waste from the cells of the outer layers of the blood vessels which are too large to obtain nutrients by diffusion from luminal blood. In larger veins, the vasa vasorum will penetrate more deeply into the tunica media because of the lower concentration of oxygen which flows through the veins.

Now let's take a quick look at how the tunica externa differs between arteries and veins. In arteries, it is usually thinner than the tunica media except in the case of the largest arteries. It also has elastic fibers present within its makeup. In veins, however, the tunica externa is usually the most developed layer of the vessel wall and doesn't bear large amounts of elastic fibers.

Alright, you're now a pro at the vessel layers and how they differ between arteries and veins. But there's one thing still left to do, have a look at the clinical significance.

An aneurysm is the disorder of the circulatory system caused by a localized weak spot in the blood vessel wall. Aneurysms can occur in any blood vessel and although the exact causes are unclear, it may be influenced by hereditary factors or certain acquired diseases.

An aneurysm can be classified by location. For example, a renal aneurysm occurs in the renal artery or vein, and also by type. A fusiform aneurysm bulges from all sides of the vessel while a saccular aneurysm bulges from one side only. An aneurysm affects all three tunicae of an artery.

A lot of aneurysms are asymptomatic unless they burst or compress important surrounding structures. If an aneurysm burst, a classic symptom is hypovolemic shock caused by the reduction of blood volume in the circulatory system. This can lead to ischemia of vital organs and death if not treated rapidly. If the aneurysm is identified before it bursts, it may not need immediate treatment and the watch and wait technique.

Surgical treatment typically involves fixing a stent to the affected artery so that blood bypasses the aneurysm. Once burst, the only treatment is emergency surgery to stop the bleeding and seal the affected artery.

Now before you run off, let's just recap what we learned today. We started by discussing the three tunicae which formed the walls of the arteries and veins.

First, we looked at the tunica intima which consists of an endothelium, basement membrane, and subendothelial layer. We saw that it was thicker in arteries and had a wavy appearance. In veins, the tunica intima can be reflected into the vessel lumen to form valves.

The tunica media characterized by smooth muscle layers separated by elastic laminae and type three collagen was much thicker in arteries.

Finally, we looked at the tunica externa which is characterized by large amounts of type one collagen, vasa vasorum, and nervi vasorum. It was slightly thicker in arteries.

We also discussed the internal elastic membrane between the tunica intima and the tunica media and the external elastic membrane between the tunica media and the tunica externa. And on that note, we'll conclude this tutorial.

We've covered a lot of information so give yourself a big pat on the back. Thanks for watching and happy studying!