Written by Ma. Disa Ricafort

The circulatory system! It is the system responsible for the circulation of blood around the body. This system comprises the heart and an incredible number of vessels that carry blood to every extremity of your body.

There is a lot to discuss here. Let’s begin by having a close peek at the heart.

We often use the heart as the “symbol of love,” “seed of our soul,” or the “core of our being.” Sorry to burst your bubble, but no. That is not the heart’s business. It does not make you love. It doesn’t break apart if you get dumped.

Yet, your heart is still remarkable. It keeps you alive. It is the “engine of life.” Even if it is about the size of your fist, it can pump blood to your body via a network of vessels to bring nutrients and hormones to your cells.

Take a small part of the heart as shown in the picture and zoom it in. You’ll have a closer look at the wall of the heart. The heart wall itself has several layers. It has three of them. The epicardium is the outermost layer. The others are the myocardium and endocardium. Each of these plays a definite but different role in the body.

Let’s go through all these in more detail, starting with the endocardium.

Endocardium

As its name suggests, the endocardium is the innermost layer of the wall of the heart. This layer serves as a barrier that all the blood cells are bumping up against. It houses the heart’s conduction system, which makes your heart pump.

The endocardium has three sublayers that define its function. These are as follows:

• Endothelium

This sublayer regulates material exchange between circulation and the heart muscles. Specialized endothelial cells make up this structure. It is very similar in many ways to the inner lining of the blood vessels,

• Fibroelastic tissue layer with smooth muscle cells
• Subendocardial layer

It is the endocardium’s outermost sublayer that connects to the heart muscle. In addition to nerves and arteries, it contains fibrous collagen cells that give structure and stability. It also has Purkinje fibers that send electrical impulses to the myocardium.

Given its vital function, endocardium disorders can have serious health consequences.

The most noteworthy of them is endocarditis. It is the inflammation of the inner chambers and valves of the heart. It has two types. One of them is the infective type. It is more common and caused by bacteria or others. The other one is non-infective; you can get this type by mechanical stress or chemical agents. Whatever the cause is, this can be fatal.

Myocardium

The myocardium is the heart’s principal functional element.

It consists of muscles or muscle fibers that allow the heart to contract. But you will find a certain amount of connective tissue as well. It is the thickest of the three layers, and its thickness varies across the heart. The myocardium in the ventricles is thicker than in the atria.

This is because the myocardium in the atria has only two layers. It consists of a superficial layer of muscle fibers arranged circularly and a deep layer. The latter consists of longitudinal muscle fibers, forming the inner muscle. This inner muscle is the pectinate muscle.

Meanwhile, the ventricles have three. We have the superficial muscle layer and the longitudinal muscle layer. Between those, there is a middle layer of circular muscle fiber.

You will notice that the left ventricles have the thickest myocardium. Why is that so? The reason is that it needs more strength to give a powerful contraction, for it pumps blood to the entire body.

Cardiomyocytes make up the myocardium. These muscle cells look different but still contract like others. They are shorter with fewer nuclei than skeletal muscle cells. It also has striations, like skeletal muscle tissue.

Epicardium

It also goes by its other name, visceral pericardium.

Since its primary function is protection, it consists of the following:

• Mesothelial cells. These are the same cells as in the parietal pericardium.
• A layer of connective tissue. Elastic fibers and fatty tissue comprise this layer. It provides direct contact with the epicardium to the myocardium.

To add to its function, the epicardium aids in producing the pericardial fluid. This fluid decreases friction between the layers, helping the heart pump more smoothly.

You can also find the coronary vessels and nerves that supply the heart in this layer.

What is the histology of heart valves?

Now, let’s go into more detail about heart valves.

Heart valves. They are the one-way mini gates between your heart chambers. They open and close to enable blood to circulate. They make sure that it goes from the atria into the ventricles and not vice versa.

There are two types: atrioventricular and semilunar valves.

Atrioventricular (AV) Valves

A fibrous structure separates the atria and the ventricles into two functional units. The said structure is made from endocardium and connective tissue. Embedded within these fibrous structures are the atrioventricular valves.

Each valve consists of an aperture enclosed by a ring and two or three leaflets that extend to shut the opening.

Histologically, three distinct layers of connective tissue comprise the said leaflets. These are as follows:

• Atrialis layer – made of elastin; contained more elastic fibers than the ventricularis
• Spongiosa layer – contains sparsely cells embedded in ground substance, made of glycosaminoglycans
• Fibrosa layer – enriched with large bundles of fibrous Type I collagen

We have strands mostly made of collagen and elastin, called chordae tendineae. It is also colloquially known as the heartstrings. It anchors the leaflets to papillary muscles that keep the flaps tight. These strands prevent them from everting back into the atria when our heart is under strain from pumping.

The tricuspid and mitral valves are the right and left atrioventricular valves, respectively.

Tricuspid valve

As the name implies, it consists of three irregularly shaped cusps or flaps. The cusps comprise endocardium folds attached to the heartstrings.

Each flap has many heartstrings connected with it. It consists of around 80% collagen fibers, and the rest comprises elastic fibers and endothelium. It binds each flap to a papillary muscle extending from the inferior ventricular surface.

The leaflets are named based on the margin or the papillary muscle they are attached to. Thus, there are septal, anterior, and posterior muscles and similarly named cusps of the valves.

Mitral or bicuspid valve

It is named as such for it has two flaps. Like in the tricuspid, the bicuspid valve is anchored by the heartstrings. But it is more robust and thicker. The left ventricle requires more power to pump blood under high pressure.

Semilunar valves

They have a half-moon shape, thus the term “semilunar.” These valves are the “doorways” that prevent backflow into the heart. As the ventricles relax, the blood will flow back from the arteries and press against its cusps, forcing them to close.

There are two of these valves. On the right or the pulmonary side, there is the pulmonary valve. Meanwhile, on the aorta side, there is the aortic semilunar valve.

They share the same construction as AV valves. Yet, unlike AV valves, semilunar valves lack heartstrings and papillary muscles. Instead, these valves comprise cusps made up of endocardium supported with connective tissue.

The semilunar valves also comprise fibrosa and spongiosa layers. However, it has a ventricularis layer rather than an atrialis layer.

• Ventricularis layer – composed of radially oriented elastin with a trace of collagen
•  Spongiosa layer
• Fibrosa layer

The semilunar valves consist of the pulmonary and aortic valves. These valves separate the ventricles from the pulmonary artery and aorta.

Pulmonary valve

This valve has three leaflets, separating the right ventricle from the pulmonary artery. This functions to prevent the blood from flowing back to the right ventricle.

It is positioned in an oblique plane, pointing in a posterior and superior manner toward the left- hand side. At the origin of the pulmonary artery, the pulmonary valve’s cusps are connected to the half-moon arches of the cardiac skeleton.

Aortic valve

This valve has three leaflets: the left, the right, and the non-coronary cusp (named after its well- defined sinuses). These cusps prevent the backflow from the aorta to the left ventricle.

The aortic valve lacks a continuous collagenous ring. Instead, there are three fibrous, triangular arches. These structures act as attachment sites for the cusps.

Moreover, the aortic and mitral valves interact with one another. As left ventricle contraction happens, the mitral valve shuts; meanwhile, the aortic valve opens. This allows the blood to flow via the aorta and then out to the body parts.

What is the histology of arteries?

An artery is a large thick-walled muscular vessel distributing blood to an area. It has all three layers (also known as tunics) of blood vessels (except capillaries). These tunics surround the open space, or lumen, that holds the blood.

The three tunics or layers are described further below.

Tunica Intima

Your circulatory underwear. It comprises an endothelium and is continuous with the lining of the heart. The endothelium consists of simple squamous epithelium tissue. A delicate elastic and collagenous layer of variable thickness supports it.

Tunica Media

The middle layer surrounds the tunica intima. It consists of a layer composed of variable smooth muscle cells and elastin ratios. Nerve fibers govern the smooth muscles, allowing them to constrict or dilate. That makes the tunica media an essential part since it plays a crucial role in blood flow and pressure.

Tunica Externa

The coat of your vessels. This layer is made of loosely woven collagen fibers to protect and reinforce the whole blood vessel.

Since they are closer to the heart and receive blood flowing at a much higher pressure, arteries have thicker walls than veins. In addition, arteries have narrower lumina than veins. These narrow lumina help keep blood pressure constant as it moves through the system.

As a result, arteries appear to have a rounded appearance in cross-section cuts.

What is the histology of a vein?

There is a series of veins and venules to return blood to the heart. The veins still have the same fundamental layers: tunica externa, tunica media, and tunica intima.

But in comparison to arteries, veins have proportionately less elastic and muscular components. This is because it does not need pressure on the blood to flow. Moreover, veins have much wider lumens and thinner walls than corresponding arteries.

The layering in the venular wall is not as precise as it is in arteries. The tunica intima is relatively thin. Only the larger veins include significant subendothelial connective tissue. Internal and exterior elastic laminae are either missing or thin.

The tunica media is thinner than the tunica externa. Under the microscope, it seems that the two layers tend to blend.

Furthermore, the morphological appearance of the vein wall is also affected by its location. The walls of veins in the lower extremities usually are thicker than those in the upper parts of your body.

Veins, unlike arteries, have valves to prevent backflow against the force of gravity. Arteries do not need any valves because of the intense pressure from the heart that makes blood flow in one direction.

What is the function of arteriole?

Arterioles. These are the smaller versions of your arteries. They share a similar structure with the arteries, containing all three tunics. But as they get smaller and thinner, they end up being mostly a single layer of smooth muscle surrounding endothelial cells.

They regulate the flow from those high-pressured arteries into the tiny capillaries. They dramatically slow the flow of blood. Hence, a maximum pressure drop is mainly seen in the arterioles. That is why arterioles also go by their other name, the resistance vessels.

Amazing, right? How does it work?

Arterioles can actively respond to physical stimuli. They alter blood flow as it goes into our capillary beds via vasodilation and vasoconstriction of their smooth muscle.

When there is excessive intravascular pressure, they constrict and maintain a smaller diameter. Meanwhile, they dilate when the blood flow increases and becomes broader and open.

What is the function of vasa Vasorum?

Vasa vasorum are the specialized vessels discovered in your vessel walls. The role of vasa vasorum in your body is to give you the nutrients and oxygen needed for your blood vessel walls.

Also, it eliminates waste products generated by the cells in the walls of your vessels. When diffusion from your luminal surface fails to meet the nutritional demands, the vasa vasorum externa steps in.

Vasa vasorum from your tunica externa develops into your tunica media of big arteries and veins. It actively controls the blood flow to your vessel wall.

Vasa vasorum proliferates into your intima-media of atherosclerotic arteries. This part is where vasa vasorum offers sustenance into your thickened artery. Anyhow, your neovascular channels have weak walls. This may lead to intraplaque bleeding, plaque rupture, and mural thrombosis.

What is the difference between arteriole and venule?

Arterioles and venules are the smaller versions of arteries and veins. These vessels transport your blood to and from your capillary beds. Your arterioles connect your arteries and capillaries.

Meanwhile, your venules connect your capillaries and veins.

They also differ in their sizes.

Arterioles are 0.01-0.3mm in diameter. They consist of a single smooth muscle layer overlapping endothelial cells when they are near your capillaries. On the other hand, venules range from 8 to 100 mm in diameter. They have a thin tunica externa and a tunica media consisting of two or three layers of smooth muscle cells.

Furthermore, arterioles are the ones that carry your blood that is rich in oxygen. They deliver blood from the left side of your heart into your smallest vessels, your capillaries. In contrast, the venules are the ones that carry your blood with low oxygen from your capillaries back to the right side of your heart.

Another difference is that the lumina of your venule are substantially more prominent. It also has thinner walls than your arterioles.

What is the difference between arterioles and capillaries?

Arterioles are the ones that connect your arteries and capillaries. Meanwhile, capillaries are the ones that touch your arteries to your veins. In addition, arterioles distribute blood rich in oxygen into your capillaries. In contrast, capillaries return waste-rich blood into your venules connected to your veins and the vena cava.

Your arterioles and capillaries also differ in the number of their tunics. The arterioles have three layers: the tunica externa, tunica media, and tunica intima, while the walls in your capillaries only have one. Capillaries only have tunica intima, which is made entirely of endothelial cells.

Arterioles control the flow into your capillaries by vasoconstriction and vasodilation.

On the other hand, capillaries use diffusion to allow an exchange of substances. They enable oxygen and nutrients to diffuse out of your blood into your tissues. At the same time, they are allowing carbon dioxide and wastes to pass from your tissues into your blood.

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