Written by Angelica Mari Kiroquero
Reviewed by Dr. Reuben J C. los Baños, Ph.D.
Understanding the circulatory system is a crucial step in understanding oneself.
Blood and blood cells are being pumped and managed by the circulatory system. It carries the blood to all the tissues in your body. Did you know that the total length of an adult’s blood vessel, in estimation, is between 100,00 and 500,00km?
Cells that are similar in structure work together to perform specialized functions. These are what we call tissues.
The circulatory system consists of many structures and comprises different tissues. The heart consists of cardiac muscles and epithelial and connective tissues.
The heart plays a vital role in our lives because it handles many body functions. Some may say the heart is a shape in the color red, but it is not a shape, nor is it inside of your body for the reason of loving. Let’s learn more about the heart and its function!
The main functions are pumping blood, supplying oxygen, and removing metabolic wastes. Amazing, right? The human body is capable of doing wonders. Some of you might also know this, but the size of your fist is the average heart, which is the size of a heart.
Now, try clenching your fist and find out the size of your heart!
Before we delve deeper into the circulatory system, Let’s discuss what makes up the heart. It will help you understand its significance to humans as long as we live.
Cardiac muscles compose a heart wall that divides into four chambers that help the body pump blood. The two upper chambers of your heart are the atrium, and the two lower chambers are the ventricles. The septum is an internal heart wall that divides your heart into the left and right.
On the upper chambers, your right and left atria act as intersections or junctions on the road. They receive blood from different body parts and the pulmonary veins. The redistribution of the blood throughout the parts is also done after receiving it.
The lower chambers are in charge of your pulmonary and systematic circulations. It is like pumping chambers, pumping blood while providing proper circulation. It circulates the blood like the motor pistons that allow fluid movement.
The composition of the heart does not end here, as it is a very complex organ as much as its importance. It comprises many chambers, veins, and arteries that work together to function.
Surrounding the heart is the pericardium. A fibrous, fluid-filled sac lined by a serous mesothelium as the protective layer.
It comprises the Fibrous pericardium (outer layer) and Serous pericardium (inner layer). Your serous pericardium is further divided into the parietal and visceral layers.
Three significant layers divide the heart walls’ four (4) chambers. The layers are the internal endocardium, the middle myocardium, and the external epicardium.
Endocardium
The lining endothelium comprises the internal endocardium or tunica intima. “Endo” is defined as internal or within. The name already suggests that it is the innermost layer wall of your heart. It is lining inner surfaces, such as heart valves.
Connective tissues, a fibroblastic layer with smooth muscle cells, and a subendocardial layer support them.
The subendocardial layer is a connective tissue that contains an impulse-conducting system. Conducts electrical impulses in your heart that contain specialized cardiac cells.
What is the thickest histologic layer of the heart?
Myocardium
Known as tunica media, it is the middle and muscular layer. The contractional function for blood pumping happens here and is considered the principal functional element.
Cardiomyocytes comprise the myocardium and generate the contractile force. Like us humans, communication is essential; we communicate using language. Intercalated discs are a unique intercellular bridge for their communication.
Did you know that cardiomyocytes are like cells found in the skeletal muscle? These are myocytes containing similar striations. Yet these cardiomyocytes are branching mononucleated containing intercalated disks.
The number of nuclei differentiates it from skeletal muscles.
It may contain varying amounts of elastic fibers, smooth muscle cells, and collagen fibers.
It is arranged spirally in a contractile cardiac muscle fibers chamber—the thickest layer due to the strong force needed to pump blood in circulations. The thickness varies in each location, for ventricles are denser than in the atria.
The atria are thinner due to the passive blood flow. The left ventricle requires the production of enough force to propel blood in circulation. This explains why its wall is thicker than the right ventricle.
What is the histology of the heart epicardium?
Epicardium
It is made up of simple squamous mesothelium and supported with a layer of loose connective tissue containing blood vessels and nerves. The membrane surrounding the heart is a visceral layer of the serous pericardium.
During heart movements, the;
- Adipose tissue(epicardium) cushions the underlying structures.
- Production of lubricant fluid by both layers of the serous mesothelial cells. The fluid prevents friction within the pericardium.
The pericardium subdivides into;
- The fibrous layer is comprised of many fibrous connective tissue.
- The serous layer. Contains an inseparable outer layer(fibrous pericardium) and an overlying inner (myocardium )layer.
What is the histology of the blood and blood vessels?
Blood is fluid connective tissue, while the endothelial cells line the blood vessels. The vessel’s size and job determine how much connective tissue and smooth muscle are in its wall.
Blood vessels are channels that carry blood throughout your body. Think of a road system with different vehicles. The roads and highways are the blood vessels, and the cars traveling are the blood.
Blood vessels are further divided into three parts. They are your Arteries, veins, and capillaries.
Arteries
The blood vessels that carry oxygen-rich blood to your body are your arteries. The main functions are the management of oxygen, nutrients, and hormone transportation in your body.
Transporting blood is done not only by the arteries themselves but by your heart! Your heart pumps the blood into your aorta, the most prominent artery.
There are two types of arteries: elastic (aorta and pulmonary) and muscular (Femoral, radial, and brachial arteries.
Veins
Veins are the collectors! Located throughout your body, it is in charge of returning oxygen-poor blood after its collection.
Another function is carrying oxygen-rich blood to the heart coming from your lungs. Did you know that during this cycle, it is the only time they contain oxygen-rich blood?
Although they function similarly, arteries have thick walls with muscle tissue, while veins use valves and thinner walls to keep your blood flowing throughout the body.
Capillaries
Capillaries are the smallest and most delicate type of blood vessel in your body; due to their small and narrow size, only a single line of substances can pass through at a time.
This medium transports cells, nutrients, and oxygen throughout your body. It is a connector for the veins and arteries to complete the circulatory system.
What is the histological structure of the heart valve?
Before the blood can leave each chamber, it has to pass through a valve. A valve is like your water faucet; it controls the water flow as valves control the flow of blood passing through each chamber.
When the valve opens, it allows blood to pass through, but when it closes, it prevents blood from leaving or returning to the other chamber.
Each aortic valve comprises three leaflets (cusps) made mainly of collagen; the valve is placed on a muscle ring and connected to the heart wall.
The valves usually occur in this sequence:
- As the left ventricle relaxes, the mitral valve opens once the aortic valve closes. This allows the flow of blood from your left atrium into the left ventricle.
- Contraction of the left atrium occurs, which allows more blood flow into your left ventricle.
- The mitral valve closes, and the aortic valve opens as soon as the left ventricle contracts, allowing blood to flow into the aorta and out to the rest of your body.
- Your left and right ventricle relax and cause the tricuspid valve to open and the pulmonary to close. Allowing blood to flow into the right ventricle that had returned to the right atrium from the body.
- Your left and right ventricles contract and open and close the pulmonary and tricuspid valve, respectively. Blood flows from the right ventricle to the lungs and then returns to the left atrium as fresh, oxygenated blood.
An extracellular matrix highly organizes valves into layers. Arranged according to the flow of your blood.
The semilunar valves carry blood away from the heart to pulmonary and systemic circulations—the two valves are pulmonary and aortic.
The semilunar valves divide into:
- Ventricularis – direct contact with pulsatile blood and directed toward the ventricles.
- Spongiosa located between the ventricularis and fibrosa layers. It is made of proteoglycans with collagen fibers.
- Fibrosa directs toward the outflow vessel lumen.
Atrioventricular valves are like the semilunar valves. But, they have a layer called the ventricularis. It allows blood to flow from the atria into the ventricles.
It has two valves. Each valve lies between an atrium and a ventricle. The right atrioventricular valve (tricuspid valve) and left atrioventricular valve (mitral valve). A ring surrounds the orifice, with two or three cusps lining the valves.
The layers of the atrioventricular valves arrange themselves in a way that:
- The atrialis.
- The spongiosa located in the middle.
- The fibrosa layer is directed toward the ventricles.
A sheath of endocardial endothelial cells interlaced with valve interstitial cells encases each layer of the valves.
The four valves of the heart are aortic, mitral, pulmonary (or pulmonic), and tricuspid.
As the name suggests, the aortic valve allows blood to flow from the heart’s left ventricle to the aorta. It distributes oxygen-rich blood from the heart to other parts of your body and is the final destination encountered by oxygenated blood before it leaves the heart.
Located in between your left ventricle and aorta, when facing the heart it is located near the middle of your heart. For healthy individuals, the leaflets allow blood flow by opening wide and clasping tightly to prevent the backflow of blood.
In some cases, some individuals may be born with only two leaflets. A bicuspid aortic valve is called if it only consists of two leaflets.
Similar in function to the valves but different in structure. The mitral valve or left atrioventricular valve, has two leaflets, while the others have three leaflets. The leaflets are referred to as the anterior leaflet and the posterior leaflet.
They allow blood to flow from your left atrium to your left ventricle and prevent backward flow. This valve ensures the blood flows in the proper direction as it opens and closes. It is located in between the upper left chamber of your heart and the lower left chamber.
Now, let’s recap: the upper-left chamber is the left atrium, and the lower-left chamber is the left ventricle.
Your pulmonary valve helps manage blood flow and controls the flow of oxygen-poor blood from your heart to your lungs. Once your blood enters your main pulmonary artery, it travels to your lungs from your left and right pulmonary arteries.
The main function of your pulmonary valve is managing the blood from your right ventricle into your main pulmonary artery. However, it manages oxygen-poor blood.
Located on the right and in front of your aortic valve. Your pulmonary artery passes through the arch of the aorta as it carries your blood to the right and left lungs.
Lastly, the tricuspid valve ensures that blood flows to your right ventricle from the right atrium. Like the other three valves, it also prevents blood from backflowing to the different chambers.
As the blood reaches the right atrium and fills it up, it is allowed into the right ventricle as the tricuspid valve opens up. Contraction of the right ventricle provides blood to follow to the lungs. The tricuspid valve closes tightly to avoid backflow.
As your heart beats, the sound comes from the leaflets. The opening and closing of valves make up the sound of your heartbeat!
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