LAYERS OF THE HEART: HISTOLOGY AND FUNCTIONS - ANATOMY AND PHYSIOLOGY - 2025

The layers of the heart are the tissues that make up the wall of this organ and are the endocardium, myocardium and pericardium. Scientific texts suggest that these three layers are similar to the layers of blood vessels, known as the tunica intima, media, and adventitia, respectively.

In humans the heart, the main organ of the cardiovascular system, is about the size of a fist and is located in the left-middle area of ​​the chest (mediastinum), between both lungs.

Diagram of the wall of the human heart (Source: Blausen 0470 HeartWall.png: BruceBlaus (BruceBlaus. When using this image in external sources it can be cited as: Blausen.com staff (2014). «Medical gallery of Blausen Medical 2014). WikiJournal of Medicine 1 (2). DOI: 10.15347 / wjm / 2014.010. ISSN 2002-4436.) Derivative work: Miguelferig (translation to Galician) via Wikimedia Commons)

It is made up of four hollow "chambers" made up of myocardial tissue (heart muscle) known as the atria and ventricles. The two atria are the upper chambers, meanwhile the two ventricles are the lower chambers. Each ventricle, left and right, is connected to an atrium, left and right, respectively.

The connection of the atrium and the left ventricle and of the atrium and the right ventricle occurs through the pulmonary circulation, that is, they are not directly connected to each other, from right to left or vice versa.

Heart anatomy

The right atrium receives deoxygenated systemic blood and conducts it to the right ventricle, from where it is pumped into the lungs. The left atrium receives oxygenated blood from the lungs and carries it to the left ventricle, which pumps it through the aorta artery throughout the body. In the following image you can see how a heart beats:

There are unidirectional valves that separate each atrium from its respective ventricle and each ventricle from the arteries to which it connects. In addition, the contraction of the heart muscle (myocardium) depends on the electrical impulses generated by special groups of cells that characterize cardiac activity.

Layers of the human heart

From the inside out, the heart has the following layers: endocardium, myocardium, and pericardium.

- Endocardium

The endocardium is the innermost layer of the heart wall and is continuous with the intima of the blood vessels entering and leaving it.

In blood vessels and arteries, the tunica intima is composed of several layers of epithelial connective tissue known collectively as the endothelium, which is continuous throughout the vascular system, including the inner lining of the heart.

In the cardiac organ, this endothelium consists of four layers:

- A simple squamous epithelium

- A layer of connective tissue with scattered fibroblasts

- A layer of dense connective tissue, rich in elastic fibers that mix with myocardial cells

- A “subendocardial” layer, formed by loose connective tissue with abundant irrigation (presence of blood vasculature) and nerve fibers (this is the “deepest” layer of the endocardium)

The subendocardial fall is also rich in cells specialized in conducting electrical impulses, known as Purkinje fibers.

The endothelial cells present in the endocardium are in permanent contact with the blood that circulates in the cardiac lumen (the internal space of the heart) and this interaction represents an important control point for the organic functioning of the heart.

The endocardium covers the entire internal surface of the cardiac chambers, including the septa that separate the atria and ventricles from each other.

In addition, it continues with the fibrous skeleton that forms the one-way valves arranged between the ventricles and atria (atrioventricular valves) and between the ventricles and arteries (aortic semilunar valve and pulmonary valve).

The endocardium is thought to be involved in preventing compression of the subendocardial blood vessels by controlling the patency of the Tebesium vessels, which are one of the heart's draining veins.

- Myocardium

The myocardium is the middle layer of the heart wall, that is, it is the layer that lies between the endocardium and the epicardium, and is the thickest of the three.

In this layer are the cells of the heart muscle that make possible the contraction and relaxation of the atria and ventricles during the pumping of blood to and from the rest of the body tissues.

In the myocardium, muscle cells are arranged in spirals around the openings of the chambers and have different functions.

Some of them are responsible for the attachment of the muscular layer to the fibrous cardiac skeleton, while others participate in the secretion of hormones and others are responsible for the generation or conduction of electrical impulses that stimulate contraction.

The most numerous and important cells in the myocardial layer are cardiac myocytes, which are in charge of the sequential contraction of the cardiac chambers for the pumping of blood or cardiac output.

The three layers of the heart: endocardium, myocardium and epicardium (Source: OpenStax College via Wikimedia Commons)

Arrangement of cardiac muscle fibers

The myocytes or cardiac muscle fibers are in permanent contact with each other through their ends and through structures called “intercalary discs”. Its organization and communication is such that the flow of ions and excitation between one cell and another is extremely fast, since the tissue functions as a syncytium.

A syncytium is a structure or tissue composed of cells that are attached to each other and that communicate in such a way that they behave as a unit. This is especially true of the transmission of electrical potential, since stimulation of a single myocyte involves direct transmission of the stimulus to all other cardiac fibers.

- Epicardium

The epicardium is the outermost layer of the heart; In some texts it is also known as the “visceral layer of the pericardium” and it is composed of a simple squamous epithelium called the mesothelium.

Between the epicardium and the myocardium is a space known as the "subepicardium" or "subepicardial" space where many mesenchymal cells can be found.

This layer contributes to the storage of fat on the cardiac tissue and in the subepicardial portion are many coronary vessels, ganglia and nerve cells. In addition, the epicardium acts as an important source of trophic signals that help support the development, growth, and continued differentiation of the heart during development.

At the roots of the vessels entering and leaving the heart, the epicardium (visceral pericardium) continues with the serous layer of the parietal pericardium. Both layers enclose the pericardial cavity, which contains a small amount of serous fluid that lubricates the outer surface of the epicardium and the inner surface of the parietal pericardium.

References

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