The heart weighs 250-300 g, is located in the chest on the left side. The main task of this organ is to pump blood through the human body. Below we will analyze in detail the structure and work of the heart at both the macro and micro levels. We will also discuss whether it has any additional functions.
A little history
The study of the functions and structure of the human heart began long before our days. The first to describe this organ most accurately was the Arab physician Ibn an-Nafis (1213-1288). In the XVII century, the English doctor William Harvey (1587-1657) showed that contractions promote blood through the circulatory system. In the following centuries, scientists described the conductive system of the heart and the mechanisms of contractile impulses, as well as various theories of the occurrence of diseases. The study of this organ and the diseases associated with it continues to this day.
The human heart is formed and functions in the womb. By the end of the 3rd week of pregnancy, passive oxygen diffusion becomes insufficient to maintain the metabolism of the developing embryo. The fetal heart becomes vital for the distribution of oxygen and nutrients. The initiation of the first heartbeat through the primitive heart tube begins on the 22nd day of intrauterine development. By the end of the 4th week, active blood circulation of the fetus is established, and by the 7th week, a four-chambered heart is formed.
The human heart is divided into four chambers: two atria and two ventricles. This structure provides blood flow in the systemic and pulmonary circulation. The first starts from the left ventricle and ends in the right atrium. It serves to carry various substances into and out of tissues. In a small circle, which begins in the right ventricle and ends in the left atrium, blood passes through the lungs, is enriched with oxygen, and gets rid of excess carbon dioxide.
In the heart, a base is distinguished (occupies the upper part, directed up and to the right, corresponds to the plane of the valves of large vessels) and the apex (pointed and turned down and to the left). In addition, this internal organ has several surfaces, which include:
- anterior (sternocostal) – mainly formed by the right ventricle;
- back (vertebral) – includes the surfaces of the left and right atria;
- lower (diaphragmatic) – adjacent to the pleura and formed by all four chambers;
- lateral (pulmonary) – often described as the left and right edges of the heart.
The heart is located in the pericardial cavity in the lower middle mediastinum.
In all four orifices of the heart, there are corresponding valves that regulate the direction of blood flow, preventing it from returning from the great arteries to the ventricles or from the ventricles to the atria. On the left side of the heart, between the ventricle and the atrium, is the mitral valve. It consists of two valves (therefore it is often called bivalve).
In the right part of the heart, there is a tricuspid valve, consisting of three leaflets. The bicuspid and tricuspid valves prevent blood from flowing from the ventricles to the atria. On the border of the aorta and the left ventricle is the aortic valve, and between the right ventricle and the pulmonary artery is the pulmonary valve.
The valves are connected to the annulus fibrosus, which contains collagen fibers.
Muscles of the heart and its work
Inside the chambers are the so-called papillary muscles. From them, the heart chords depart, which prevent the bending of the valve cusps.
The atria and ventricles alternately have a phase of systole (contraction) and distole (relaxation). When the atria fill with blood, the chambers throw it out to the periphery. The chambers of the heart then relax and blood flows through the right and left atrioventricular outlets from the atria to the ventricles.
When you listen to your heart, these phenomena show up as heart murmurs, which we call heart sounds. The first tone, that is, the systolic tone, mainly comes from the contracting ventricles, and the second tone is due to the return of blood entering the closed aortic and pulmonary valves.
Veins and arteries
The vessels that carry blood to the heart are called veins. All vessels through which blood flows from the heart are arteries – regardless of the composition of the blood that circulates in them. Thus, it is a mistake to believe that the arteries carry exclusively oxygenated blood, while the veins are rich in carbon dioxide-rich blood.
Coronary vessels and blood circulation of the heart
The left and right coronary arteries branch off from the first part of the aorta, called the aortic bulb. They supply the heart muscle with oxygenated blood. Initially, they merge in the coronal groove. The left coronary artery then runs in the anterior interventricular sulcus and the right coronary artery in the posterior interventricular sulcus. The right coronary artery is represented by a branch of the sinus node, which vascularizes (supplies blood) to the sinus node. The branch of the right ventricle and the right vestibular branch depart from the latter. In the next section, a marginal artery and a posterior descending artery are distinguished.
The left coronary artery is divided into the anterior interventricular branch and the surrounding branch.
Heart wall structure
The heart has three layers: endocardium, myocardium, and epicardium.
The endocardium is lined with a single-layer squamous epithelium located on its own connective tissue plate of the endocardium.
The myocardium is the most massive layer due to which the heart contracts. It is abundantly supplied with blood and consists of muscle cells – cardiomyocytes.
The epicardium is the inner (visceral plate) of the pericardium. It consists of a monolayer squamous epithelium located on the epicardial plate and the underlying subepicardial tissue, which contains numerous adipocytes (adipose tissue cells).
Microscopic structure of the heart muscle
There are two types of cells in the heart: cardiomyocytes and pacemaker cells (pacemaker cells).
Cardiomyocytes are the true cells of the heart muscle that form the myocardium of the atria of the ventricles. These cells can shorten and lengthen their myofibrils, creating the pumping power of the heart. Human cardiomyocytes are about 100 µm in length and 10–25 µm in diameter.
Pacemaker cells are modified cardiomyocytes and control the heartbeat. They spontaneously generate and send electrical impulses to cardiomyocytes. The pacemaker cells create rhythmic impulses of the heartbeat. In other words, they directly control your heart rate.
Conductive system of the heart
Pacemaker cells form the basis for the successful functioning of the so-called cardiac conduction system, which consists of several nodes and pathways.
The sinoatrial node (SA node) is located in the wall of the right atrium near the outlet of the superior vena cava. It has the properties of spontaneous generation of electrical impulses, that is, it has its own automatism. Usually, the sinoatrial node, also called the sinus node, produces electrical stimuli with the fastest rhythm. In healthy people, it is the main center responsible for the heart rate.
Impulses from the SA node propagate along the intra-atrial and interatrial pathways, causing both atria to contract. Then they reach the atrioventricular node (AV node). The latter slows down the conduction of the impulse so that the ventricles do not contract simultaneously with the atria. Then, from the AV node, impulses enter the ventricular parts of the conducting system, that is, into the bundle of His, which is divided into right and left branches running on both sides of the interventricular septum.
The final element of the system is the Purkinje fibers, which are located in the thickness of the left and right ventricles of the heart. The muscle cells themselves also tend to automatically contract with the destruction of the above centers (AV-, SA-nodes), as well as the His bundle of Purkinje fibers.
A properly functioning cardiac conduction system determines the appropriate heart rate and successive contractions of the atria and ventricles. The sinoatrial node normally sets the rhythm from 60 to 100 beats per minute. Pathology of the conducting system at any level leads to arrhythmias.
The easiest way to assess the effectiveness of the conduction system is to perform electrocardiography (ECG), which is a record of the electrical activity of the heart.
What other function does the heart have?
Above, we mentioned two types of heart muscle cells: working cardiomyocytes and pacemakers. However, there is a third group – secretory cardiomyocytes. They secrete atrial natriuretic peptide (ANP). ANP dramatically reduces plasma volume through three main mechanisms:
- increased excretion of salts and water through the kidneys;
- increasing vascular permeability.
Thus, the heart, in addition to the pumping function, also plays the role of an endocrine organ.
In this article, we have analyzed aspects of the structure and work of the heart, blood circulation circles. The heart is used for blood flow throughout the body. It consists of three layers and has four cavities: two ventricles and two atria. The organ contracts due to the presence of special pacemaker cells and a conduction system. It also performs endocrine functions. Heart structure, vascular anatomy, and circulatory physiology play an important role in understanding how various cardiovascular diseases develop.
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- Tan CMJ, Lewandowski AJ. The Transitional Heart: From Early Embryonic and Fetal Development to Neonatal Life. Fetal Diagn Ther. 2020;47(5):373-386. doi: 10.1159/000501906. Epub 2019 Sep 18. PMID: 31533099; PMCID: PMC7265763.
The main function of the heart is pumping. That is why this organ is made up mostly of muscle fibers.
The heart consists of three layers and has four cavities: two ventricles and two atria. The organ contracts due to the presence of special pacemaker cells and a conduction system.
There are 4 chambers in the human heart: two ventricles and two atria.
There are two circles of blood circulation in the human body: large and small.
Three: endocardium, myocardium, and epicardium.