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Heart

The heart (Latin: cor) is a thick, muscular organ with four cavitated parts located in the middle portion of the inferior mediastinum. The heart acts as a central pump of the cardiovascular system that maintains the unidirectional flow of blood.

The heart is a muscular organ with four cavities and it is located in the middle mediastinum, which is the middle part of the inferior mediastinum.

It has a cone or pyramid shape with the base projected upward, backward, and to the right, while the apex projects forward, downward, and to the left.

The heart has five external surfaces formed by different internal parts:

  • diaphragmatic surface (inferiorly),
  • sternocostal surface (anteriorly),
  • right and left pulmonary surfaces (laterally),
  • base of the heart (posteriorly).

Heart surfaces

The base of the heart is quadrilateral, oriented posteriorly and consists of the left atrium, a small portion of the right atrium, and the proximal parts of the great veins (superior and inferior venae cavae, and the pulmonary veins). 

The base of the heart is fixed posteriorly to the pericardial wall, opposite the bodies of 5th to 8th thoracic vertebrae (6th to 9th when standing). Immediately posterior to the base of the heart, lies the esophagus. The apex of the heart is typically located deep to the left 5th intercostal space, 8 to 9 cm from the mid-central line. The apex is formed by the inferolateral part of the left ventricle.

The sternocostal (anterior) surface of the heart is directed anteriorly, superiorly, and slightly to the left. It consists mostly of the right ventricle, with part of the right atrium on the right and part of the left ventricle on the left.

The heart in its anatomical position rests on the diaphragmatic surface, which faces inferiorly and lies on the diaphragm. This surface is formed by the left ventricle and a small part of the right ventricle separated by the posterior interventricular groove. It is separated from the base of the heart by the coronary sinus.

The right and left pulmonary surfaces are broad and convex and face the lungs. The left pulmonary surface is formed by the left ventricle and a part of the left atrium, while the right pulmonary surface consists of the right atrium.

Margins of the heart

The margins of the heart are considered the following:

  • right and left margins of the heart - these are the same as the right and left pulmonary surfaces,
  • inferior margin - the sharp edge separating the anterior and diaphragmatic surfaces of the heart, and
  • obtuse margin - it is rounded and extends from the left auricle to the apex of the heart, it separates the anterior surface of the heart from the left pulmonary surface.

The external sulci

The internal partitions that form the four chambers of the heart produce grooves on the external surface called external sulci, which include the coronary sulcus, and the anterior and posterior interventricular sulci.

The coronary sulcus is a groove that circles the heart, marking the separation between the atria and the ventricles. It contains the right coronary artery, the small cardiac vein, the coronary sinus, and the circumflex branch of the left coronary arte

The anterior and posterior interventricular sulci mark the separation of the two ventricles of the heart. The anterior interventricular sulcus is found on the anterior surface of the heart and it contains the anterior interventricular artery and the great cardiac vein. The posterior interventricular sulcus is located on the diaphragmatic surface and contains the posterior interventricular artery, as well as the middle cardiac vein.

All of these external sulci are continuous with each other inferiorly, just to the right of the apex of the heart. The area on the lower backside of the heart where the coronary sulcus and the posterior interventricular sulcus meet is called the cardiac crux or crux of the heart.

Internal anatomy of heart

Internally, the heart is divided into four chambers: right and left atria, and right and left ventricles. 

The chambers are separated by septa, namely, the interatrial (between atria), interventricular (between ventricles) and atrioventricular (between atria and ventricles) septa.

Functionally, the heart consists of two pumps, each formed by an atrium and a ventricle separated by a valve. The right pump receives deoxygenated blood and pumps it into the lungs, and the left pump receives oxygenated blood from the lungs and sends it to the body. 

The atria have relatively thin walls and they receive blood, while the ventricles with relatively thick walls pump blood out of the heart.

Right atrium

The right atrium is situated in the superior right corner of the heart above the right ventricle. The systemic circulation ends in the right atrium, as deoxygenated blood from the tissues of the body enters it through the superior and inferior vena cava, and the coronary sinus.

The right atrium has a cuboid shape, therefore it has six walls: superior, inferior, anterior, posterior, lateral and medial. 

  • The superior vena cava opens to the superior wall.
  • On the inferior wall, the right atrioventricular orifice is located, which is an oval aperture between the right atrium and the right ventricle.
  • On the anterior wall, the right auricle is placed. The main function of the right auricle is to increase the volume of the right atrium. The muscular layer of the heart creates parallel ridges in the anterior wall and the auricle called the pectineal muscles.
  • On the posterior wall, the opening of the inferior vena cava is located.
  • On the lateral wall, the pectineal muscles are located.
  • The medial wall is formed by the interatrial septum. In the interatrial septum lies an oval fibrous depression called the fossa ovalis that covers the foramen ovale during fetal development. Around the fossa ovalis is an oval margin named the annulus ovalis or limbus of the fossa ovalis. Small openings for the smallest cardiac veins are also located on the medial wall of the right atrium.
  • Between the medial and the posterior wall there is a small opening for the coronary sinus to the right atrium. The coronary sinus collects venous blood from the heart muscle and is protected by a semicircular fold of the lining membrane of the auricle, called the valve of the coronary sinus.

The Right Ventricle

The right ventricle is one of the heart chambers located in the inferior right portion of the heart under the right atrium and opposite to the left ventricle. The main function of the right ventricle is to pump blood up through the pulmonary valve and artery into the lungs, thus providing pulmonary circulation.

The right ventricle has a pyramidal shape with the base directed upwards, and the apex - downwards. 

  • The right atrioventricular orifice and the opening of the pulmonary trunk are located on the base of the right ventricle.
  • The medial wall is formed by the interventricular septum.
  • The inferior wall lies close to the central tendon of the diaphragm.
  • The anterior wall of the right ventricle is directed towards the inner surfaces of the sternum and ribs.

The part of the heart from where the pulmonary trunk arises is called the infundibulum (or conus arteriosus). It is a conical extension formed from the upper and left angle of the right ventricle in the cordate heart. The inner surface of the right ventricle is smooth only in the infundibulum, elsewhere the muscular layer of the heart creates papillary muscles and irregular muscular columns called trabeculae carneae. The papillary muscles fix the tricuspid valve. 

Venous blood from the right atrium enters the right ventricle through the right atrioventricular orifice. Around the right atrioventricular orifice is the tricuspid valve. The tricuspid valve has three leaflets - the anterior, posterior and septal leaflets. The main function of the tricuspid valve is to prevent backflow of the blood from the ventricles into the atria.

There are three papillary muscles in the right ventricle. The chordae tendineae arise from one leaflet and inserts into two adjacent papillary muscles.

Around the opening of the pulmonary trunk is the pulmonary valve, which is a semilunar valve lying between the right ventricle and the pulmonary artery. It has three cusps and it opens up during the ventricular systole.

The Left atrium

The left atrium is one of the chambers of the heart located in the superior left corner, above the left ventricle, and opposite to the right atrium. The pulmonary circulation ends in the left atrium, as oxygenated blood from the lungs enter it through the pulmonary veins.

The left atrium has a cuboid shape, it is thicker, but smaller in the volume than the right atrium. 

There are five openings found in the walls of the left atrium, and these include: four openings for the pulmonary veins, and the left atrioventricular orifice. 

  • The four openings for the pulmonary veins are located on the superior and posterior walls of the left atrium. The pulmonary veins carry oxygenated blood from the lungs to the left atrium.
  • The left atrioventricular orifice is found on the inferior wall of the left atrium and carries oxygenated blood to the left ventricle.

The medial wall of the left atrium is formed by the interatrial septum, which separates the left and right atrium. 

On the anterior surface of the left atrium lies the left auricle, a flap of the heart wall. The left auricle has an irregular shape with many tiny ridges created by the pectinate muscles. Elsewhere, the inner surface of the left atrium is smooth. The main function of the left auricle is to increase the volume of the left atrium.

The Left Ventricle

The left ventricle is one of the heart chambers located in the lower left portion of the heart below the left atrium, opposite to the right ventricle. The main function of the left ventricle is to pump blood into the aorta, providing the systemic circulation. 

The left ventricle has a cone shape with a base directed upward, while the apex is directed inferiorly. 

There are two openings found on the base of the left ventricle: the left atrioventricular orifice and the aortic orifice. There are fibrous rings and valves around both openings. Around the left atrioventricular orifice there is the left atrioventricular valve (also called the mitral valve, bicuspid valve). The mitral valve has two cusps - the anterior and posterior cusps. The opening of the mitral valve is surrounded by a fibrous ring called the mitral annulus. The chordae tendineae are inelastic tendons that are attached to the valve cusps and to the papillary muscles within the left ventricle. The chordae tendineae prevent the valve leaflets from prolapsing into the left atrium. 

The aortic orifice is guarded by the aortic semilunar valve. The aortic valve has three cusps - the left, right and posterior cusps. The aortic orifice is surrounded by a fibrous ring called the annulus fibrosus, which fixes the aortic valve. 

The anterosuperior portion of the left ventricle below the aortic orifice is called the aortic vestibule. The inner surface of the heart is smooth at the aortic vestibule, while elsewhere the inner surface is covered by papillary muscles and trabeculae carneae.

Layers of the heart wall

Layers of the heart wall

The wall of the heart is formed by three layers:

  • endocardium - the inner layer,
  • myocardium - the middle layer, and
  • epicardium - the outer layer.

The endocardium is the epithelial layer lining all the cavities of the heart. It covers the pectinate and papillary muscles, trabeculae carneae, and chordae tendineae. 

The structure of the endocardium is mostly similar throughout the heart, however, in the left atrium and ventricle the endocardium is thicker than in the right atrium and ventricle, and the atrial endocardium is more prominent than the ventricular endocardium.

All of the heart valves are derived from the endocardium.

Myocardium

The myocardium is a thick muscular layer between endocardium and epicardium, which is formed by cardiac muscle fibers, also known as cardiomyocytes. The main function of the myocardium is to contract and pump blood out of the heart to provide organs with blood. 

The myocardial layer differs in the atria and ventricles. In the atria the myocardium has two layers:

  • an outer circular layer that is common in both atria, and
  • an inner longitudinal layer that is separate in each atria. The longitudinal layer forms the pectinate muscles.

In the ventricles the myocardium is formed by three layers:

  • an outer and an inner layer that are longitudinal and are common in both ventricles, and
  • a middle layer that is circular and separate in each ventricle. In the ventricles, the inner layer forms the papillary muscles and the trabeculae carneae.

The epicardium, also called the visceral or inner layer of the serous pericardium, is the outer serous layer of the heart. It is a thin, clear layer that is composed of loose connective tissue, elastic fibers and adipose tissue. 

The epicardium covers not only the heart, but also the beginnings of the large vessels: the ascending aorta, the pulmonary trunk, the superior vena cava, the inferior vena cava, as well as the pulmonary veins before they return to the heart. From these blood vessels the epicardium further continues as the parietal layer of the serous pericardium. 

The epicardium protects the inner heart layers and participates in the production of the pericardial fluid.

Pericardium

The heart is surrounded by a closed fibrous sac called the pericardium. The pericardium has an outer (fibrous) layer and an inner (serous) layer. 

The serous layer has two lamina - the parietal and the visceral laminae. The visceral lamina of the serous pericardium is also known as the epicardium, which forms the outer layer of the heart. 

The space between the parietal and visceral laminae of the serous pericardium is called the pericardial cavity, and it contains small amount of serous fluid that decreases the surface tension and lubricates, allowing free movement of the heart during contraction.