Cuore

Pericardium

The pericardium is a bi-serous sac surrounding the heart and the roots of the great vessels; it contains the heart and the juxta-cardiac parts of its great vessels.

Parts of the pericardium

It consists of two parts:

• Fibrous pericardium: It is a tough connective tissue that defines the boundaries of the middle mediastinum.
• Serous pericardium: It consists of three layers. Both of them are arranged in a closed sac made of tough connective tissue, completely surrounding but unattached to the heart.

Serous pericardium

The serous pericardium consists of two parts:

• The parietal layer: It lines the inner surface of the fibrous pericardium.
• The visceral layer (epicardium): It adheres to the heart and forms its outer covering.

Except at their points of reflection, the two serous pericardial membranes are separated by a narrow space, the pericardial cavity, which contains a thin film of fluid that provides complete separation between the heart and its surroundings, allowing freedom of cardiac motion and shape change.

Functions of the pericardium

Many functions have been attributed to the pericardium, these include:

• Intrathoracic cardiac stability
• Reducing friction between the heart and the mediastinal structures
• Preventing cardiac distension and chamber dilation and so preventing hypertrophy
• Limiting uniform cardiac distribution of pericardial cavity hydrostatic pressure
• Ensuring ventriculoatrial regurgitations under increased ventricular end-diastolic pressure

Fibrous pericardium

The fibrous pericardium is a cone-shaped bag with its base attached to the central tendon of the diaphragm and its apex continuous with the adventitia (external membrane) of the great vessels. Anteriorly, it is attached to the posterior surface of the sternum by sterno-pericardial ligaments.

The phrenic nerves, which innervate the diaphragm and originate from spinal cord levels C3 to C5, pass through the fibrous pericardium. Laterally, the mediastinal parts of the parietal pleura cover the mediastinal surfaces of the lungs. The phrenic nerve and pericardiophrenic vasculature descend on each side between the fibrous pericardium and mediastinal part of the parietal pleura. Inferiorly, the pericardium is separated from the liver and gastric fundus by the respiratory diaphragm covering its inferior (abdominal) surface.

Serous pericardium

The serous pericardium forms the closed pericardial cavity (sac) within the fibrous pericardium. It has:

• Visceral layer (epicardium): It covers the heart and great vessels and at specific points folds back on itself to become continuous with the parietal layer.
• Parietal layer: It lines the internal surface of the fibrous pericardium.

The zone of reflection surrounding the veins is J-shaped, and the sac formed within the J, posterior to the left atrium, is the oblique pericardial sinus.

Sinus

A passage between the two sites of reflected serous pericardium is the transverse pericardial sinus. The sinus lies posteriorly to the ascending aorta and the pulmonary trunk, anteriorly to the superior vena cava, and superiorly to the left atrium.

The cardiac subserosal layer contains fat, especially along the ventricular side of the inferior cardiac border. The amount of epicardial adipose tissue may be a cardiometabolic risk factor, with increasing thickness linked to metabolic syndrome and diabetes mellitus.

Structure of the heart wall

The wall of the heart has three distinct layers:

• Epicardium: An outer (visceral pericardium). It forms the external surface of the heart.
• Myocardium: A middle layer consisting of multiple, interlocking layers of cardiac muscle tissue, with associated connective tissues, blood vessels, and nerves.
• Endocardium: An inner layer covering the inner surfaces of the heart, including the valves, and is made by a simple squamous epithelium.

Heart

The heart has a complicated, spiral, three-dimensional structure. The oblique portion of the heart in the thorax may be conceptualized by comparing it to a deformed pyramid, with the base facing posteriorly to the right and the apex facing anteriorly to the left.

Surfaces and borders

Its surfaces are designated as:

• Anterior (sternocostal)
• Inferior (diaphragmatic)
• Right and left pulmonary

Its borders are termed as:

• Upper
• Inferior (acute margin/border)
• Left (obtuse margin/border)

Although the heart is placed obliquely in the thorax, the atrial and ventricular septal structures are virtually in line, inclined anteriorly to the left to the sagittal plane. The right ventricle forms most of the anterior aspect of the ventricular mass. The left atrium forms most of the posterior aspect of the heart, whereas the left ventricle is only prominent inferiorly, running along the left border to reach the cardiac apex.

The right heart (right-sided chambers), while forming the right border and pulmonary surface, follows a gentle curve and also covers most of the anterior aspect of the left heart.

General organization

The heart is a fused pair of valve-containing muscular pumps circulating blood in series through the systemic (left heart) and pulmonary (right heart) circulations. Despite the functional disposition in series, the two atria receive venous blood and serve as weakly contractile reservoirs for final filling of the two ventricles, which then provide the powerful expulsive contraction that forces blood into the main arterial trunk.

The right heart starts at the right atrium, which receives the main myocardial venous inflow from the superior and inferior venae cavae and via the coronary sinus. The systemic venous blood traverses the right atrioventricular orifice, guarded by the tricuspid (right atrioventricular) valve, to enter the inlet part of the right ventricle. Contraction of the ventricle closes the tricuspid valve and, with increasing pressure, ejects the blood through the muscular right ventricular outflow tract into the pulmonary trunk.

The left heart starts at the left atrium, which receives all the pulmonary inflow of oxygenated blood and some coronary venous inflow. It contracts to rapidly fill the left ventricle through the left atrioventricular mitral valve, increasing the pressure in the apical trabecular part, closing the mitral valve, and opening the aortic valve. This enables the ventricle to eject blood via the left ventricular outflow tract into the systematic arterial tree, ascending aorta, and aortic sinuses.

Cardiac size, shape, and external features

The heart is a hollow, fibromuscular organ of a pyramidal form, with a base, an apex, and a series of surfaces and borders (margins). Enclosed in the pericardium, it occupies the middle mediastinum between the lungs and their pleural coverings. It is placed obliquely behind the body of the sternum, adjoining costal cartilages, ribs, and intercostal muscles.

Sulci on the cardiac surface

The division of the heart into four chambers produces boundaries that are visible externally as sulci. The extensive interatrial sulcus (or Waterston) separates the two atria. Internally, the ventricles are separated by the interventricular septum; the ventricular sulcus extends from the interatrioventricular sulcus to the cardiac apex on the inferior border (acute margin).

Cardiac base, apex, surfaces, and borders

The cardiac base is quadrilateral, with curved lateral extensions. It faces posteriorly and to the right, separated from the bodies of the thoracic vertebrae and their intervertebral discs by the pericardium, right pulmonary veins, esophagus, azygos venous system, thoracic duct, and coronary arterial branches.

Apex of the heart (cardiac apex)

The conical apex of the left ventricle is directed anteroinferiorly and to the left and is overlapped...