Anatomy of the heart

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Assistant Editor(s)-in-Chief: Rim Halaby

Overview

• The mediastinum occupies the center of the thoracic cavity between the pulmonary viscera. It spans superiorly at the level of the thoracic aperture, inferiorly at the diaphragm, and from the sternum to the costal cartilages and bodies of vertebrae T1 to T12 antero-posteriosly.
• With the exception of the lungs, the mediastinum involves all thoracic viscera.
• For the purpose of description, the mediastinum is divided into superior, middle and inferior sections.
• The heart, its roots of the great vessels (ascending aorta, pulmonary trunk, superior vena cava), and the pericardium occupy the middle mediastinum.

The Pericardium

• The pericardium is a sac that embeds the heart. It is pierced by the great vessels. Morphologically, it is a conical-shaped, double-walled fibro-serous membrane.
• The pericardial sac rests posteriorly to the sternum at the level of second to sixth costal cartilages and T5-T8 vertebrae.
• The pericardium is made of two layers:
  • Fibrous Pericardium
    • Hard protective external layer
    • Attached to sternum anteriorly by sterno-pericardial ligaments and fused with the central tendon of the diaphragm and great vessels to allow mobility of the pericardial sac against sudden cardiac overfilling.
  • Serous Pericardium
    • Smooth internal layer made of 2 components:
      • Parietal: reflects onto fibrous pericardium
      • Visceral: reflects onto heart and great vessels and forms the epicardium, the external layer of the heart wall.
• Pericardial cavity: Potential space between parietal and visceral layers. It contains a serous fluid film that occupies the cavity and functions as lubricant against friction by all chest movements.

Pericardial Sinuses

• Pericardial sinuses are channels or chambers between the visceral and parietal pericardia.
• Transverse sinus:
  • Located posterior to the pulmonary trunk and ascending aorta at the level between the superior vena cava and aortic arch.
  • Formed after dorsal mesocardium rupture embryonically
  • Functional role is to allow the unhindered expansion of great arteries posteriorly during cardiac systole.
  • Utilized surgically to pass surgical clamps or place ligatures around great arteries.
• Oblique sinus:
  • A blind recess (cul-de-sac) posterior to the left atrium between superior vena cava, right and left pulmonary veins inferior to the transverse sinus.
  • Formed embryonically by the incorporation of the pulmonary vein tributaries into the left atrium.
  • Functional role believed to be the expansion of the left atrium upon normal collapse of the thorax.

Below are images showing the pericardium and pericardial sinuses.

• 
  The pericardium
• 
  The pericardial sinuses

External and Internal Features of the Heart

Layers of the Heart

• The heart wall consists of 3 layers:
  • Epicardium: thin external layer formed by visceral layer of serous pericardium
  • Myocardium: thick middle layer made of cardiac muscle
  • Endocardium: thin internal membrane that lines the heart and its valves. It is composed of endothelium and sub-endothelial connective tissue, similar to intimal component of blood vessels.

External Features

• The heart and roots of great vessels are embedded in the pericardial sac, approximately in the center of the thorax. The size of the heart is slightly larger than a clenched fist. The heart is surrounded laterally and posteriorly by the lungs, and anteriorly bound by the sternum and medial sections of the ribs and sterno-costal joints.
• It has the shape of a three-sided pyramid with an apex (left anterior), a base (posterior), and 4 surfaces: Sterno-costal/anterior (formed by right ventricle), diaphragmatic/inferior (formed by left ventricle and part of right ventricle), left pulmonary (formed by left ventricle, in contact with left lung), and right pulmonary (formed by right atrium).
• The heart appears trapezoid in the posterior and anterior views. As such, it consists of 4 borders: Right (convex), Inferior (horizontal), left (oblique), and superior.
• The external surface of the heart is notable for 3 main sulci (grooves):
  • Coronary (atrioventricular) sulcus: runs around the heart, and separates atria from ventricles
  • Anterior interventricular sulcus: runs along the interventricular septum anteriorly
  • Posterior interventricular sulcus: runs along the interventricular septum posteriorly

Internal Features

• The heart has 4 chambers: Right and left atria and right and left ventricles.
• Blood flows normally in the right to left direction: Right atrium to right ventricle to left atrium, and finally to left ventricle.
• Oxygen-depleted blood reaches the right atrium via the coronary sinus and superior and inferior vena cava and evacuates the right ventricle via the pulmonary artery towards the lungs.
• Oxygen enriched blood then re-enters the left atrium via 4 pulmonary veins and evacuates the left ventricle via the aorta.

Right Atrium

• The right atrium forms the right border of the heart.
• The superior vena cava and the inferior vena cava drain systemic venous blood into the smooth posterior wall of the right atrium.
• In contrast, the right atrium’s anterior wall is ridge-like; it is composed internally of pectinate muscles, a rough muscular wall.

Right auricle:

• The right atrium contains a right auricle, an ear-like conical muscular pouch that overlaps the ascending aorta.
• The right auricle is an embryonic structural remnant that allows the right atrium to increase its capacity.

Opening of the superior and inferior vena cava:

• The superior vena cava opens in the interior right atrium at its superior part, at the level of the right third costal cartilage.
• The inferior vena cava opens in the interior right atrium at its inferior part, almost in line with the superior vena cava at approximately the level of the fifth costal cartilage.

Coronary Sinus:

• It is a venous collection, whereby systemic blood is received at the posterior part of the coronary groove from the cardiac veins into the coronary sinus. The latter is a derivative of an embryonic structure called the venous sinus.
• The opening or orifice of the coronary sinus is located between the right atrioventricular orifice and the inferior vena cava orifice.

Sinus Venarum:

• The adult right atrium also contains posteriorly a smooth-walled pouch called sinus venarum, on which the coronary sinus, and superior and inferior vena cava bring poorly oxygenated blood to the heart.
• Sinus venarum is formed by the incorporation of the venous sinus (L. sinus venosum), an embryonic structure, into the primordial atrium. As a result, sinus venarum provides further capacity for the right atrium.
• The sinus venorum is separated from the primordial atrium by means of 2 structures: the sulcus terminalis (terminal groove) externally which extends from the front of the superior vena cava to the front of the inferior vena cava, and represents the line of union of the sinus venosus of the embryo with the primitive atrium. Internally, the separation is indicated by the crista terminalis (terminal crest).

Fossa Ovalis:

• Fossa ovalis is a depression in the inter-atrial septum.
• It is a remnant of the foramen ovale, an embryonic physiological shunt, significant for fetal oxygenation and nutrition from the mother’s placenta to fetus’s left atrium without passing through the lungs.
• The limbus of the fossa ovalis (annulus ovalis) is the prominent oval margin of the fossa ovalis.
• It is located on the medial wall of the right atrium and circumscribes the septum primum the fossa ovalis anteriorly, posteriorly, and superiorly.

Left Atrium

• The left atrium is the major component of the heart base.
• The interatrial septum is part of the left atrial wall; it runs posteriorly and to the right. Its wall is slightly thicker than that of the right atrium.
• The interior of the left atrium is characterized by two main pouches:
  • A larger smooth-walled part, believed to be formed by absorption of parts of embryonic pulmonary veins.
  • A smaller muscular auricle with pectinate muscles, believed to be remnant of left part of primordial atrium.

Pulmonary Veins:

• Blood enters the left atrium on the posterior wall via 4 valveless pulmonary veins, arranged in two pairs, left and right.

Left Auricle:

• Similar to the right atrium, the left atrium also contains a left auricle.
• In contrast to the right auricle which overlaps the ascending aorta, the left auricle forms the superior part of the left border of the heart and overlaps the pulmonary trunk.
• The left auricle is a small muscular region containing pectinate muscles inside.

Left Atrio-Ventricular Orifice:

• At the level of the left atrio-ventricular orifice, the outflow of oxygenated blood received from the pulmonary veins evacuates the left atrium.

References

Template:WikiDoc Sources