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

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.

References

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