THE CONTEXT
The heart consists of two pumps arranged in series, with chamber structure reflecting their roles in pulmonary and systemic circulation.
Organisation and pumping roles
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The right and left heart act as two serial pumps that share several electrical and mechanical components.
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The right heart circulates blood to the lungs for oxygenation, and the left heart circulates blood to the rest of the body.
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The thin-walled atria act as priming pumps for the ventricles. The thick-walled ventricles provide most of the energy needed to maintain circulation by pumping blood through large vascular beds under pressure.
Atrial position and venous drainage
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The atria lie posteriorly within the mediastinum.
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The left atrium lies anterior to the oesophagus and descending aorta. It receives four pulmonary veins, with two from each lung.
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The right atrium receives blood from the superior vena cava, inferior vena cava and coronary sinus.
Annulus fibrosus
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The annulus fibrosus separates the atria from the ventricles, forms the skeleton for the atrioventricular valves and electrically insulates the atria from the ventricles.
Right ventricular anatomy
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The right ventricle is triangular, has a wall about 2–3 mm thick, and lies anterior and to the right of the left ventricle.
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It extends from the annulus fibrosus to near the cardiac apex. Its anterosuperior surface is rounded and convex.
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The interventricular septum bounds its posterior extent and bulges into the chamber.
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Its upper extent forms the right ventricular outflow tract, from which the pulmonary artery arises.
Left ventricular anatomy
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The left ventricle is more conical in shape and nearly circular in cross-section. It extends from the left atrium to the cardiac apex.
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The left ventricular myocardium is approximately 10 mm thick because it pumps at a higher pressure than the right ventricle through a larger vascular bed.
Cardiac silhouette on chest X-ray
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Normally, the heart occupies less than 50% of the transthoracic diameter in the frontal plane.
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The left cardiac silhouette is formed by the aortic arch, pulmonary trunk, left atrial appendage and left ventricle.
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The right silhouette is described as being formed by the right atrium and the superior and inferior venae cavae, with the lower right border formed by the right ventricle.
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Hypertrophy or dilatation in disease states or congenital cardiac abnormalities may alter the silhouette.