4 External Anatomy of the Heart

External Anatomy of the Heart refers to the observable structures, contours, and spatial relationships visible on the outer surface of the heart. This includes the overall shape of the organ, its surfaces, borders, external landmarks, and the visible roots of the great vessels. Understanding the external anatomy is essential for identifying cardiac orientation within the thorax, localizing internal structures, and recognizing normal variations or pathological changes during imaging or surgery.

Overall Cardiac Morphology

The heart is a muscular, hollow organ with a conical shape, slightly flattened anteroposteriorly. It typically weighs 250–350 grams in adults and measures about 12 cm in length, 8–9 cm in width, and 6 cm in thickness. The heart is situated in the middle mediastinum, obliquely within the thoracic cavity, with two-thirds to the left of the midline.

The external appearance is characterized by:

• A broad superior base.
• A pointed inferior apex.
• Four chambers: right atrium, right ventricle, left atrium, left ventricle (only partially visible externally).
• Prominent grooves separating the chambers and marking the paths of coronary vessels.

Cardiac Apex and Base Morphology

Apex

The apex is the pointed inferior end of the heart, formed mainly by the tip of the left ventricle. It is directed anteroinferiorly and to the left, lying at the level of the 5th left intercostal space, about 8–9 cm from the median line. The apex is the site where the apical impulse (point of maximal impulse, PMI) can be palpated.

Base

The base is the broad superior and posterior aspect of the heart, opposite the apex. It is formed mainly by the left atrium, with a smaller contribution from the right atrium. The base faces posteriorly toward the vertebral column and is separated from the esophagus by the pericardium.

Surfaces of the Heart

Sternocostal (Anterior) Surface

This surface is directed forward and upward, lying behind the sternum and costal cartilages. It is formed mainly by the right ventricle, with contributions from the right atrium and the left ventricle. The anterior interventricular groove runs along this surface, housing the anterior interventricular artery and the great cardiac vein.

Diaphragmatic (Inferior) Surface

This surface rests on the central tendon of the diaphragm. It is formed primarily by the left ventricle and partly by the right ventricle. The posterior interventricular groove, containing the posterior interventricular artery and the middle cardiac vein, marks this surface.

Pulmonary (Left and Right) Surfaces

• The left pulmonary surface faces the left lung and is formed mainly by the left ventricle, with a small contribution from the left atrium.
• The right pulmonary surface faces the right lung and is formed chiefly by the right atrium.

Cardiac Borders

The heart has four borders that outline its silhouette on imaging and dissection:

• Right border: Formed by the right atrium, extending from the superior vena cava (SVC) to the inferior vena cava (IVC).
• Inferior border: Formed mainly by the right ventricle, with a minor contribution from the left ventricle near the apex.
• Left border: Formed predominantly by the left ventricle, and partly by the left auricle.
• Superior border: Formed by both atria and the origins of the great vessels.

External Atrial Morphology

Right Atrium

The right atrium is visible on the right side of the heart’s anterior surface. Its external features include the right auricle—a muscular, ear-shaped appendage that overlaps the root of the aorta. The sulcus terminalis is a shallow groove marking the boundary between the smooth and rough portions of the atrial wall.

Left Atrium

Most of the left atrium is located posteriorly, forming the base of the heart. Only the left auricle is visible from the anterior view, projecting forwards and to the left, overlapping the pulmonary trunk.

External Ventricular Morphology

Right Ventricle

Forms most of the anterior (sternocostal) surface and the entire inferior border. It is separated externally from the left ventricle by the anterior and posterior interventricular grooves.

Left Ventricle

Forms the left border and a significant portion of the diaphragmatic surface. Its muscular wall gives the heart its rounded left contour.

Cardiac Grooves and External Separations

The heart’s external grooves (sulci) serve as landmarks separating the chambers and housing major coronary vessels:

• Coronary (Atrioventricular) Sulcus: Encircles the heart, separating the atria from the ventricles. It contains the right coronary artery, circumflex artery, and coronary sinus.
• Anterior Interventricular Sulcus: Marks the boundary between the right and left ventricles on the sternocostal surface.
• Posterior Interventricular Sulcus: Marks the boundary on the diaphragmatic surface.

Externally Visible Great Vessel Roots

Several major vessels are visible at the superior aspect of the heart:

• Aorta: Emerges from the left ventricle, ascending before arching posteriorly.
• Pulmonary Trunk: Arises from the right ventricle, splitting into right and left pulmonary arteries.
• Superior Vena Cava (SVC): Drains into the right atrium from above.
• Inferior Vena Cava (IVC): Enters the right atrium from below.
• Pulmonary Veins: Four veins (usually) enter the left atrium posteriorly.

External Chamber Mapping

Although the heart's four chambers are internal structures, their outlines can be mapped externally based on grooves and landmarks.

• The right atrium lies to the right of the sternocostal surface.
• The right ventricle forms most of the anterior surface.
• The left ventricle is found leftward and posterior.
• The left atrium is mostly posterior, with only the auricle visible anteriorly.

External Cardiac Variation and Integration

There is natural variation in cardiac external anatomy due to individual differences in heart orientation (axis), size, and shape. Age, sex, physical conditioning, and pathology (such as cardiomegaly or congenital anomalies) may alter external features. The heart’s external anatomy is integrated with the pericardium, adjacent mediastinal structures, and the roots of the great vessels, which together form the cardiac silhouette observed in imaging.