The heart is a remarkable muscular organ that serves as the central pump of the circulatory system. This tutorial provides a comprehensive overview of the heart’s position, size, orientation, and external anatomical features.
Position within the Thorax
The heart occupies the middle mediastinum, a compartment within the thoracic cavity. It is situated between the lungs (with the left lung overlapping its left side), posterior to the sternum, anterior to the vertebral column (T5-T8 vertebrae), and superior to the diaphragm.
The heart is positioned obliquely within the chest, with approximately two-thirds of its mass lying to the left of the midline. Its orientation can be described as:
- Base: Directed posteriorly, superiorly, and to the right
- Apex: Directed anteriorly, inferiorly, and to the left
This oblique position means the right side of the heart is more anterior, while the left side is more posterior. The apex of the heart is located at the level of the fifth intercostal space, approximately 9 cm from the midline.
Size and Weight
The size of the heart correlates with body size and mass:
- Average adult heart weight: 250-350 grams (approximately 0.45% of body weight)
- Length: Approximately 12 cm
- Width: Approximately 8-9 cm at its widest point
- Anteroposterior diameter: Approximately 6 cm
The heart of a well-conditioned athlete may be slightly larger and heavier due to myocardial hypertrophy. Pathological conditions can significantly alter heart size — hypertension may cause left ventricular hypertrophy, while heart failure may result in overall cardiac enlargement (cardiomegaly).
External Anatomical Features
Surfaces of the Heart
The heart has three primary surfaces:
Sternocostal (Anterior) Surface: Formed mainly by the right ventricle, with the right atrium contributing on the right side. This surface lies directly behind the sternum and ribs.
Diaphragmatic (Inferior) Surface: Formed primarily by the left ventricle, with a small contribution from the right ventricle. This surface rests on the diaphragm.
Pulmonary (Left) Surface: Formed mainly by the left ventricle. This surface is in contact with the left lung.
Borders of the Heart
- Right Border: Formed by the right atrium, extending from the superior vena cava to the inferior vena cava
- Left Border: Formed mainly by the left ventricle, with a small contribution from the left atrium superiorly
- Inferior Border: Formed primarily by the right ventricle, extending from the right atrium to the apex
- Superior Border: Formed by the atria and the great vessels
External Grooves (Sulci)
The heart has several external grooves that mark the internal divisions:
Coronary (Atrioventricular) Sulcus: A deep groove that encircles the heart, separating the atria from the ventricles. It contains the coronary arteries, cardiac veins, and adipose tissue.
Anterior Interventricular Sulcus: Located on the anterior surface, this groove marks the septum between the right and left ventricles and contains the left anterior descending (LAD) coronary artery.
Posterior Interventricular Sulcus: Located on the diaphragmatic surface, this groove marks the posterior ventricular septum and contains the posterior descending artery (PDA).
Great Vessels
The heart connects to several great vessels:
Superior Vena Cava: Returns deoxygenated blood from the upper body to the right atrium.
Inferior Vena Cava: Returns deoxygenated blood from the lower body to the right atrium.
Pulmonary Trunk: Carries deoxygenated blood from the right ventricle to the lungs. It divides into left and right pulmonary arteries.
Pulmonary Veins: Four veins (two from each lung) return oxygenated blood to the left atrium.
Aorta: The largest artery in the body, carries oxygenated blood from the left ventricle to the systemic circulation. It branches into the ascending aorta, aortic arch, and descending aorta.
Pericardial Relationships
The heart is enclosed within the pericardial sac, which consists of the fibrous pericardium (outer layer) and the serous pericardium (inner layer). The pericardial cavity contains approximately 15-50 mL of serous fluid that lubricates the heart surface and reduces friction during contraction.
Surface Projection
On the anterior chest wall, the heart projects onto an area bounded by:
- Second intercostal space on the right (superior right border)
- Second intercostal space on the left (superior left border)
- Fifth intercostal space in the midclavicular line on the left (apex)
- Sixth costal cartilage on the right (inferior right border)
This projection is important for cardiac auscultation, percussion, and emergency procedures such as pericardiocentesis.
Clinical Significance
Understanding heart position is critical for:
- Cardiac auscultation: Knowing where each valve is best heard
- Pericardiocentesis: Accessing the pericardial space safely
- Chest tube placement: Avoiding cardiac injury
- Cardiopulmonary resuscitation: Effective chest compression requires knowing heart location
- Interpretation of chest radiographs: Evaluating heart size and contour