External Features of the Heart — Surfaces and Borders
Position
The heart is a hollow muscular organ situated in the middle mediastinum, enclosed in the pericardium. It lies obliquely behind the body of the sternum and adjoining costal cartilages such that one-third lies to the right and two-thirds to the left of the median plane.
Shape and Measurements
- Shape: Pyramidal/conical
- Length: 12 cm | Width: 9 cm
- Weight: 300 g (males); 250 g (females)
- Roughly the size of the owner’s clenched fist
Apex
- Formed entirely by the left ventricle
- Directed: downwards, forwards, and to the left
- Location: Left 5th intercostal space, 9 cm (3.5 inches) from the midsternal line, just medial to the midclavicular line
- Apex beat (point of maximum cardiac impulse/PMCI): palpable and visible at this site during ventricular systole
In children below 7 years: apex lies in the 4th intercostal space just lateral to the midclavicular line.
Base (Posterior Surface)
- Formed mainly by the left atrium (2/3rd) and partly by the right atrium (1/3rd)
- Directed: backwards and to the right (opposite to apex)
- Vertebral relations: lies opposite T5–T8 in lying posture; descends one level in erect posture (T6–T9)
- Separated from the vertebral column by: oblique pericardial sinus, oesophagus, and aorta
- Receives: 4 pulmonary veins (into left atrium) + SVC and IVC (into right atrium)
Surfaces
| Surface | Formation | Key Feature |
|---|---|---|
| Sternocostal (anterior) | Mainly right atrium + right ventricle; partly left auricle + left ventricle | Part uncovered by lung (cardiac notch of left lung) = area of superficial cardiac dullness |
| Diaphragmatic (inferior) | Left ventricle (left 2/3rd) + right ventricle (right 1/3rd) | Rests on central tendon of diaphragm; traversed by posterior interventricular groove |
| Left surface | Mainly left ventricle + left auricle at upper end | Related to left phrenic nerve and pericardiacophrenic vessels |
Left atrium is hidden anteriorly by the ascending aorta and pulmonary trunk — not visible on sternocostal surface.
Borders
| Border | Formation | Extent |
|---|---|---|
| Right | Right atrium (vertical) | From opening of SVC above to IVC below |
| Left | Left ventricle (mainly) + left auricle | From left auricle to apex |
| Inferior | Right ventricle (mainly) + small part right atrium near IVC | From IVC to apex; nearly horizontal |
| Upper | Both atria, mainly left atrium | From 2nd left costal cartilage to 3rd right costal cartilage |
Upper border is obscured anteriorly by the ascending aorta and pulmonary trunk.
Sulci Demarcating Chambers
| Sulcus | Location | Contents |
|---|---|---|
| Coronary sulcus (atrioventricular groove) | Encircles heart; separates atria from ventricles; deficient anteriorly at root of pulmonary trunk | Right AV groove: right coronary artery + small cardiac vein; Left AV groove: circumflex branch; Posterior AV groove: coronary sinus |
| Anterior interventricular sulcus | Sternocostal surface | Anterior interventricular artery + great cardiac vein |
| Posterior interventricular sulcus | Diaphragmatic surface | Posterior interventricular artery + middle cardiac vein |
Crux of the heart: Meeting point of the posterior interventricular sulcus, coronary sulcus, and interatrial groove on the diaphragmatic surface.
Area of Superficial Cardiac Dullness
The part of the sternocostal surface not covered by lung (behind the cardiac notch of the left lung) is dull on percussion — called the area of superficial cardiac dullness. It overlies the right ventricle directly.