Surfaces and borders
The heart is shaped like a tilted pyramid, with its base facing back and its apex pointing forwards, down and left.
- Apex: formed by the left ventricle, usually in the left fifth intercostal space in the midclavicular line.
- Base (posterior surface): mainly the left atrium, with the pulmonary veins entering it.
- Anterior (sternocostal) surface: mainly the right ventricle.
- Inferior (diaphragmatic) surface: mainly the left ventricle, with part of the right, resting on the central tendon.
| Border | Formed by |
|---|---|
| Right | Right atrium, between the superior and inferior venae cavae |
| Inferior | Right ventricle, with the left ventricle at the apex |
| Left | Left ventricle, with the left auricle above |
| Superior | Both atria and auricles, with the great vessels emerging |
On the chest wall the heart's outline runs from the third right costal cartilage down to the sixth, across to the apex, and back up to the second left costal cartilage.
Chambers and valves
Each chamber has internal features that identify it on an opened specimen.
Right atrium
It receives the superior and inferior venae cavae and the coronary sinus. A ridge, the crista terminalis, separates the smooth posterior part from the anterior part and the auricle, which are lined by pectinate muscles. The sinuatrial node lies at the upper end of the crista. The fossa ovalis on the septum is the remnant of the fetal foramen ovale.
Right ventricle
Its wall is lined by coarse muscular ridges, the trabeculae carneae. The septomarginal trabecula (moderator band) crosses from the septum to the base of the anterior papillary muscle, carrying the right bundle branch. The outflow tract, the infundibulum or conus arteriosus, is smooth-walled and leads up to the pulmonary valve.
Left atrium and ventricle
The left atrium receives four pulmonary veins. The left ventricle has a much thicker wall and two papillary muscles, anterior and posterior, anchoring the mitral valve by chordae tendineae.
| Valve | Cusps | Best heard |
|---|---|---|
| Tricuspid | Anterior, posterior, septal | Lower left sternal edge |
| Pulmonary | Three semilunar | Second left intercostal space, parasternal |
| Mitral | Anterior, posterior | Apex |
| Aortic | Right, left, posterior (non-coronary) | Second right intercostal space, parasternal |
The anterior mitral cusp is the larger and is continuous with the aortic valve, separating the ventricle's inflow and outflow paths.
Relations and great vessels
The heart sits in the fibrous pericardium between the lungs, with the great vessels leaving its upper part. The ascending aorta arises from the left ventricle behind the pulmonary trunk and passes up and right for about 5 cm to the sternal angle, where it becomes the arch. The pulmonary trunk leaves the right ventricle in front and divides under the arch into right and left pulmonary arteries. The superior vena cava forms from the two brachiocephalic veins; the left brachiocephalic vein crosses in front of the arch branches to join it.
Behind the heart the oesophagus lies directly against the left atrium, with the descending aorta beside it. The phrenic nerves run down on each side of the pericardium in front of the lung roots, and the vagus nerves pass behind the roots.
Inside the pericardium, the transverse sinus is a passage behind the aorta and pulmonary trunk and in front of the superior vena cava; the oblique sinus is a recess behind the left atrium.
Blood supply and innervation
The heart is supplied by two coronary arteries arising from the aortic sinuses just above the aortic valve.
- Right coronary artery: arises from the right (anterior) aortic sinus, passes between the right auricle and the pulmonary trunk and runs in the right atrioventricular groove. It gives a conus branch, usually the sinuatrial nodal artery, the right marginal artery and, in most hearts, the atrioventricular nodal and posterior interventricular arteries.
- Left coronary artery: arises from the left posterior aortic sinus, passes behind the pulmonary trunk and divides into the anterior interventricular artery (left anterior descending) and the circumflex artery. The anterior interventricular artery gives diagonal branches over the left ventricle and septal branches into the septum.
The posterior aortic sinus gives no artery and is called the non-coronary sinus. Dominance is defined by which artery gives the posterior interventricular artery; right dominance is the common pattern.
Most venous blood returns by the great, middle and small cardiac veins to the coronary sinus, which opens into the right atrium. Sympathetic fibres from T1–T4 and vagal fibres form the cardiac plexus. Cardiac pain travels with the sympathetic fibres, so it is referred to the chest wall and inner arm.
Clinical relevance
Coronary anatomy predicts the territory of a myocardial infarct and its complications.
- Anterior interventricular artery occlusion: anterior and septal infarction.
- Circumflex occlusion: lateral wall infarction.
- Right coronary occlusion: inferior infarction, often with bradycardia or heart block because it usually supplies the nodes.
- Type A aortic dissection involves the ascending aorta and is a surgical emergency.
- Pericardiocentesis is performed from below the xiphoid, aiming towards the left shoulder.
- Atrial septal defect most often lies at the fossa ovalis.
On the specimen
Heart stations use both the heart in situ and opened hearts, so identify the view before the pins.
- Borders in situ: the right border is the right atrium, the inferior border is mostly right ventricle, and the left border is left ventricle.
- Ascending aorta or pulmonary trunk: the pulmonary trunk lies in front and to the left, arising from the infundibulum; the aorta lies behind and to the right.
- Brachiocephalic trunk or left brachiocephalic vein: the vein is thin-walled and crosses obliquely in front; the artery is thick-walled and runs up behind it to the right, dividing into the right common carotid and subclavian arteries.
- Right or left ventricle: the right has a thin wall, coarse trabeculae carneae, a moderator band and a smooth infundibulum; the left has a thick wall and two large papillary muscles.
- Mitral cusps: the anterior cusp is larger and lies next to the aortic outflow; the posterior cusp is a smaller crescent.
- Anterior interventricular artery and diagonal branches: the artery runs in the groove between the ventricles; diagonals leave it towards the left border.
- Phrenic or vagus nerve: the phrenic runs in front of the lung root on the pericardium; the vagus runs behind it.
The tendon of the infundibulum (conus ligament) is a fibrous band linking the back of the infundibulum to the aortic root.