Medical School · Year 1 · Anatomy Thorax Abdomen · includes a quiz and discussion video
Lecture 3: Heart - External Features and Pericardium
Unit 1.4: Human Gross Anatomy II - Thorax and Abdomen
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the structure and layers of the pericardium
- Identify the surface projections of the heart and great vessels
- Describe the external features of the heart (surfaces, borders, sulci)
- Identify the chambers and great vessels from an external view
- Describe the coronary circulation and its distribution
- Explain the innervation of the heart
Lecture Content
I. The Pericardium
The pericardium is a fibroserous sac that encloses the heart and the proximal portions of the great vessels. Located in the middle mediastinum, this structure provides protection, anchoring, and a friction-reducing environment for cardiac function.
The fibrous pericardium forms the tough outer layer of this sac. This dense connective tissue layer fuses inferiorly with the central tendon of the diaphragm, anchoring the heart to this structure. Anteriorly, the sternopericardial ligaments attach the fibrous pericardium to the posterior surface of the sternum. Superiorly, the fibrous pericardium becomes continuous with the adventitia of the great vessels. The fibrous pericardium serves to protect the heart, prevent overdistension during volume overload, and maintain the heart's position within the mediastinum.
The serous pericardium consists of two continuous layers that form the pericardial cavity. The parietal layer lines the inner surface of the fibrous pericardium, adhering to it so closely that the two layers appear as one. The visceral layer, also called the epicardium, covers the outer surface of the heart itself. These two layers are continuous with each other at the reflections around the great vessels, creating a closed sac similar to the relationship between visceral and parietal pleura.
The pericardial cavity lies between the parietal and visceral layers of the serous pericardium. This potential space normally contains fifteen to fifty milliliters of serous fluid that lubricates the surfaces and allows frictionless cardiac movement during the continuous cycles of contraction and relaxation.
<image>Panel A: Fibrous pericardium as a thick outer layer fusing with the diaphragm inferiorly and great vessel adventitia superiorly. Panel B: Parietal serous pericardium lining the inner surface of the fibrous pericardium. Panel C: Visceral serous pericardium (epicardium) covering the heart surface with myocardium visible beneath. Panel D: Pericardial cavity between the serous layers containing a thin lubricating fluid layer.</image>
II. Pericardial Sinuses
The pericardial sinuses are recesses formed where the serous pericardium reflects between the great vessels. Two sinuses have clinical significance.
The transverse pericardial sinus lies behind the ascending aorta and pulmonary trunk, and in front of the superior vena cava. This sinus forms because the arterial vessels (aorta and pulmonary trunk) share a common reflection of serous pericardium, separate from the venous vessels behind them. A finger can be passed through this sinus behind the great arteries, which has important surgical implications. During cardiac surgery, a clamp or ligature can be passed through the transverse sinus to temporarily occlude the aorta and pulmonary trunk, allowing control of arterial outflow from the heart.
The oblique pericardial sinus lies behind the left atrium, bounded by the reflections of serous pericardium around the pulmonary veins and inferior vena cava. This sinus forms a cul-de-sac or blind pouch because the four pulmonary veins and the inferior vena cava enter the heart in close proximity, creating a U-shaped reflection. Access to this sinus from the front is limited, but it becomes important during posterior pericardial procedures.
<image>Panel A: Posterior view of the heart showing the transverse sinus as a passage behind the ascending aorta and pulmonary trunk. Panel B: Transverse sinus relationship with the SVC posteriorly, demonstrating finger passage through the sinus. Panel C: Oblique sinus behind the left atrium bounded by four pulmonary veins and IVC as a blind cul-de-sac. Panel D: Surgical application showing clamp placement through the transverse sinus for arterial control.</image>
III. Surface Projections of the Heart
Understanding the surface projections of the heart enables accurate physical examination and interpretation of cardiac pathology. The heart lies predominantly to the left of the midline, with approximately one-third on the right and two-thirds on the left.
The borders of the heart project onto the anterior chest wall in a predictable pattern. The right border extends from the third right costal cartilage to the sixth right costal cartilage, formed by the right atrium and receiving the superior and inferior venae cavae. The left border extends from the second left intercostal space near the sternum to the apex, formed superiorly by the left atrial appendage and inferiorly by the left ventricle. The inferior border extends from the sixth right costal cartilage to the apex, formed primarily by the right ventricle with the left ventricle contributing at the apex. The superior border lies at the level of the second costal cartilages bilaterally, formed by the atria and the roots of the great vessels.
The apex lies in the fifth intercostal space at the midclavicular line, approximately eight to nine centimeters from the midline. The left ventricle forms the apex, which produces the point of maximal impulse palpable during physical examination.
The anatomical positions of the cardiac valves differ from their optimal auscultation sites due to the direction of blood flow. The mitral valve lies behind the fourth left costal cartilage but is best heard at the apex in the fifth intercostal space at the midclavicular line. The tricuspid valve lies at the fourth to fifth intercostal space to the left of the sternum but is best heard at the left lower sternal border. The aortic valve lies behind the sternum at the third intercostal space but is best heard at the second intercostal space at the right sternal border. The pulmonary valve lies at the left third costal cartilage but is best heard at the second intercostal space at the left sternal border.
<image>Panel A: Anterior chest wall showing the four cardiac borders with corner points marking right border along the right sternal edge from third to sixth cartilages. Panel B: Left border from second intercostal space to the apex at fifth ICS MCL, with inferior border extending to the apex. Panel C: Anatomical valve positions clustered centrally behind the sternum at the third to fifth intercostal spaces. Panel D: Auscultation sites spread to optimal listening positions with connecting lines to anatomical positions and chambers contributing to each border.</image>
IV. External Features of the Heart
The heart is a muscular pump approximately the size of a closed fist, weighing between 250 and 350 grams. Its orientation places the apex pointing toward the left, anteriorly, and inferiorly, while the base faces posteriorly.
The heart has four surfaces named for the structures they face. The anterior or sternocostal surface faces the sternum and costal cartilages. The right ventricle forms most of this surface, with contributions from the right atrium to the right and the left ventricle toward the apex. The posterior surface, also called the base, faces the vertebral column and is formed mainly by the left atrium receiving the four pulmonary veins. The inferior or diaphragmatic surface rests on the central tendon of the diaphragm, formed mainly by the left ventricle with a smaller contribution from the right ventricle. The left or pulmonary surface faces the left lung, formed predominantly by the left ventricle, and creates the cardiac impression on the mediastinal surface of the left lung.
The borders of the heart correspond to specific chambers. The right border is formed entirely by the right atrium. The left border is formed by the left ventricle with the left auricle contributing superiorly. The inferior border is formed primarily by the right ventricle with the left ventricle contributing at the apex. The superior border is formed by both atria with their auricles, plus the roots of the aorta and pulmonary trunk.
<image>Panel A: Anterior (sternocostal) surface view with right ventricle predominant, right atrium on the right side, and left ventricle at the apex. Panel B: Posterior view (base) showing the left atrium with four pulmonary vein entries. Panel C: Inferior (diaphragmatic) surface view showing predominantly left ventricle with right ventricle contribution. Panel D: Left lateral (pulmonary) surface view showing the left ventricle creating the cardiac impression on the left lung.</image>
V. Sulci of the Heart
The external sulci or grooves of the heart mark the boundaries between chambers and contain the major coronary vessels.
The coronary sulcus, also called the atrioventricular sulcus, circles the heart like a crown, marking the boundary between the atria and ventricles. The coronary sulcus is not visible on the anterior surface because it is covered by the origin of the pulmonary trunk and the aorta, but it is clearly seen on the posterior and inferior surfaces. This sulcus contains the right coronary artery on the right side and anteriorly, the circumflex branch of the left coronary artery on the left side and posteriorly, and the coronary sinus draining into the right atrium posteriorly.
The anterior interventricular sulcus runs on the sternocostal surface from the coronary sulcus toward the apex, marking the boundary between the right and left ventricles anteriorly. This groove contains the left anterior descending artery, also called the anterior interventricular artery, and the great cardiac vein.
The posterior interventricular sulcus runs on the diaphragmatic surface from the coronary sulcus toward the apex, marking the posterior boundary between the ventricles. This groove contains the posterior descending artery, also called the posterior interventricular artery, and the middle cardiac vein.
<image>Panel A: Coronary (atrioventricular) sulcus encircling the heart containing the RCA on the right side and circumflex artery on the left. Panel B: Coronary sinus in the posterior portion of the coronary sulcus draining into the right atrium. Panel C: Anterior interventricular sulcus with the LAD artery and great cardiac vein descending toward the apex. Panel D: Posterior interventricular sulcus with the PDA and middle cardiac vein on the diaphragmatic surface.</image>
VI. The Chambers - External View
The external appearance of each chamber reveals characteristic features that allow identification.
The right atrium forms the right border of the heart and receives systemic venous return. Externally, the superior vena cava enters superiorly and the inferior vena cava enters inferiorly. The right auricle projects anteriorly from the main chamber, wrapping around the right side of the ascending aorta. The sulcus terminalis on the external surface marks the internal crista terminalis, the ridge separating the smooth sinus venarum from the rough pectinate muscle region.
The right ventricle forms most of the anterior surface and the inferior border. The conus arteriosus or infundibulum forms the smooth-walled outflow tract leading to the pulmonary trunk, visible as an elevation on the anterior surface below the pulmonary valve.
The left atrium forms most of the posterior surface or base of the heart. Externally, four pulmonary veins enter this chamber, two from each lung. The left auricle projects anteriorly, visible at the left side of the pulmonary trunk.
The left ventricle forms the apex and left border, as well as most of the inferior surface. The walls of this chamber are noticeably thicker than the right ventricle, reflecting the higher pressures required for systemic circulation. The ascending aorta arises from this chamber, visible as the large vessel ascending from behind the pulmonary trunk.
<image>Panel A: Right atrium external features showing SVC entry superiorly, IVC entry inferiorly, right auricle projecting anteriorly, and sulcus terminalis marked. Panel B: Right ventricle with anterior surface prominence and conus arteriosus leading to the pulmonary trunk. Panel C: Left atrium posterior view showing four pulmonary vein entries with left auricle projecting anteriorly. Panel D: Left ventricle showing thicker walls, apex formation, and ascending aorta arising from the chamber.</image>
VII. Great Vessels
The great vessels enter and leave the heart, connecting the cardiac chambers to the systemic and pulmonary circulations.
Vessels entering the heart carry blood toward the cardiac chambers. The superior vena cava brings deoxygenated blood from the head, neck, upper limbs, and thorax to the right atrium. The inferior vena cava brings deoxygenated blood from the abdomen, pelvis, and lower limbs to the right atrium. The coronary sinus drains deoxygenated blood from the heart wall itself into the right atrium. Four pulmonary veins, two from each lung, bring oxygenated blood from the lungs to the left atrium.
Vessels leaving the heart carry blood away from the cardiac chambers. The pulmonary trunk carries deoxygenated blood from the right ventricle, bifurcating into the right and left pulmonary arteries that deliver blood to the lungs for oxygenation. The ascending aorta carries oxygenated blood from the left ventricle, continuing as the aortic arch and then the descending aorta to supply the entire body.
The spatial arrangement of these vessels is important for understanding cardiac anatomy. The aorta and pulmonary trunk lie anteriorly, with the pulmonary trunk to the left and the aorta to the right before the trunk bifurcates. The venae cavae and pulmonary veins lie posteriorly, entering the atria from behind.
<image>Panel A: Venous inflow showing SVC and IVC entering the right atrium with coronary sinus entering posteriorly. Panel B: Four pulmonary veins entering the left atrium from the lungs carrying oxygenated blood. Panel C: Arterial outflow with pulmonary trunk from right ventricle bifurcating into right and left pulmonary arteries. Panel D: Ascending aorta arising from the left ventricle with spatial arrangement showing arteries anterior and veins posterior.</image>
VIII. Coronary Circulation
The coronary arteries supply the heart muscle with oxygenated blood, arising from the aortic sinuses immediately above the aortic valve. The two main coronary arteries and their branches provide the entire blood supply to the myocardium.
The right coronary artery arises from the right aortic sinus and descends in the coronary sulcus, passing between the right atrium and right ventricle. The sinoatrial nodal artery branches early and supplies the SA node in approximately fifty-five percent of individuals. The right marginal artery descends along the right border of the heart, supplying the right ventricular free wall. In approximately seventy percent of individuals, the right coronary artery gives rise to the posterior descending artery, also called the posterior interventricular artery, which runs in the posterior interventricular sulcus toward the apex. The AV nodal artery arises near this junction and supplies the AV node in approximately eighty percent of individuals. Overall, the right coronary artery supplies the right atrium, most of the right ventricle, the SA node in most people, the AV node in most people, and the posterior third of the interventricular septum.
The left coronary artery arises from the left aortic sinus and passes behind the pulmonary trunk for a short distance before bifurcating into its two main branches. The left anterior descending artery, also called the anterior interventricular artery, descends in the anterior interventricular sulcus toward the apex. This critical vessel supplies the anterior left ventricle, the anterior interventricular septum, the apex of the heart, and a small portion of the anterior right ventricle. The left circumflex artery continues in the coronary sulcus around the left border of the heart to reach the posterior surface. It supplies the left atrium, the posterior left ventricle, and gives rise to the SA nodal artery in approximately forty-five percent of individuals. In about thirty percent of individuals, the left circumflex artery gives rise to the posterior descending artery.
<image>Panel A: RCA arising from right aortic sinus, descending in coronary sulcus with SA nodal branch and right marginal branch. Panel B: RCA continuing to give the PDA in the posterior interventricular sulcus in the right-dominant pattern. Panel C: LCA arising from left aortic sinus and bifurcating behind the pulmonary trunk into LAD and circumflex branches. Panel D: LAD in the anterior interventricular sulcus to apex and circumflex in the coronary sulcus with supply territories color-coded.</image>
IX. Coronary Dominance
Coronary dominance refers to which artery gives rise to the posterior descending artery, which supplies the inferior interventricular septum and inferior left ventricular wall.
Right dominance is the most common pattern, present in approximately seventy percent of individuals. In this pattern, the right coronary artery gives rise to the posterior descending artery and typically also supplies the AV node.
Left dominance occurs in approximately fifteen percent of individuals. The left circumflex artery gives rise to the posterior descending artery and usually supplies the AV node in these individuals.
Co-dominance, where both arteries contribute to the posterior descending supply, occurs in approximately fifteen percent of individuals.
Understanding coronary dominance has clinical implications. The dominant artery supplies the inferior septum and AV node, so occlusion of the dominant artery may cause both inferior wall infarction and conduction disturbances including heart block.
X. Cardiac Veins
The cardiac veins drain deoxygenated blood from the myocardium, with most venous return entering the coronary sinus.
The coronary sinus is the main venous drainage channel of the heart, lying in the posterior portion of the coronary sulcus. It receives tributaries from around the heart and empties directly into the right atrium near the opening of the inferior vena cava.
The great cardiac vein travels in the anterior interventricular sulcus alongside the left anterior descending artery, then continues around the left border to enter the coronary sinus. This vessel drains the territories supplied by the left coronary artery system.
The middle cardiac vein travels in the posterior interventricular sulcus alongside the posterior descending artery, draining into the coronary sinus. It drains the inferior portions of both ventricles.
The small cardiac vein travels in the coronary sulcus alongside the right coronary artery, draining into the coronary sinus. It drains the posterior right atrium and right ventricle.
Additional tributaries include the posterior vein of the left ventricle draining the posterior left ventricular wall, and the oblique vein of the left atrium, a small vessel representing the remnant of the embryonic left superior vena cava.
Not all cardiac venous drainage enters the coronary sinus. The anterior cardiac veins drain the anterior right ventricle directly into the right atrium without passing through the coronary sinus. The venae cordis minimae, also called thebesian veins, are tiny vessels that drain directly into all four cardiac chambers.
<image>Panel A: Coronary sinus in the posterior coronary sulcus receiving the great cardiac vein from the anterior interventricular sulcus. Panel B: Middle cardiac vein from the posterior interventricular sulcus and small cardiac vein along the right coronary artery draining to the coronary sinus. Panel C: Anterior cardiac veins draining directly into the right atrium without passing through the coronary sinus. Panel D: Coronary sinus opening into the right atrium near the IVC orifice with venous territories color-coded.</image>
XI. Nerve Supply of the Heart
The heart receives autonomic innervation from the cardiac plexus, a network of sympathetic and parasympathetic fibers located at the base of the heart. This plexus is divided into superficial and deep portions.
The superficial cardiac plexus lies below the aortic arch, between the arch and the pulmonary trunk bifurcation. The deep cardiac plexus lies between the aortic arch and the tracheal bifurcation, posterior to the aortic arch.
Sympathetic innervation reaches the heart from the cervical and upper thoracic sympathetic ganglia. These cardioaccelerator fibers increase heart rate, increase force of contraction, increase conduction velocity through the conduction system, and produce coronary artery dilation. Sympathetic activation prepares the heart for increased output during exercise or stress.
Parasympathetic innervation arrives via the vagus nerve, the tenth cranial nerve. These cardioinhibitory fibers decrease heart rate, decrease atrial contractility, decrease conduction velocity producing a longer PR interval, and produce mild coronary vasoconstriction. Parasympathetic or vagal tone predominates at rest, maintaining the resting heart rate below the intrinsic rate of the SA node.
Sensory afferent fibers from the heart travel with both sympathetic and parasympathetic pathways. Pain fibers travel primarily with the sympathetic nerves to spinal cord segments T1 through T4. This pathway explains referred cardiac pain to the medial arm, jaw, and neck, as these areas share the same spinal segments. The dermatomal distribution of T1-T4 includes the medial arm and forearm. Vagal afferents carry information for cardiovascular reflexes, including baroreceptor and chemoreceptor responses.
<image>Panel A: Cardiac plexus at the heart base receiving sympathetic fibers from cervical and upper thoracic ganglia (T1-T4) producing cardioaccelerator effects. Panel B: Parasympathetic fibers from the vagus nerve producing cardioinhibitory effects including decreased heart rate and conduction velocity. Panel C: Afferent pain pathways to T1-T4 spinal segments with sympathetic nerves carrying cardiac pain sensation. Panel D: Referred pain distribution mapped on the arm, shoulder, neck, and jaw corresponding to T1-T4 dermatomes.</image>
XII. Clinical Correlations
Pericarditis, inflammation of the pericardium, produces characteristic clinical features. Patients experience sharp chest pain that typically worsens with inspiration and when lying flat, improving when sitting forward. The inflamed pericardial surfaces rub against each other, producing a friction rub audible with the stethoscope. The ECG shows diffuse ST elevation and PR depression.
Cardiac tamponade occurs when fluid accumulates in the pericardial cavity rapidly enough to compress the heart and impair filling. The fibrous pericardium limits acute distension, so relatively small volumes of rapidly accumulating fluid can be life-threatening. Beck's triad describes the classic findings: hypotension from reduced cardiac output, jugular venous distension from impaired venous return, and muffled heart sounds from the fluid surrounding the heart. Treatment requires emergent pericardiocentesis, needle drainage of the pericardial fluid, using a subxiphoid approach that avoids the pleura and lungs.
Coronary artery disease results from atherosclerotic narrowing of the coronary arteries, reducing myocardial blood supply. The left anterior descending artery is most commonly affected by significant stenosis. Proximal LAD occlusion, sometimes called the "widow maker," produces large anterior myocardial infarctions due to the extensive territory supplied by this vessel.
Myocardial infarction patterns correlate with the occluded coronary artery. Right coronary artery occlusion produces inferior myocardial infarction and may affect the conduction system since the RCA supplies the SA and AV nodes in most individuals. Left anterior descending occlusion produces anterior infarction, typically the largest infarct territory. Left circumflex occlusion produces lateral or posterior infarction.
Referred pain from cardiac ischemia follows the sympathetic afferent pathway to spinal segments T1-T4. Patients experience pain in areas sharing these dermatomal segments, including the left arm particularly the medial aspect, the jaw, and the neck. Understanding this referral pattern helps recognize atypical presentations of acute coronary syndrome.
<image>Panel A: Pericarditis showing inflamed pericardium with friction rub auscultation position and typical patient posture sitting forward. Panel B: Cardiac tamponade with Beck's triad illustrated including hypotension, JVD, and muffled heart sounds with pericardiocentesis needle approach from subxiphoid. Panel C: Coronary artery territories with infarct distributions marked showing RCA-inferior, LAD-anterior, and LCx-lateral patterns. Panel D: Referred pain dermatomes T1-T4 mapped on the upper body with arm, shoulder, neck, and jaw distribution.</image>
Summary
- The pericardium has fibrous (outer) and serous (parietal and visceral) layers creating the pericardial cavity
- The transverse sinus lies behind the aorta and pulmonary trunk; the oblique sinus lies behind the left atrium
- The heart has sternocostal (anterior), base (posterior), and diaphragmatic (inferior) surfaces
- The right coronary artery supplies the right heart, SA node in 55%, and AV node in 80% of individuals
- The left coronary artery divides into the LAD and circumflex, supplying the left heart and most of the septum
- The coronary sinus is the main venous drainage, emptying into the right atrium
- Sympathetic activation increases heart rate and contractility; parasympathetic activation decreases them
Key Terms
| Term | Definition |
|---|---|
| Pericardium | Fibroserous sac enclosing the heart and proximal great vessels |
| Transverse sinus | Pericardial passage behind the aorta and pulmonary trunk; allows surgical clamping |
| Coronary sulcus | Atrioventricular groove containing the right coronary and circumflex arteries |
| LAD | Left anterior descending artery; supplies anterior heart; occlusion causes anterior MI |
| Coronary sinus | Main venous drainage channel emptying into right atrium |
| Cardiac tamponade | Heart compression from pericardial fluid; causes Beck's triad |
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