Medical School · Year 1 · Cardiovascular · includes a quiz and discussion video

Lecture 1: Cardiac Anatomy and Histology

Unit 1.7: Cardiovascular System

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the anatomical position, surfaces, and borders of the heart
2. Identify the chambers of the heart and their structural features
3. Describe the cardiac valves and their anatomical relationships
4. Explain the coronary arterial supply and venous drainage
5. Describe the histological features of cardiac muscle
6. Identify the components of the cardiac conduction system

Gross Anatomy of the Heart

The heart occupies the middle mediastinum, situated within the pericardial sac between the lungs. Roughly the size of a closed fist and weighing between 250 and 350 grams, the heart is oriented obliquely with its apex pointing to the left, anteriorly, and inferiorly. The base, formed by the atria and great vessels, lies posteriorly and superiorly. This orientation means the heart does not sit symmetrically in the chest but rather projects predominantly to the left of the midline.

The heart presents four surfaces defined by their anatomical relationships. The sternocostal (anterior) surface faces forward and is formed predominantly by the right ventricle, which explains why right ventricular enlargement can be detected by increased cardiac contact with the anterior chest wall. The diaphragmatic (inferior) surface rests on the central tendon of the diaphragm and is formed by both ventricles, with the left ventricle contributing the larger portion. The left pulmonary surface, formed by the left ventricle, produces the cardiac impression on the left lung. The right pulmonary surface, formed by the right atrium, creates a smaller impression on the right lung.

The borders of the heart are similarly defined by the chambers. The right border is formed by the right atrium, the left border by the left ventricle and left atrial appendage, the superior border by the atria and great vessels, and the inferior border by both ventricles resting on the diaphragm.

<image>Panel A: Anterior view of the thorax with transparent ribcage showing the heart positioned in the middle mediastinum with apex pointing left and inferiorly toward the fifth intercostal space. Panel B: The sternocostal surface formed predominantly by the right ventricle highlighted in orange with the diaphragmatic surface showing both ventricles in red and blue. Panel C: Surrounding structures including lungs, sternum, and diaphragm demonstrating anatomical relationships. Panel D: Superior view showing the great vessels (aorta, pulmonary trunk, superior vena cava) emerging from the cardiac base.</image>

The Pericardium

The pericardium encloses the heart in a double-walled sac that anchors it to surrounding structures while permitting free movement during the cardiac cycle. The outer fibrous pericardium is a tough, inelastic layer that attaches superiorly to the adventitia of the great vessels and inferiorly to the central tendon of the diaphragm. This fibrous layer prevents acute overdistension of the heart and maintains cardiac position within the mediastinum.

The serous pericardium consists of two layers that form a closed sac around the heart. The parietal serous pericardium lines the inner surface of the fibrous pericardium, while the visceral serous pericardium (also called the epicardium) adheres directly to the heart surface. Between these layers lies the pericardial cavity, a potential space containing approximately 20 to 50 milliliters of serous fluid that lubricates the heart during its constant motion.

Two important recesses exist within the pericardial cavity. The transverse sinus is a passage behind the aorta and pulmonary trunk but anterior to the superior vena cava. Surgeons can pass a finger or surgical clamp through this sinus to access and control the great arterial vessels during cardiac surgery. The oblique sinus is a blind pouch behind the left atrium, formed by the reflection of pericardium around the pulmonary veins and inferior vena cava.

The pericardium receives its blood supply primarily from the pericardiacophrenic artery, a branch of the internal thoracic artery. The phrenic nerve, which provides the only motor supply to the diaphragm, travels alongside these vessels on its course through the thorax.

<image>Panel A: Sagittal section showing the tough white fibrous pericardium as the outermost layer with the thin parietal serous pericardium lining its interior. Panel B: The visceral serous pericardium (epicardium) in yellow adherent to the myocardium in red with the pericardial cavity depicted as a thin fluid-filled space. Panel C: Transverse section at the great vessels level showing the transverse sinus as a passage between the aorta and pulmonary trunk anteriorly and the SVC posteriorly. Panel D: The oblique sinus shown as a recess posterior to the left atrium bounded by the pulmonary veins.</image>

Cardiac Chambers

The heart contains four chambers: two thin-walled atria that receive blood and two thick-walled ventricles that eject blood into the great arteries.

The right atrium receives deoxygenated systemic venous blood through three openings: the superior vena cava superiorly, the inferior vena cava inferiorly, and the coronary sinus, which drains the coronary circulation. The interior of the right atrium is divided into a smooth-walled posterior portion derived embryologically from the sinus venosus and a rough-walled anterior portion including the right atrial appendage. The crista terminalis is a vertical muscular ridge that marks this division, visible on the surface as the sulcus terminalis. Pectinate muscles, which are parallel muscular ridges, extend anteriorly from the crista terminalis into the auricle. The fossa ovalis is a depression in the interatrial septum marking the closed foramen ovale, the fetal communication between the atria.

The right ventricle receives blood from the right atrium through the tricuspid valve and ejects it through the pulmonary valve into the pulmonary trunk. Its interior is heavily trabeculated with muscular ridges called trabeculae carneae. Three papillary muscles—anterior, posterior, and septal—anchor the tricuspid valve leaflets via the chordae tendineae, preventing valve prolapse during ventricular systole. The moderator band is a distinctive muscular trabecula that crosses from the interventricular septum to the base of the anterior papillary muscle, carrying the right bundle branch of the conduction system. The outflow region, called the conus arteriosus or infundibulum, is smooth-walled and leads to the pulmonary valve.

The left atrium receives oxygenated blood from four pulmonary veins entering its posterior wall. Most of the left atrium is smooth-walled, with pectinate muscles confined to the left atrial appendage. The wall is slightly thicker than the right atrium despite experiencing similar pressures.

The left ventricle has the thickest walls of any cardiac chamber, approximately three times thicker than the right ventricle, reflecting its role in generating systemic arterial pressure. Blood enters through the mitral (bicuspid) valve and exits through the aortic valve. Two large papillary muscles, the anterolateral and posteromedial, attach via chordae tendineae to the two mitral valve leaflets. The outflow region, called the aortic vestibule, has smooth walls and lies immediately below the aortic valve. The interventricular septum separates the ventricles and consists of a thick muscular portion and a thin membranous portion superiorly.

<image>Panel A: Right atrium in blue showing the crista terminalis, pectinate muscles, fossa ovalis, and openings for the SVC, IVC, and coronary sinus. Panel B: Right ventricle in blue displaying thick trabeculae carneae, the moderator band crossing to the anterior papillary muscle, and chordae tendineae attached to the tricuspid valve. Panel C: Left atrium in red showing smooth walls and the four pulmonary vein openings. Panel D: Left ventricle in red demonstrating thick myocardium, anterolateral and posteromedial papillary muscles with chordae to the mitral valve, smooth aortic vestibule, and the membranous and muscular portions of the interventricular septum.</image>

Cardiac Valves

The cardiac valves ensure unidirectional blood flow through the heart. Two atrioventricular valves regulate flow from atria to ventricles, while two semilunar valves guard the ventricular outflow tracts.

The tricuspid valve, positioned between the right atrium and right ventricle, has three leaflets—anterior, posterior, and septal—each attached by chordae tendineae to its corresponding papillary muscle. The mitral (bicuspid) valve guards the left atrioventricular orifice and has two leaflets, the larger anterior leaflet and the smaller posterior leaflet. Both atrioventricular valves are complex structures whose competence depends on the integrated function of the annulus, leaflets, chordae tendineae, papillary muscles, and ventricular wall.

The semilunar valves differ structurally from the atrioventricular valves. Each consists of three cup-shaped cusps without chordae tendineae or papillary muscles. The pulmonary valve has anterior, right, and left cusps, each with a corresponding pulmonary sinus in the vessel wall. The aortic valve has right, left, and posterior (non-coronary) cusps. The aortic sinuses (sinuses of Valsalva) behind the right and left cusps contain the ostia of the right and left coronary arteries, respectively.

The cardiac skeleton provides structural support for the valves. This framework of dense fibrous tissue surrounds all four valve orifices and separates the muscular walls of the atria and ventricles. The skeleton serves multiple functions: it provides firm attachment for the valve leaflets, anchors the myocardial fibers, and electrically insulates the atria from the ventricles, ensuring that electrical impulses can only pass through the specialized conduction system.

<image>Panel A: Superior view of the heart with atria removed showing the fibrous skeleton as a white framework connecting all four valve rings. Panel B: The tricuspid valve on the right with three leaflets (anterior, posterior, septal) in blue and the mitral valve on the left with two leaflets (anterior and posterior) in red. Panel C: The pulmonary valve and aortic valve shown as three-cusped semilunar structures with a cross-sectional inset of a thin avascular valve leaflet. Panel D: Close-up of the aortic valve showing coronary artery ostia visible in the right and left aortic sinuses behind the corresponding cusps.</image>

Coronary Circulation

The heart's metabolic demands are supplied by the right and left coronary arteries, which arise from the aortic sinuses immediately above the aortic valve.

The right coronary artery emerges from the right aortic sinus and travels in the right atrioventricular groove. In approximately 60 percent of individuals, the sinoatrial nodal artery arises as an early branch of the RCA, supplying the primary pacemaker of the heart. The right marginal artery courses along the inferior border of the heart to supply the right ventricular free wall. The RCA continues in the posterior atrioventricular groove, where it gives rise to the atrioventricular nodal artery in 90 percent of individuals. In about 70 percent of the population (right-dominant circulation), the RCA gives rise to the posterior descending artery, which runs in the posterior interventricular groove and supplies the posterior one-third of the interventricular septum and the inferior wall of the left ventricle.

The left coronary artery arises from the left aortic sinus and passes behind the pulmonary trunk before dividing into two major branches. The left anterior descending artery (LAD) travels in the anterior interventricular groove toward the cardiac apex, supplying the anterior wall of the left ventricle and the anterior two-thirds of the interventricular septum. Diagonal branches from the LAD supply the anterolateral left ventricular wall. The left circumflex artery (LCx) continues in the left atrioventricular groove, giving off obtuse marginal branches to supply the lateral wall of the left ventricle. In left-dominant circulation (10 percent of individuals), the circumflex gives rise to the posterior descending artery. In codominant circulation (20 percent), both the RCA and LCx contribute to posterior supply.

Coronary venous drainage largely parallels the arterial supply. The great cardiac vein accompanies the LAD, receiving tributaries from the anterior left ventricle. It then courses in the left atrioventricular groove with the circumflex artery before draining into the coronary sinus. The middle cardiac vein accompanies the posterior descending artery. The small cardiac vein drains the right ventricular wall. These veins converge into the coronary sinus, which lies in the posterior atrioventricular groove and empties into the right atrium. Anterior cardiac veins drain directly from the right ventricular wall into the right atrium without passing through the coronary sinus. The thebesian veins are small vessels that drain directly into any of the four cardiac chambers.

<image>Panel A: Anterior heart view showing the left main coronary artery bifurcating into the LAD in the anterior interventricular groove and the circumflex in the left AV groove with their branches. Panel B: The RCA in the right AV groove with the right marginal branch on anterior view, and posterior view showing the posterior descending artery and AV nodal branch. Panel C: Color-coded coronary territories with LAD territory in dark red, circumflex in medium red, and RCA in orange. Panel D: Venous drainage in blue showing the great cardiac vein, middle cardiac vein, and small cardiac vein draining into the coronary sinus which empties into the right atrium.</image>

Cardiac Histology

The cardiac wall consists of three layers. The endocardium lines the chambers and is continuous with the endothelium of blood vessels. It consists of endothelium, a subendothelial layer of connective tissue, and a deeper subendocardial layer containing Purkinje fibers of the conduction system. The myocardium, the thickest layer, is composed of cardiac muscle and is responsible for contraction. The epicardium (visceral pericardium) covers the external surface and consists of mesothelium overlying connective tissue that contains the coronary vessels and variable amounts of adipose tissue.

Cardiac muscle cells, or cardiomyocytes, are branched cylindrical cells approximately 100 micrometers long and 15 micrometers in diameter. Unlike skeletal muscle fibers, they typically contain one or two centrally located nuclei. The cytoplasm is densely packed with myofibrils arranged in sarcomeres, giving the cells their striated appearance. T-tubules penetrate the cells at the level of the Z-lines, forming dyads rather than the triads seen in skeletal muscle. The sarcoplasmic reticulum is less extensive than in skeletal muscle, reflecting the greater reliance on extracellular calcium for contraction.

The most distinctive feature of cardiac muscle is the intercalated disc, the specialized junction between adjacent cardiomyocytes. These step-like structures appear as dark lines crossing the cells in longitudinal sections. Each intercalated disc contains three types of junctions. Fascia adherens junctions anchor actin filaments of the terminal sarcomeres, transmitting contractile force between cells. Desmosomes provide mechanical coupling, resisting the shear forces generated during contraction. Gap junctions, formed by connexin proteins, provide low-resistance electrical coupling, allowing action potentials to spread rapidly from cell to cell. This electrical coupling allows the heart to function as a syncytium, contracting in a coordinated wave.

<image>Panel A: Longitudinal section of cardiac muscle showing branched cardiomyocytes with central nuclei, prominent cross-striations, and intercalated discs as dark step-like lines. Panel B: Electron micrograph of an intercalated disc showing fascia adherens anchoring actin filaments, desmosomes providing mechanical connections, and gap junctions as closely apposed membranes. Panel C: Skeletal muscle comparison showing fibers with peripheral nuclei, cylindrical non-branching morphology, and triads at A-I junctions. Panel D: T-tubule arrangement comparison showing cardiac diads at Z-lines versus skeletal triads at A-I junctions.</image>

Cardiac Conduction System

The cardiac conduction system is a specialized network of modified cardiomyocytes that initiates and coordinates the rhythmic contraction of the heart.

The sinoatrial (SA) node is the heart's primary pacemaker, located in the right atrium near the junction with the superior vena cava. It consists of small, pale cells with few myofibrils that spontaneously depolarize at a rate of 60 to 100 beats per minute. The SA node is supplied by the sinoatrial nodal artery (from the RCA in 60 percent of individuals, from the LCx in 40 percent) and is richly innervated by autonomic nerves that modulate heart rate.

From the SA node, impulses spread through the atrial myocardium, reaching the atrioventricular (AV) node located in the interatrial septum within the Triangle of Koch. This triangle is bounded by the tendon of Todaro, the septal leaflet of the tricuspid valve, and the orifice of the coronary sinus. The AV node delays impulse transmission for approximately 100 milliseconds, allowing atrial contraction to complete before ventricular activation. The AV node is supplied by the AV nodal artery (from the RCA in 90 percent of individuals) and represents the only normal electrical connection between the atria and ventricles.

The bundle of His emerges from the AV node and penetrates the fibrous cardiac skeleton to enter the interventricular septum. It divides into the right and left bundle branches. The right bundle branch travels down the right side of the septum, crossing to the anterior papillary muscle via the moderator band. The left bundle branch divides into anterior and posterior fascicles that spread across the left ventricular endocardium.

The terminal branches of the conduction system are the Purkinje fibers, which spread beneath the endocardium and penetrate into the myocardium. These are the largest cells in the conduction system, pale in appearance with few myofibrils and abundant glycogen. They conduct impulses at the fastest rate in the heart (2 to 4 meters per second), ensuring near-simultaneous activation of the ventricular myocardium from apex to base.

<image>Panel A: The SA node as a crescent-shaped structure near the SVC-right atrial junction in yellow with an inset showing small pacemaker cells with sparse myofibrils. Panel B: The AV node within the Triangle of Koch bounded by the coronary sinus, tendon of Todaro, and tricuspid valve septal leaflet with internodal pathways shown as three distinct routes. Panel C: The bundle of His penetrating the fibrous skeleton and dividing into right and left bundle branches traveling down the interventricular septum with the moderator band highlighted. Panel D: The Purkinje fiber network as a fine yellow mesh beneath the ventricular endocardium with a histological inset showing large pale cells with peripheral myofibrils and abundant central glycogen.</image>

Innervation of the Heart

The heart receives both sympathetic and parasympathetic innervation through the cardiac plexus, a network of nerve fibers located at the base of the heart.

Sympathetic innervation arises from the T1 through T4 spinal segments. Preganglionic fibers synapse in the cervical and upper thoracic sympathetic ganglia, and postganglionic fibers travel to the heart via cardiac nerves. The sympathetic nerves release norepinephrine, which acts on beta-1 adrenergic receptors to increase heart rate (positive chronotropy), increase conduction velocity (positive dromotropy), increase contractility (positive inotropy), and enhance the rate of relaxation (positive lusitropy).

Parasympathetic innervation reaches the heart through the vagus nerve (cranial nerve X). Preganglionic fibers synapse in ganglia within the cardiac plexus or in the atrial walls, and short postganglionic fibers release acetylcholine. Vagal effects are most prominent on the SA node, AV node, and atrial myocardium, producing decreased heart rate and slowed AV conduction. Parasympathetic effects on ventricular myocardium are minimal.

At rest, parasympathetic tone predominates. This is why the resting heart rate of approximately 70 beats per minute is lower than the SA node's intrinsic rate of 100 beats per minute. Cutting the vagus nerves results in tachycardia.

Afferent (sensory) fibers from the heart travel with both sympathetic and parasympathetic nerves. Pain fibers, activated by ischemia, travel with sympathetic afferents to spinal segments T1 through T4, explaining the referred pain patterns of angina pectoris to the chest, left arm, and jaw. Reflexogenic fibers involved in cardiovascular reflexes travel with vagal afferents.

<image>Panel A: Sympathetic pathway in green originating from T1-T4 spinal segments, synapsing in cervical and upper thoracic ganglia, and projecting to the cardiac plexus. Panel B: Parasympathetic pathway in blue with the vagus nerve descending from the brainstem to the cardiac plexus showing both superficial and deep components. Panel C: Distribution of nerve fibers to the SA node, AV node, atrial myocardium, and ventricular myocardium with receptor types labeled (beta-1 for sympathetic, M2 for parasympathetic). Panel D: Afferent pain pathways returning via sympathetic routes to T1-T4 dermatomes with a human figure showing referred pain distribution in chest, left arm, and jaw.</image>

Clinical Correlations

Coronary artery disease affects different territories depending on which vessel is occluded. Left anterior descending artery occlusion causes anterior wall infarction, manifesting with ST elevation in leads V1 through V4 and potentially causing left ventricular dysfunction and heart failure. Right coronary artery occlusion typically causes inferior wall infarction with ST elevation in leads II, III, and aVF, and may cause conduction abnormalities if the AV nodal artery is involved. Left circumflex occlusion affects the lateral wall with changes in leads I, aVL, V5, and V6.

Valvular heart disease produces characteristic hemodynamic consequences. Mitral stenosis impedes left atrial emptying, causing left atrial enlargement and pulmonary congestion. Aortic stenosis creates a pressure gradient across the valve, leading to left ventricular hypertrophy and symptoms of syncope, angina, and heart failure. Mitral regurgitation allows backflow into the left atrium during systole, causing both atrial and ventricular volume overload.

Pericarditis, inflammation of the pericardium, causes chest pain typically relieved by leaning forward and produces a pericardial friction rub on auscultation. Pericardial effusion may accumulate in the pericardial cavity. If fluid accumulates rapidly, it can compress the heart and impair filling, producing cardiac tamponade. Beck's triad of hypotension, jugular venous distension, and muffled heart sounds characterizes this life-threatening condition.

Conduction system disease causes various arrhythmias. SA node dysfunction produces sick sinus syndrome with bradycardia and pauses. AV nodal disease causes heart block of varying degrees, from prolonged PR interval to complete dissociation between atrial and ventricular activity. Bundle branch block produces characteristic widened QRS patterns on the electrocardiogram.

<image>Panel A: Coronary artery territories mapped onto a 12-lead ECG with LAD territory (V1-V4), RCA territory (II, III, aVF), and LCx territory (I, aVL, V5-V6) color-coded on heart and ECG leads. Panel B: Cardiac tamponade with pericardial effusion compressing chambers and an inset showing Beck's triad (hypotension, JVD, muffled sounds). Panel C: ECG patterns of heart block including first-degree (prolonged PR), second-degree Mobitz I and II, and third-degree (complete AV dissociation). Panel D: Bundle branch block patterns showing RBBB (RSR' in V1) and LBBB (broad R in V6) QRS morphologies.</image>

Summary

The heart is located in the middle mediastinum, oriented with its apex pointing left, anterior, and inferior. The pericardium consists of fibrous and serous layers with transverse and oblique sinuses providing surgical access. The four chambers have distinct structural features: the right atrium receives systemic venous return and has the crista terminalis and fossa ovalis; the right ventricle has trabeculations, papillary muscles, and the moderator band; the left atrium receives pulmonary veins and is mostly smooth-walled; and the left ventricle has the thickest walls with two papillary muscles supporting the mitral valve.

The atrioventricular valves have leaflets attached via chordae tendineae to papillary muscles, while semilunar valves consist of three cusps without supporting structures. The coronary arteries arise from the aortic sinuses, with the RCA supplying the right side and often the inferior wall, and the LCA dividing into the LAD and circumflex to supply the anterior and lateral walls.

Cardiac muscle is striated and branched, with central nuclei and intercalated discs containing gap junctions for electrical coupling. The conduction system consists of the SA node (primary pacemaker), AV node (delays impulse for atrial contraction), bundle of His, bundle branches, and Purkinje fibers providing rapid ventricular activation.

Key Terms

This content is subject to the MIT License. © 2024–2026 Hibbert School of Medicine.

Read this lecture as Markdown