Anatomy of the Human Heart: Chambers, Valves & Myocardial Dynamics

1. Gross Morphology and Topographical Orientation in the Thorax

The human heart is a hollow, muscular, cone-shaped organ roughly the size of a closed adult fist, measuring approximately 12 cm in length, 8 to 9 cm in breadth at its widest point, and weighing between 250 and 300 grams in adult females and 300 to 350 grams in adult males. It resides centrally within the thoracic cavity inside the middle mediastinum, protected anteriorly by the sternum and costal cartilages, flanked laterally by the lungs and pleurae, resting inferiorly on the central tendon of the diaphragm, and bounded posteriorly by the esophagus and descending thoracic aorta.

The organ is oriented obliquely in three dimensions: approximately two-thirds of its mass lies to the left of the median sagittal plane. It presents a broad superior-posterior base (formed predominantly by the left atrium) where the great vessels emerge, and an anterior-inferior apex directed downward, forward, and to the left. The cardiac apex is formed entirely by the muscular tip of the left ventricle, situated deep to the fifth left intercostal space approximately 8 to 9 cm lateral to the midsternal line, where the apical cardiac impulse (point of maximal impulse, PMI) is readily palpated.

Coronal section of the human heart displaying the right and left atria, thick-walled ventricles, atrioventricular valves, and semilunar outlets. Via Wikimedia Commons.

2. The Cardiac Wall: Epicardium, Myocardium, and Endocardium

The wall of the human heart is composed of three distinct histological layers encased within a protective fibroserous sac termed the pericardium:

1. The Pericardial Sac: Consisting of an outer tough, inelastic fibrous pericardium that prevents acute cardiac over-distension and anchors the heart within the mediastinum, and an inner serous pericardium subdivided into a parietal layer lining the fibrous sac and a visceral layer (epicardium) adhering to the myocardium. The narrow pericardial cavity between them contains 15 to 50 mL of clear serous pericardial fluid that eliminates frictional resistance during rhythmic cardiac contractions.

2. The Epicardium: The visceral serous layer composed of a simple squamous mesothelium overlying a fibroelastic subepicardial layer containing adipose tissue, autonomic cardiac nerve plexuses, and the main branches of the coronary arteries and cardiac veins.

3. The Myocardium: The thick, contractile middle layer comprising specialized branching cardiac myocytes arranged in complex figure-eight and helical spiral bundles around the ventricular chambers. This helical architectural orientation causes the ventricles to wring and twist during systole, maximizing ejection fraction efficiency.

4. The Endocardium: A smooth, glistening luminal lining consisting of simple squamous endothelial cells continuous with the vascular endothelium of the great vessels, overlying a subendocardial layer containing the Purkinje conduction network.

Cardiac Chamber	Wall Thickness & Geometry	Venous / Arterial Inflow	Outflow Valve & Conduit	Hemodynamic Role
Right Atrium	2 - 3 mm; pectinate muscles	Superior & Inferior Venae Cavae, Coronary Sinus	Tricuspid Valve into Right Ventricle	Low-pressure (0 - 4 mmHg) reservoir receiving systemic deoxygenated venous return.
Right Ventricle	4 - 5 mm; crescent-shaped lumen	From Right Atrium via Tricuspid Valve	Pulmonary Semilunar Valve into Pulmonary Trunk	Compliant low-pressure pump (systolic 15 - 25 mmHg) supplying pulmonary gas exchange.
Left Atrium	2 - 3 mm; smooth-walled interior	Four Pulmonary Veins (2 left, 2 right)	Bicuspid (Mitral) Valve into Left Ventricle	Receives oxygen-rich blood from lungs; contracts to top off left ventricular end-diastolic volume.
Left Ventricle	10 - 15 mm; thick circular cylinder	From Left Atrium via Mitral Valve	Aortic Semilunar Valve into Ascending Aorta	High-pressure muscular pump (systolic 100 - 140 mmHg) driving systemic arterial circulation.
Fibrous Skeleton	Dense collagenous rings (annuli fibrosi)	Surrounds all 4 valve orifices	Electrically isolates atria from ventricles	Provides structural anchorage for valve leaflets and origins for myocardial muscle bundles.

Structural Dimensions and Hemodynamic Profiles of the Four Cardiac Chambers

Gross anatomical exterior view illustrating the coronary sulcus, anterior interventricular artery, and great vessel roots. Via Wikimedia Commons.

3. Internal Cavitary Architecture and the Four Cardiac Valves

The internal volume of the heart is completely partitioned into right and left halves by continuous septa, each side possessing a receiving atrium and a pumping ventricle:

Atrioventricular (AV) Valves: Tricuspid and Mitral

1. The Tricuspid Valve: Positioned between the right atrium and right ventricle, composed of three flexible fibrous cusps (anterior, posterior, and septal). The free margins of these cusps are tethered by fibrous cords called chordae tendineae to muscular papillary muscles projecting from the ventricular wall. During ventricular systole, papillary muscles contract synchronously, exerting tension on the chordae tendineae to prevent valve prolapse into the atrium.

2. The Mitral (Bicuspid) Valve: Located between the left atrium and left ventricle, consisting of two heavy, durable cusps (anterior and posterior). The mitral valve withstands immense retrograde pressures during left ventricular systole (up to 140 mmHg), supported by robust paired papillary muscles.

Semilunar Valves: Aortic and Pulmonary

1. The Aortic Valve: Located at the root of the ascending aorta, comprising three pocket-like semilunar cusps (right coronary, left coronary, and posterior non-coronary). Immediately superior to the cusps lie the aortic sinuses of Valsalva, from which the coronary arteries originate.

2. The Pulmonary Valve: Located at the entrance to the pulmonary trunk, consisting of three thin semilunar cusps (anterior, right, and left). When ventricular systole ceases and ventricular pressure falls below arterial pressure, backward blood flow fills the crescent pockets, snapping the cusps shut to prevent regurgitation.

The Fibrous Skeleton of the Heart

The four valve orifices are embedded within a dense, tough connective tissue framework called the fibrous skeleton (annuli fibrosi). Beyond providing mechanical structural support and preventing valve ring dilation, the fibrous skeleton acts as a critical electrical insulator between atrial and ventricular myocardia, ensuring that action potentials can only traverse from atria to ventricles via the specialized atrioventricular (AV) bundle of His.

Thoracic surface anatomy mapping the apex beat and sternocostal cardiac borders relative to the rib cage. Via Wikimedia Commons.

4. Coronary Circulation: The Myocardial Blood Supply

Because the thick, working myocardium cannot extract nutrients or oxygen directly from blood flowing through its internal chambers, the heart possesses its own specialized microvascular network termed the coronary circulation:

1. Left Coronary Artery (LCA): Arising from the left aortic sinus, the LCA quickly divides into the anterior interventricular artery (left anterior descending, LAD) and the circumflex artery. The LAD travels down the anterior interventricular sulcus, supplying the anterior wall of both ventricles and the anterior two-thirds of the interventricular septum. The circumflex artery circles around the left border to supply the left atrium and posterolateral left ventricular wall.

2. Right Coronary Artery (RCA): Arising from the right aortic sinus, the RCA courses through the coronary sulcus, giving off the marginal branch supplying the right ventricle and the posterior descending artery (PDA) supplying the inferior-posterior myocardium and the sinoatrial and atrioventricular nodal pacemaker tissues.

3. Unique Hemodynamic Perfusion Profile: Because myocardial contraction compresses intramural coronary arterioles during systole, more than 80% of coronary blood flow to the left ventricle occurs during ventricular diastole when the muscle relaxes.

4. Venous Drainage: Deoxygenated coronary venous blood collects in the great, middle, and small cardiac veins, converging into the broad coronary sinus on the posterior surface of the heart, which empties directly into the right atrium.

5. Cardiac Innervation and Visceral Sensory Neuroanatomy

Although intrinsic auto-rhythmic pacemaker cells initiate every heartbeat, the rate, contractile force, and conduction velocity of the heart are modulated continuously by the autonomic cardiac plexus situated beneath the aortic arch:

1. Sympathetic Postganglionic Fibers: Originating from the superior, middle, and inferior cervical ganglia and upper thoracic sympathetic chain ganglia (T1-T4). Sympathetic adrenergic terminals release norepinephrine onto beta-1 adrenergic receptors throughout the SA node, AV node, and working ventricular myocardium, increasing heart rate (positive chronotropy), increasing contractile force (positive inotropy), and dilating coronary arteries.

2. Parasympathetic Preganglionic Fibers: Carried by the right and left vagus nerves (cranial nerve X), synapsing in tiny intrinsic ganglia within the subepicardium near the SA and AV nodes. Vagal postganglionic terminals release acetylcholine onto muscarinic M2 receptors, hyperpolarizing nodal cells and exerting powerful negative chronotropic inhibition over resting heart rate.

3. Visceral Afferent Pain Pathways and Angina: Ischemia of cardiac myocytes activates chemosensitive and mechanosensitive unmyelinated sympathetic sensory afferents. These visceral fibers travel backward through the cardiac plexus and upper thoracic sympathetic chain to enter spinal cord segments T1 through T4. Because somatic sensory fibers from the chest wall, neck, and medial aspect of the left arm enter these identical dorsal horn spinal segments, visceral cardiac pain is perceived centrally as radiating somatic pain—the classic clinical phenomenon of referred angina pectoris.

Key Terms & Functional Definitions

A quick-reference glossary defining key biological terms, cellular structures, and transport mechanisms covered in this guide.

Mediastinum	The central anatomical compartment of the thoracic cavity between the pleural sacs housing the heart and great vessels.
Pericardium	A double-walled fibroserous sac that encloses the heart and root of the great vessels, containing serous fluid.
Myocardium	The thick, contractile middle layer of the heart wall composed of specialized striated cardiac muscle cells.
Endocardium	The smooth simple squamous endothelial lining of the heart chambers continuous with vascular endothelium.
Tricuspid Valve	The right atrioventricular valve composed of three fibrous cusps preventing regurgitation into the right atrium.
Mitral Valve	The left atrioventricular valve composed of two robust cusps; also termed the bicuspid valve.
Chordae Tendineae	Inelastic fibrous cords connecting papillary muscles to the free margins of atrioventricular valve cusps.
Papillary Muscles	Muscular projections arising from the ventricular walls that contract during systole to tighten chordae tendineae.
Fibrous Skeleton	Dense connective tissue rings surrounding valve orifices providing structural support and electrical insulation.
Coronary Sinus	A broad venous vascular channel on the posterior surface of the heart that returns myocardial venous blood to the right atrium.

Core Scientific Terminology in Human Cardiac Anatomy

Key Takeaways

• The human heart is a four-chambered muscular pump located in the middle mediastinum, surrounded by a double-walled pericardium.
• The cardiac wall consists of three histological layers: visceral epicardium, contractile myocardium, and luminal endocardium.
• Atrioventricular valves (tricuspid and mitral) are anchored by chordae tendineae to papillary muscles to prevent systolic regurgitation.
• Semilunar valves (aortic and pulmonary) prevent retrograde flow into ventricles during diastole without chordal tethering.
• The fibrous skeleton provides structural valve support and acts as an electrical insulator between atria and ventricles.

Frequently Asked Questions

Why is the left ventricular myocardium three times thicker than the right?

The left ventricle must generate enough systolic pressure (120 mmHg) to overcome high peripheral vascular resistance across the entire systemic circulation, whereas the right ventricle only pumps against low resistance in pulmonary vessels (20 mmHg).

What prevents the atrioventricular valves from inverting during systole?

Papillary muscles contract simultaneously with the ventricles, pulling on chordae tendineae anchored to the valve cusps, bracing them against high retrograde systolic pressures and preventing eversion into the atria.

Why does left coronary blood flow occur mostly during diastole?

During ventricular systole, the powerful contraction of surrounding myocardial muscle fibers compresses intramural coronary capillaries, dramatically elevating resistance. When the myocardium relaxes during diastole, vessels open and blood rushes in from the aorta.

What is the clinical significance of the "widow-maker" artery?

The anterior interventricular artery (left anterior descending, LAD) supplies over 50% of left ventricular myocardium including the septum and apex. Sudden acute occlusion of the proximal LAD typically causes massive myocardial infarction and sudden death.

How does the fibrous skeleton ensure coordinated heart contractions?

By acting as a non-conductive electrical barrier between atria and ventricles, the fibrous skeleton prevents electrical impulses from passing directly across the muscle wall, forcing impulses to route through the atrioventricular node and bundle of His.

What is pericardial tamponade?

Pericardial tamponade is a life-threatening medical emergency where excess fluid, blood, or pus accumulates rapidly inside the rigid fibrous pericardium, compressing the cardiac chambers and preventing normal diastolic filling, leading to acute circulatory collapse.

Key references
• Standring, S.
• Moore, K. L., Dalley, A. F., & Agur, A. M. R.
• Hall, J. E., & Hall, M. E.
• Netter, F. H.

Human cardiac gross anatomy, histological myocardial architecture, valve biomechanics, and coronary microcirculation compiled from standard medical anatomy and cardiovascular reference textbooks.