# Lecture 3: The Heart — Anatomy and Conduction System
## Anatomy and Physiology II
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Describe the location, size, and orientation of the heart in the thoracic cavity
2. Identify the layers of the heart wall and the pericardium
3. Describe the external and internal anatomy of the four chambers
4. Trace the pathway of blood through the heart and identify all valves
5. Describe the coronary circulation
6. Explain the components of the cardiac conduction system and the path of electrical impulses
7. Interpret a normal electrocardiogram (ECG)
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## Lecture Content
### I. Location and Orientation of the Heart
The heart sits in the mediastinum, the central compartment of the thoracic cavity, with approximately two-thirds of its mass lying to the left of the midsternal line. It is roughly the size of a closed fist, measuring about 12 cm in length, 9 cm in width, and weighing between 250 and 350 grams. The apex, the pointed inferior tip of the heart, is directed anteriorly, inferiorly, and to the left, landing at the fifth intercostal space along the midclavicular line. The base, which is the broad, flat superior portion, faces posteriorly and is where the great vessels attach. The heart is bordered laterally by the lungs, posteriorly by the vertebral column, and inferiorly by the diaphragm.
### II. Pericardium
The heart is enclosed within a double-walled sac called the pericardium, which protects it and anchors it in place. The outer **fibrous pericardium** is a tough layer of dense connective tissue that attaches the heart to the diaphragm, the great vessels, and the sternum. It serves the important function of preventing the heart from overstretching.
Deep to the fibrous pericardium lies the **serous pericardium**, a double-layered membrane. Its **parietal layer** lines the inner surface of the fibrous pericardium, while its **visceral layer**, also called the epicardium, adheres directly to the heart surface. Between these two layers lies the **pericardial cavity**, a potential space containing 15 to 50 mL of serous pericardial fluid that reduces friction with each heartbeat. When fluid accumulates pathologically in this space, the resulting compression of the heart is called **cardiac tamponade**, a potentially life-threatening condition.
### III. Layers of the Heart Wall
The heart wall consists of three layers. The **epicardium**, the outermost layer and the visceral pericardium itself, is a serous membrane containing fat, coronary blood vessels, and nerves. The **myocardium**, the thick middle layer, is composed of cardiac muscle tissue and is responsible for the heart's pumping action. It is thickest in the left ventricle, reflecting that chamber's need to generate the highest pressures. Cardiac muscle fibers are branched and striated, and they are connected to one another by specialized junctions called intercalated discs. These discs contain desmosomes for mechanical coupling and gap junctions for electrical coupling. The gap junctions allow ions to flow rapidly from cell to cell, enabling the heart to function as a functional syncytium in which electrical impulses spread seamlessly through the muscle. The **endocardium**, the innermost layer, consists of a thin endothelium that lines the heart chambers, is continuous with the endothelium of the blood vessels, and covers the heart valves.
A cross-sectional diagram of the heart wall and pericardium. The layers from outermost to innermost are shown in a magnified wedge section: fibrous pericardium, parietal layer of serous pericardium, pericardial cavity (with fluid indicated), visceral layer of serous pericardium (epicardium) containing adipose tissue and coronary vessels, thick myocardium showing cardiac muscle fibers with intercalated discs highlighted in an inset, and endocardium lining the chamber interior. Each layer is distinctly colored and clearly labeled.
### IV. External Anatomy of the Heart
Several surface grooves, or sulci, mark the boundaries between the heart's chambers. The **coronary sulcus** (atrioventricular groove) encircles the heart and separates the atria from the ventricles. The **anterior interventricular sulcus** runs along the anterior surface, marking the boundary between the right and left ventricles, while the **posterior interventricular sulcus** marks the same boundary on the posterior surface. Small, ear-shaped appendages called **auricles** project from each atrium and serve to increase atrial volume.
### V. Internal Anatomy — Chambers and Valves
#### Atria (Right and Left)
The atria are the heart's receiving chambers and have relatively thin walls. The **right atrium** receives deoxygenated blood from three sources: the superior vena cava (draining the head, neck, and upper limbs), the inferior vena cava (draining the trunk and lower limbs), and the coronary sinus (draining the heart wall itself). Internally, it features pectinate muscles along its anterior wall and the fossa ovalis, a remnant of the foramen ovale from fetal circulation. The **left atrium** receives oxygenated blood from the four pulmonary veins and has a smooth posterior wall.
#### Ventricles (Right and Left)
The ventricles are the heart's pumping chambers and have substantially thicker walls. The **right ventricle** pumps blood to the lungs via the pulmonary trunk. Its wall is thinner than that of the left ventricle because it pumps against the relatively low resistance of the pulmonary circuit. Internally, it displays trabeculae carneae (muscular ridges), papillary muscles, and chordae tendineae. The **left ventricle** pumps blood to the entire body via the aorta and possesses the thickest myocardium, roughly three times thicker than that of the right ventricle. It shares the same internal features as the right ventricle. The two ventricles are separated by the muscular **interventricular septum**.
#### Heart Valves
The heart contains two types of valves that prevent the backflow of blood. The **atrioventricular (AV) valves** sit between the atria and ventricles. On the right side, the **tricuspid valve** has three cusps, while on the left, the **bicuspid (mitral) valve** has two. The cusps of both AV valves are anchored by chordae tendineae to papillary muscles, which contract during ventricular systole to prevent the valve leaflets from prolapsing (inverting) into the atria.
The **semilunar (SL) valves** guard the exits from the ventricles into the great arteries. The **pulmonary semilunar valve** sits at the base of the pulmonary trunk, and the **aortic semilunar valve** sits at the base of the aorta. Each semilunar valve has three pocket-like cusps that are self-supporting and do not require chordae tendineae.
### VI. Blood Flow Through the Heart
Blood flows through the heart in two parallel circuits. On the **right side**, deoxygenated blood enters the right atrium via the superior and inferior venae cavae and the coronary sinus, passes through the tricuspid valve into the right ventricle, and is ejected through the pulmonary semilunar valve into the pulmonary trunk. The pulmonary trunk splits into right and left pulmonary arteries carrying blood to the lungs, where gas exchange occurs in the pulmonary capillaries. Oxygenated blood then returns to the left atrium via four pulmonary veins.
On the **left side**, oxygenated blood enters the left atrium from the pulmonary veins, passes through the bicuspid (mitral) valve into the left ventricle, and is ejected through the aortic semilunar valve into the aorta. From the aorta, blood is distributed to body tissues via the systemic arteries, and deoxygenated blood eventually returns via systemic veins to the right atrium, completing the circuit.
An anterior view of the heart in cross-section showing all four chambers. Panel A: The full pathway of blood flow is traced with arrows — blue arrows for deoxygenated blood entering the right atrium from the SVC and IVC, flowing through the tricuspid valve into the right ventricle, then through the pulmonary semilunar valve into the pulmonary trunk and arteries to the lungs; red arrows for oxygenated blood returning from the lungs via pulmonary veins to the left atrium, through the mitral valve into the left ventricle, then through the aortic semilunar valve into the aorta. Panel B: Enlarged views of the AV valves from above, showing the tricuspid valve with three cusps and the mitral valve with two cusps, with chordae tendineae and papillary muscles visible. Panel C: Enlarged view of a semilunar valve showing the three pocket-shaped cusps in open and closed positions.
### VII. Coronary Circulation
The heart muscle requires its own dedicated blood supply, which is provided by the coronary circulation. The **coronary arteries** originate from the base of the aorta, just above the aortic semilunar valve. The **left coronary artery (LCA)** divides into the **left anterior descending artery (LAD)**, which supplies the anterior interventricular septum and the anterior walls of both ventricles, and the **circumflex artery**, which supplies the left atrium and posterior left ventricle. The **right coronary artery (RCA)** branches into the **right marginal artery**, which supplies the lateral right side of the heart, and the **posterior descending artery (PDA)**, which supplies the posterior interventricular septum and posterior ventricular walls.
Venous drainage from the heart muscle flows through the great cardiac vein, middle cardiac vein, and small cardiac vein into the coronary sinus, which empties into the right atrium. An important physiological detail is that the coronary arteries fill primarily during ventricular diastole (relaxation), because during systole the contracting myocardium compresses the coronary vessels and impedes flow. A **myocardial infarction (MI)** occurs when a coronary artery becomes occluded, typically by atherosclerosis or a thrombus, causing death of the heart tissue it supplies.
### VIII. Cardiac Conduction System
The heart possesses intrinsic automaticity, meaning it can generate its own electrical impulses without external nervous input. This property resides in specialized autorhythmic (pacemaker) cells that have unstable resting membrane potentials and undergo spontaneous depolarization, known as pacemaker potentials.
#### Components and Pathway of Conduction
The electrical impulse originates at the **sinoatrial (SA) node**, a small cluster of pacemaker cells located in the right atrial wall near the opening of the superior vena cava. The SA node serves as the primary pacemaker and sets the heart rate. Its intrinsic firing rate is approximately 100 beats per minute, though this is modulated down to about 75 bpm at rest by the autonomic nervous system. The impulse generated by the SA node spreads through both atria via gap junctions and internodal pathways.
The impulse then reaches the **atrioventricular (AV) node**, located in the inferior interatrial septum near the tricuspid valve. Here, conduction is deliberately delayed by approximately 0.1 seconds, a pause called the AV delay that allows the atria to finish contracting before the ventricles begin. The AV node has an intrinsic firing rate of 40 to 60 bpm and can serve as a backup pacemaker if the SA node fails.
From the AV node, the impulse travels through the **atrioventricular (AV) bundle** (Bundle of His), the only electrical connection between the atria and ventricles, as the two are otherwise insulated from each other by the fibrous skeleton of the heart. The AV bundle passes through the interventricular septum and divides into the **right and left bundle branches**, with the left bundle branch further splitting into anterior and posterior fascicles.
Finally, the impulse reaches the **Purkinje fibers** (subendocardial conducting network), terminal branches that spread throughout the ventricular myocardium. These fibers conduct impulses extremely rapidly, ensuring that the ventricles contract in a coordinated fashion from apex to base, which optimizes ejection of blood. The Purkinje fibers have an intrinsic firing rate of 20 to 40 bpm.
### IX. The Electrocardiogram (ECG/EKG)
An electrocardiogram is a recording of the heart's electrical activity as detected by electrodes placed on the body surface. A standard 12-lead ECG uses electrodes on the limbs and chest to capture the electrical events of the cardiac cycle from multiple angles.
#### Normal ECG Waveform
The **P wave** represents atrial depolarization, the moment when the SA node fires and the impulse spreads through the atria. The **QRS complex** represents ventricular depolarization as the impulse spreads through the ventricles via the bundle branches and Purkinje fibers. Atrial repolarization occurs simultaneously but is hidden within the larger QRS complex. The **T wave** represents ventricular repolarization. The **P-R interval**, measured from the start of the P wave to the start of the QRS complex, reflects the time required for atrial depolarization plus the AV delay, and normally ranges from 0.12 to 0.20 seconds. The **Q-T interval** spans from the start of ventricular depolarization to the end of ventricular repolarization. The **S-T segment**, between the end of the QRS complex and the beginning of the T wave, should be isoelectric (flat). Elevation or depression of the ST segment may indicate myocardial ischemia or infarction.
#### Common Arrhythmias
**Sinus tachycardia** is a normal rhythm with a heart rate exceeding 100 bpm, while **sinus bradycardia** is a normal rhythm with a rate below 60 bpm. **Atrial fibrillation** involves chaotic atrial electrical activity with an irregular ventricular response and no distinct P waves. **Ventricular fibrillation** is a medical emergency in which chaotic ventricular activity eliminates effective pumping and requires immediate defibrillation. **Heart block** refers to impaired conduction through the AV node and comes in three degrees: first-degree heart block presents with a prolonged PR interval; second-degree heart block shows some P waves that are not followed by QRS complexes; and third-degree (complete) heart block occurs when the atria and ventricles beat entirely independently of each other.
A diagram of the cardiac conduction system and ECG. Panel A: An anterior view of the heart showing the locations of the SA node, internodal pathways, AV node, Bundle of His, right and left bundle branches, and Purkinje fibers, with arrows showing the direction of impulse propagation. Panel B: A normal ECG tracing on a grid showing a clearly labeled P wave, PR interval, QRS complex, ST segment, T wave, and QT interval, with time and voltage scales indicated. Panel C: A timeline correlating the conduction system activity with each wave of the ECG — SA node firing corresponding to the P wave, AV delay corresponding to the PR segment, ventricular conduction corresponding to the QRS complex, and ventricular repolarization corresponding to the T wave.
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