Anatomy of the Heart

• The heart is a muscular organ responsible for pumping blood throughout the body via the circulatory system, providing oxygen and nutrients to tissues and removing carbon dioxide and other wastes.
• Its size is approximately that of a closed fist and weighs about 250-350 grams in adults.
• The heart is located in the mediastinum, the central compartment of the thoracic cavity, between the lungs.
• Approximately two-thirds of the heart's mass lies to the left of the midline.

Layers of the Heart Wall

• The heart wall consists of three layers: the epicardium, myocardium, and endocardium.
• Epicardium: The outermost layer, also known as the visceral layer of the serous pericardium. It contains blood vessels, lymphatics, and nerves that supply the heart.
• Myocardium: The middle and thickest layer, composed of cardiac muscle tissue responsible for the heart's pumping action.
• Endocardium: The innermost layer, lining the heart chambers and covering the heart valves, made of a thin layer of endothelium and connective tissue.

Heart Chambers

• The heart has four chambers: two atria (right and left) and two ventricles (right and left).
• Right Atrium: Receives deoxygenated blood from the superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus.
• Left Atrium: Receives oxygenated blood from the pulmonary veins (typically four: two from the right lung and two from the left lung).
• Right Ventricle: Receives deoxygenated blood from the right atrium and pumps it to the lungs via the pulmonary artery.
• Left Ventricle: Receives oxygenated blood from the left atrium and pumps it to the body via the aorta. It is the thickest chamber of the heart.

Heart Valves

• The heart has four valves that ensure unidirectional blood flow: the tricuspid valve, the mitral valve, the pulmonary valve, and the aortic valve.
• Tricuspid Valve: Located between the right atrium and right ventricle, it has three cusps.
• Mitral Valve: Located between the left atrium and left ventricle, also known as the bicuspid valve, it has two cusps.
• Pulmonary Valve: Located between the right ventricle and the pulmonary artery, it has three semilunar cusps.
• Aortic Valve: Located between the left ventricle and the aorta, it also has three semilunar cusps.

Blood Flow Through the Heart

• Deoxygenated blood enters the right atrium through the SVC, IVC, and coronary sinus.
• Blood passes through the tricuspid valve into the right ventricle.
• The right ventricle pumps blood through the pulmonary valve into the pulmonary artery, which carries it to the lungs for oxygenation.
• Oxygenated blood returns to the left atrium via the pulmonary veins.
• Blood passes through the mitral valve into the left ventricle.
• The left ventricle pumps blood through the aortic valve into the aorta, which distributes it to the systemic circulation.

Pericardium

• The pericardium is a double-layered sac that surrounds and protects the heart. It consists of two main layers: the fibrous pericardium and the serous pericardium.
• Fibrous Pericardium: The outer layer, made of tough, inelastic connective tissue, which anchors the heart in the mediastinum and prevents overfilling.
• Serous Pericardium: The inner layer, divided into two layers: the parietal layer (fused to the fibrous pericardium) and the visceral layer (epicardium), which adheres to the heart's surface.
• Pericardial Cavity: The space between the parietal and visceral layers of the serous pericardium, containing a small amount of serous fluid that reduces friction during heart contractions.

Coronary Circulation

• The heart receives its blood supply from the coronary arteries, which arise from the aorta near the aortic valve.
• Right Coronary Artery (RCA): Supplies the right atrium, right ventricle, and part of the left ventricle. It typically gives rise to the posterior descending artery (PDA), which supplies the posterior aspect of the heart.
• Left Coronary Artery (LCA): Divides into the left anterior descending artery (LAD) and the circumflex artery.
• LAD: Supplies the anterior wall of the left ventricle, the anterior interventricular septum, and part of the right ventricle.
• Circumflex Artery: Supplies the left atrium, the lateral and posterior walls of the left ventricle.
• Coronary Veins: After blood circulates through the heart, it is collected by coronary veins, which drain primarily into the coronary sinus, located on the posterior aspect of the heart, and then into the right atrium.

Cardiac Muscle Tissue

• Cardiac muscle cells (cardiomyocytes) are striated, similar to skeletal muscle, but are shorter and branched.
• Intercalated Discs: Specialized cell junctions that contain desmosomes (for structural integrity) and gap junctions (for electrical coupling) allowing rapid spread of action potentials.
• Autorhythmicity: Cardiac muscle has the ability to generate its own electrical impulses, allowing the heart to beat independently.
• Long Refractory Period: Prevents tetanus and ensures efficient pumping.

Cardiac Conduction System

• The cardiac conduction system consists of specialized cardiac muscle cells that initiate and distribute electrical impulses throughout the heart.
• Sinoatrial (SA) Node: Located in the right atrium, it is the heart's primary pacemaker, generating impulses at a rate of 60-100 beats per minute.
• Atrioventricular (AV) Node: Located in the interatrial septum, it delays the impulse from the SA node, allowing the atria to contract before the ventricles.
• Bundle of His: Located in the interventricular septum, it transmits the impulse from the AV node to the bundle branches.
• Right and Left Bundle Branches: Conduct the impulse through the interventricular septum towards the apex of the heart.
• Purkinje Fibers: Distribute the impulse rapidly and uniformly throughout the ventricular myocardium, causing ventricular contraction.

Innervation of the Heart

• The heart is innervated by both the sympathetic and parasympathetic nervous systems, which regulate heart rate and contractility.
• Sympathetic Innervation: Increases heart rate and contractility via the release of norepinephrine.
• Parasympathetic Innervation: Decreases heart rate via the release of acetylcholine, primarily affecting the SA and AV nodes.
• Vagus Nerve: Carries parasympathetic fibers to the heart.

Clinical Significance

• Coronary Artery Disease (CAD): Blockage of the coronary arteries, leading to ischemia and potential myocardial infarction (heart attack).
• Arrhythmias: Irregular heart rhythms, caused by disturbances in the cardiac conduction system.
• Heart Failure: Condition in which the heart cannot pump enough blood to meet the body's needs.
• Valve Disorders: Conditions affecting the heart valves, leading to stenosis (narrowing) or regurgitation (leakage).
• Cardiomyopathy: Disease of the heart muscle, leading to impaired contractility and heart failure.