The Heart: Anatomy and Physiology Overview

Introduction to the Heart

The heart is a muscular organ essential for pumping blood throughout the body via the pulmonary and systemic circuits. Understanding its structure and function is critical for healthcare professionals, as it forms the foundation for clinical assessment and intervention.

• Stethoscope Use: Healthcare providers use stethoscopes to listen to heart sounds, which provide vital information about cardiac health.

• Heart Sounds: The normal rhythm is described as "lub-dub," corresponding to the closing of heart valves. Abnormal sounds, such as murmurs, may indicate pathology.

Cardiac Circulation: Pulmonary and Systemic Circuits

Pathways of Blood Flow

The heart pumps blood through two main circuits:

• Pulmonary Circuit: Right ventricle → pulmonary arteries → lungs (gas exchange) → pulmonary veins → left atrium.

• Systemic Circuit: Left ventricle → aorta → body tissues (oxygen delivery) → systemic veins → right atrium.

Blood flows continuously through these circuits, ensuring oxygen delivery and waste removal.

Chambers of the Heart

Structure and Function

• Atria (Receiving Chambers):

  • Right atrium: Receives oxygen-poor blood from the systemic circuit.

  • Left atrium: Receives oxygen-rich blood from the pulmonary circuit.

• Ventricles (Pumping Chambers):

  • Right ventricle: Pumps blood to the pulmonary circuit.

  • Left ventricle: Pumps blood to the systemic circuit.

Major Blood Vessels

Arteries and Veins

• Arteries: Carry blood away from the heart (regardless of oxygen content).

• Veins: Carry blood toward the heart (regardless of oxygen content).

• Examples:

  • Pulmonary arteries: Carry oxygen-poor blood to the lungs.

  • Pulmonary veins: Carry oxygen-rich blood to the left atrium.

  • Superior/inferior vena cava: Return systemic blood to the right atrium.

Location and Coverings of the Heart

Mediastinum and Pericardium

• Mediastinum: The medial cavity of the thorax, housing the heart, great vessels, esophagus, trachea, and thymus.

• Pericardium: Double-walled sac surrounding the heart.

  • Fibrous pericardium: Anchors and protects the heart; prevents overfilling.

  • Serous pericardium: Two layers:

    • Parietal layer: Lines the internal surface of the fibrous pericardium.

    • Visceral layer (Epicardium): Covers the heart surface.

  • Pericardial cavity: Space between layers containing serous fluid to reduce friction.

Layers of the Heart Wall

Three Main Layers

• Epicardium: Outer layer (visceral pericardium).

• Myocardium: Middle, muscular layer containing contractile cardiac muscle cells. Responsible for pumping action.

• Endocardium: Inner layer lining the heart chambers and valves.

Cardiac Skeleton

Structure and Function

• Network of criss-crossing collagen and elastic fibers.

• Anchors cardiac muscle cells and valves.

• Limits spread of electrical impulses to proper pathways (fibrous tissue is a poor conductor).

Heart Valves

Types and Functions

• Atrioventricular (AV) Valves:

  • Tricuspid valve: Between right atrium and right ventricle (three cusps).

  • Mitral (bicuspid) valve: Between left atrium and left ventricle (two cusps).

  • Chordae tendineae: Collagen cords anchoring valve cusps to papillary muscles, preventing valve prolapse (inversion).

• Semilunar (SL) Valves:

  • Pulmonary valve: Between right ventricle and pulmonary trunk.

  • Aortic valve: Between left ventricle and aorta.

• Valve Function: Ensure unidirectional blood flow; open and close in response to pressure changes.

Pressure Changes and Heart Sounds

Mechanism of Valve Operation

• Blood flows from high to low pressure.

• AV valves open when atrial pressure > ventricular pressure; close when ventricular pressure > atrial pressure (producing the first heart sound, "lub").

• SL valves open when ventricular pressure > arterial pressure; close when arterial pressure > ventricular pressure (producing the second heart sound, "dub").

Homeostatic Imbalances

Inflammation and Valve Disorders

• Pericarditis: Inflammation of the pericardium, causing friction rub and pain. Excess fluid can lead to cardiac tamponade (compression of the heart, medical emergency).

• Valve Disorders:

  • Incompetent valve: Fails to close properly, causing backflow (regurgitation).

  • Valvular stenosis: Valve narrows due to stiffening (e.g., from calcium deposits or infection), increasing resistance and workload on the heart.

• Heart Murmurs: Abnormal sounds due to turbulent blood flow, often from valve problems.

Coronary Circulation

Supplying the Heart Muscle

• Arterial Supply:

  • Right and left coronary arteries (arise from the aorta).

  • Left coronary artery branches: Circumflex artery (serves left atrium and posterior left ventricle), anterior interventricular artery (serves anterior walls of both ventricles).

  • Right coronary artery branches: Right marginal artery (serves lateral right heart), posterior interventricular artery (serves posterior walls of both ventricles).

• Venous Return:

  • Great, middle, and small cardiac veins drain into the coronary sinus, which empties into the right atrium.

  • Anterior cardiac veins drain directly into the right atrium.

Clinical Correlations: Angina and Myocardial Infarction

Ischemic Heart Disease

• Angina pectoris: "Choked chest"; transient chest pain due to temporary deficiency in blood delivery to the myocardium. Cells weaken but do not die.

• Myocardial infarction (MI): Heart attack; prolonged blockage leads to cell death. Dead cardiac muscle is replaced by non-contractile scar tissue (fibrosis), reducing heart function.

• Cardiac muscle cells: Are amitotic (do not divide); lost cells are not replaced by new muscle cells.

Comparative Structure of Ventricles

Right vs. Left Ventricle

• Right ventricle: Pumps blood to the nearby lungs (pulmonary circuit); thinner walls, lower pressure.

• Left ventricle: Pumps blood to the entire body (systemic circuit); thicker walls (about 3x), higher pressure (about 5x).

Key Terms and Definitions

• Mediastinum: Central compartment of the thoracic cavity.

• Pericardium: Double-walled sac enclosing the heart.

• Epicardium: Outer layer of the heart wall.

• Myocardium: Muscular middle layer of the heart wall.

• Endocardium: Inner lining of the heart chambers and valves.

• Chordae tendineae: "Heart strings" anchoring AV valve cusps to papillary muscles.

• Cardiac skeleton: Network of connective tissue fibers supporting the heart.

• Pericarditis: Inflammation of the pericardium.

• Cardiac tamponade: Compression of the heart due to fluid accumulation in the pericardial cavity.

• Angina pectoris: Chest pain from transient myocardial ischemia.

• Myocardial infarction: Heart attack; death of cardiac muscle due to prolonged ischemia.

Summary Table: Heart Valves and Their Locations

Key Equations

• Blood Flow Direction: Blood flows from regions of higher pressure to regions of lower pressure.

• Pressure-Driven Valve Operation:

  • AV valves open:

  • AV valves close:

  • SL valves open:

  • SL valves close:

Clinical Application Example

• Heart Murmur: A patient presents with a swishing sound during auscultation. This may indicate an incompetent valve, causing blood to regurgitate through a partially closed valve.

• Myocardial Infarction: Prolonged blockage of a coronary artery leads to death of cardiac muscle cells, which are replaced by non-contractile scar tissue, reducing heart function.

Additional info: Some explanations (e.g., the role of histamine in inflammation, the definition of the mediastinum, and the clinical significance of cardiac tamponade) were expanded for academic completeness and clarity.