Chapter 18: The Cardiovascular System – The Heart

Section 18.1: The Heart – Structure and Function

The heart is a muscular organ located in the mediastinum of the thoracic cavity. It functions as a dual pump, circulating blood through the pulmonary and systemic circuits to supply oxygen and nutrients to tissues and remove wastes.

• Pulmonary Pump: The right side of the heart pumps deoxygenated blood to the lungs for gas exchange.

• Systemic Pump: The left side of the heart pumps oxygenated blood to the rest of the body.

• Mediastinum: The central compartment of the thoracic cavity where the heart is located.

• Base: The broad, superior aspect of the heart where major vessels attach.

• Apex: The pointed, inferior tip of the heart, directed toward the left hip.

• Apical Impulse: The palpable heartbeat at the apex, often felt in the fifth intercostal space.

• Pericardium: A double-walled sac surrounding the heart, consisting of:

  • Fibrous Pericardium: Tough, outer layer that protects and anchors the heart.

  • Serous Pericardium: Thin, inner layer with two sublayers:

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

    • Visceral Layer (Epicardium): Covers the external heart surface.

• Pericarditis: Inflammation of the pericardium, causing pain and friction.

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

• Myocardium: The thick, muscular middle layer responsible for contraction.

• Endocardium: The smooth, inner lining of the heart chambers and valves.

• Atria: The two superior chambers that receive blood returning to the heart.

• Ventricles: The two inferior chambers that pump blood out of the heart.

Example: The right ventricle pumps blood into the pulmonary trunk, while the left ventricle pumps blood into the aorta.

Section 18.2: Heart Valves and Unidirectional Blood Flow

Heart valves ensure that blood flows in one direction through the heart, preventing backflow.

• Atrioventricular (AV) Valves: Located between atria and ventricles.

  • Tricuspid Valve: Right AV valve with three cusps.

  • Mitral (Bicuspid) Valve: Left AV valve with two cusps.

  • Chordae Tendineae: Tendinous cords that anchor AV valve cusps to papillary muscles.

• Semilunar (SL) Valves: Located at the exits of the ventricles.

  • Aortic Valve: Between left ventricle and aorta.

  • Pulmonary Valve: Between right ventricle and pulmonary trunk.

• Valve Disorders:

  • Incompetent Valves: Fail to close properly, causing regurgitation.

  • Mitral Valve Prolapse: The mitral valve bulges into the left atrium during ventricular contraction.

Example: A heart murmur may result from an incompetent valve allowing blood to leak backward.

Section 18.3: Blood Flow Through the Heart and Coronary Circulation

Blood flows from the atria to the ventricles, then to the lungs or systemic circulation. The heart muscle itself is supplied by coronary circulation.

• Pathway of Blood Flow:

  1. Right atrium → right ventricle → pulmonary trunk → lungs

  2. Lungs → left atrium → left ventricle → aorta → body

• Coronary Arteries: Supply oxygenated blood to the myocardium.

  • Left Coronary Artery

  • Right Coronary Artery

• Coronary Veins: Drain deoxygenated blood from the myocardium.

  • Great Cardiac Vein

  • Middle Cardiac Vein

  • Small Cardiac Vein

  • Coronary Sinus: Collects blood from cardiac veins and empties into the right atrium.

• Coronary Circulation Disorders:

  • Angina Pectoris: Chest pain due to temporary myocardial ischemia.

  • Myocardial Infarction (Heart Attack): Prolonged blockage causing death of cardiac muscle tissue.

Example: Blockage of the left coronary artery can cause a myocardial infarction affecting the left ventricle.

Section 18.4: Cardiac Muscle Tissue and Intercalated Discs

Cardiac muscle fibers are connected by intercalated discs, forming a functional syncytium that allows coordinated contraction.

• Cardiac Tissue Characteristics:

  • Striated, branched cells with a single nucleus

  • Intercalated discs contain gap junctions and desmosomes

• Comparison: Cardiac vs. Skeletal Muscle

  • Cardiac muscle contracts involuntarily and rhythmically; skeletal muscle contracts voluntarily

  • Cardiac muscle cells are connected by intercalated discs; skeletal muscle fibers are not

Example: Gap junctions in intercalated discs allow ions to pass, enabling rapid spread of action potentials.

Section 18.5: Pacemaker Cells and Cardiac Conduction

Pacemaker cells generate spontaneous action potentials that coordinate the heartbeat. The cardiac conduction system ensures efficient contraction.

• Pacemaker Cell Characteristics: Autorhythmic, unstable resting membrane potential, initiate action potentials.

• Action Potential Phases in Pacemaker Cells:

  • Unstable resting potential due to slow Na+ influx ("funny" current)

  • Threshold reached, Ca2+ channels open, rapid depolarization

  • Repolarization as K+ channels open

• Cardiac Conduction Pathway:

  1. Sinoatrial (SA) Node

  2. Bachmann’s Bundle (to left atrium)

  3. Atrioventricular (AV) Node

  4. AV Bundle (Bundle of His)

  5. Right and Left Bundle Branches

  6. Purkinje Fibers (Subendocardial Conducting Network)

• Contractile Cell Action Potential:

  • Resting membrane potential maintained by K+ efflux

  • Depolarization triggered by Na+ influx

  • Plateau phase due to Ca2+ influx

  • Repolarization as K+ channels reopen

  • Absolute refractory period prevents tetanus

• Disorders:

  • Ion imbalances (e.g., hyperkalemia)

  • Arrhythmias (irregular rhythms)

  • Fibrillation (uncoordinated contraction)

  • Ectopic focus (abnormal pacemaker site)

  • Heart block (impaired conduction)

  • Asystole (absence of heartbeat)

Example: The SA node sets the pace of the heart; damage to the AV node can cause heart block.

Section 18.6: The Cardiac Cycle – Mechanical Events

The cardiac cycle describes the sequence of pressure and volume changes in the heart during one heartbeat.

• Phases of the Cardiac Cycle:

  • Early Diastole: Isovolumetric relaxation; ventricles relax, all valves closed.

  • Mid-Late Diastole: Ventricular filling as AV valves open.

  • Systole:

    • Isovolumetric contraction (ventricles contract, all valves closed)

    • Ventricular ejection (SL valves open, blood expelled)

Example: During ventricular systole, pressure rises sharply, forcing the semilunar valves open.

Section 18.7: Regulation of Stroke Volume and Heart Rate

Cardiac output is determined by stroke volume and heart rate. Both are regulated to meet the body's needs.

• Stroke Volume (SV): The volume of blood pumped by one ventricle per beat.

• Equation:

  • Where EDV = End-Diastolic Volume, ESV = End-Systolic Volume

• Ejection Fraction: Percentage of EDV ejected per beat.

• Factors Affecting Stroke Volume:

  • Preload: Degree of stretch of cardiac muscle before contraction.

  • Afterload: Pressure the ventricles must overcome to eject blood.

  • Contractility: Strength of contraction at a given preload.

  • Positive Inotropic Agents: Increase contractility (e.g., epinephrine, digitalis).

  • Negative Inotropic Agents: Decrease contractility (e.g., acidosis, high K+).

• Heart Rate (HR): Number of beats per minute.

  • Positive Chronotropic Factors: Increase HR (e.g., sympathetic stimulation, fever).

  • Negative Chronotropic Factors: Decrease HR (e.g., parasympathetic stimulation, hypothermia).

  • Tachycardia: Abnormally fast HR.

  • Bradycardia: Abnormally slow HR.

• Cardiac Output (CO): Volume of blood pumped by each ventricle per minute.

• Disorders:

  • Heart palpitation (irregular or forceful heartbeat)

  • Congestive heart failure (ineffective pumping)

  • Pulmonary congestion (left heart failure)

  • Peripheral congestion (right heart failure)

  • Cor pulmonale (right ventricular failure due to lung disease)

  • Cardiac fibrosis (scarring of heart tissue)

Example: Exercise increases both heart rate and stroke volume, raising cardiac output to meet metabolic demands.

Table: Comparison of Cardiac and Skeletal Muscle

Table: Summary of Cardiac Output Regulation

Additional info: Figures and tables referenced (e.g., F18.1, T18.1) are not included here but are typically found in standard Anatomy & Physiology textbooks and illustrate heart anatomy, blood flow, and cardiac cycle events.