Chapter 18 – The Heart

Location and Size of the Heart

The heart is a muscular organ essential for pumping blood throughout the body. Its location and size are important for understanding its function and clinical relevance.

• Location: The heart is situated in the thoracic cavity, specifically within the mediastinum, which is the central compartment of the thoracic cavity.

• Size: The heart is approximately the size of a closed fist.

• Base: The base is the wider, superior portion of the heart.

• Apex: The apex is the pointed, inferior tip of the heart.

• Orientation: The heart is slightly tilted, with the apex pointing toward the left hip.

Pericardium

The pericardium is a double-walled sac that encloses the heart, providing protection and reducing friction during heart movements.

• Fibrous Pericardium: The outermost layer, composed of tough, inelastic connective tissue. It anchors the heart to surrounding structures (such as the diaphragm) and prevents overfilling.

• Serous Pericardium: A thinner, double-layered membrane:

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

  • Visceral Layer (Epicardium): Covers the heart surface and is fused to the heart wall.

• Pericardial Cavity: The space between the parietal and visceral layers, filled with serous fluid to reduce friction.

Homeostatic Imbalances

• Pericarditis: Inflammation of the pericardium, causing pain and potential damage to heart tissues.

• Cardiac Tamponade: Accumulation of fluid or blood in the pericardial cavity, which can compress the heart and lead to cardiac failure.

Heart Wall Structure

The heart wall consists of three distinct layers, each with specialized functions.

• Epicardium: The outer layer, also known as the visceral pericardium; thin and smooth.

• Myocardium: The middle layer, composed of cardiac muscle tissue responsible for contraction and pumping action.

• Endocardium: The inner layer, made of endothelium and connective tissue; provides a smooth lining for chambers and valves, continuous with blood vessel endothelium.

Cardiac Muscle

Cardiac muscle cells are specialized for involuntary, rhythmic contraction.

• Characteristics: Involuntary, striated, branched cells.

• Intercalated Discs: Specialized junctions connecting cardiac muscle cells, containing gap junctions and desmosomes.

• Gap Junctions: Allow rapid transmission of action potentials between cells.

• Desmosomes: Prevent cells from separating during contraction.

• Sliding Filament Mechanism: Cardiac muscle contracts via the same mechanism as skeletal muscle, involving actin and myosin filaments.

Surface Features of the Heart

The external anatomy of the heart includes several grooves and vessels that mark the boundaries between chambers and provide passage for coronary vessels.

• Atrioventricular (AV) Groove: Separates atria from ventricles.

• Interventricular Sulcus: Marks the boundary between right and left ventricles (anterior and posterior).

• Coronary Vessels: Run within these grooves, supplying blood to the heart muscle.

Chambers of the Heart

The heart contains four chambers that receive and pump blood.

• Atria: The two upper chambers (right and left) that receive blood.

• Ventricles: The two lower chambers (right and left) that pump blood out of the heart.

• Interatrial Septum: Separates the right and left atria.

• Interventricular Septum: Separates the right and left ventricles.

• Ventricular Wall Thickness: The left ventricle has a thicker wall than the right, as it must pump blood throughout the body against higher resistance.

Blood Flow Through the Heart

Blood flows through the heart in a specific sequence, ensuring oxygenation and circulation.

1. Right atrium receives deoxygenated blood from the superior vena cava, inferior vena cava, and coronary sinus.

2. Blood passes to the right ventricle, which pumps it into the pulmonary trunk and pulmonary arteries to the lungs (pulmonary circulation).

3. Oxygenated blood returns from the lungs via pulmonary veins to the left atrium.

4. Blood moves to the left ventricle, which pumps it into the aorta for systemic circulation.

Pulmonary Circulation

• Pulmonary Arteries: Carry deoxygenated blood from the heart to the lungs.

• Pulmonary Veins: Carry oxygenated blood from the lungs to the heart.

Systemic Circulation

• Aorta: Distributes oxygenated blood to the body.

• Vena Cavae: Return deoxygenated blood to the heart.

Myocardial Blood Supply

The heart muscle (myocardium) requires its own blood supply, provided by the coronary arteries and veins.

• Coronary Arteries: Branch off the aorta and supply oxygenated blood to the myocardium.

• Coronary Veins: Remove deoxygenated blood from the myocardium, draining into the coronary sinus and then the right atrium.

• Anastomoses: Interconnections between vessels provide alternate routes for blood flow (lateral circulation).

• Blood Flow: Most effective during diastole (relaxation phase).

Pathologies of Coronary Circulation

• Ischemia: Decreased blood supply to the myocardium.

• Hypoxia: Low oxygen supply.

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

• Myocardial Infarction (Heart Attack): Death of heart muscle tissue due to prolonged ischemia, often caused by blockage of a coronary artery.

Heart Valves: Structure and Function

Heart valves ensure unidirectional blood flow and prevent backflow.

• Atrioventricular (AV) Valves: Separate atria from ventricles.

  • Bicuspid (Mitral) Valve: Left side, two cusps.

  • Tricuspid Valve: Right side, three cusps.

  • Chordae Tendineae: Fibrous cords connecting valve cusps to papillary muscles, preventing valve prolapse.

  • Papillary Muscles: Contract to hold valves closed during ventricular contraction.

• Semilunar Valves: Located at the exits of the ventricles.

  • Aortic Semilunar Valve: Between left ventricle and aorta.

  • Pulmonary Semilunar Valve: Between right ventricle and pulmonary trunk.

Valve Pathologies

• Incompetent (Insufficient) Valve: Does not close properly, allowing backflow.

• Stenosis: Valve is narrowed or hardened, impeding blood flow.

Cardiac Muscle Action Potential

Cardiac muscle cells generate action potentials that coordinate contraction.

• Depolarization: Rapid influx of Na+ ions through fast sodium channels.

• Plateau Phase: Sustained depolarization due to Ca2+ influx and closure of K+ channels.

• Repolarization: Ca2+ channels close, K+ channels open, restoring resting potential.

• Refractory Period: Prolonged period prevents tetanus and ensures rhythmic contractions.

Key Ion Concentrations

• Extracellular: High Na+, Ca2+

• Intracellular: High K+

Intrinsic Cardiac Conduction System

The heart's electrical activity is coordinated by a specialized conduction system composed of autorhythmic cells.

• Sinoatrial (SA) Node: Primary pacemaker, initiates impulses.

• Atrioventricular (AV) Node: Delays impulse, allowing atria to contract before ventricles.

• AV Bundle (Bundle of His): Conducts impulses from AV node to ventricles.

• Right and Left Bundle Branches: Carry impulses through interventricular septum.

• Purkinje Fibers: Distribute impulses throughout ventricular myocardium.

Pacemaker Potentials

• Pacemaker cells have unstable resting membrane potentials, gradually depolarizing until threshold is reached.

• Pacemaker Potential: Slow Na+ influx.

• Depolarization: Ca2+ influx.

• Repolarization: K+ efflux.

Electrocardiogram (ECG/EKG)

An ECG records the electrical activity of the heart, providing diagnostic information.

• P Wave: Atrial depolarization.

• QRS Complex: Ventricular depolarization.

• T Wave: Ventricular repolarization.

Cardiac Cycle: Electrical and Mechanical Events

The cardiac cycle consists of alternating periods of contraction (systole) and relaxation (diastole).

• Systole: Contraction phase, blood is ejected from chambers.

• Diastole: Relaxation phase, chambers fill with blood.

Cardiac Output

Cardiac output is the volume of blood pumped by each ventricle per minute.

• Formula:

• Example: HR = 70 beats/min, SV = 70 ml/beat, CO = 4.9 L/min

• Average adult blood volume: 4–6 L

Cardiac Reserve

Cardiac reserve is the difference between maximal and resting cardiac output.

• Average individuals: 4–5 times resting CO

• Trained athletes: Up to 7 times resting CO

Regulation of Stroke Volume

Stroke volume is regulated by three main factors:

• Preload: Degree of stretch of cardiac muscle cells before contraction (related to EDV).

• Contractility: Strength of contraction at a given muscle length (influenced by sympathetic stimulation and hormones).

• Afterload: Pressure that must be overcome to eject blood from ventricles (related to arterial blood pressure).

Stroke Volume Formula:

• EDV (End Diastolic Volume): Volume of blood in ventricle after filling (~120 ml)

• ESV (End Systolic Volume): Volume of blood remaining after contraction (~50 ml)

• Each beat ejects about 60% of the blood in the ventricle

Frank-Starling Law of the Heart

• The greater the stretch (preload), the greater the force of contraction and stroke volume, up to an optimal limit.

• Relationship: Increased venous return increases EDV, which increases SV.

Contractility: Inotropic Effects

• Positive inotropic agents (e.g., sympathetic stimulation, epinephrine, thyroxine, digitalis) increase contractility.

• Negative inotropic agents (e.g., acidosis, high extracellular K+, calcium channel blockers) decrease contractility.

Afterload

• High blood pressure increases afterload, making it harder for the heart to eject blood, which can decrease stroke volume.

Regulation of Heart Rate

Heart rate is regulated by intrinsic and extrinsic factors.

• Intrinsic Controls: Atrial (Bainbridge) reflex, increased venous return stimulates SA node.

• Extrinsic Controls:

  • Autonomic Nervous System: Sympathetic (norepinephrine) increases HR; Parasympathetic (acetylcholine) decreases HR.

  • Hormones: Epinephrine, thyroxine increase HR.

  • Ions: K+, Ca2+ imbalances affect HR.

  • Other Factors: Body temperature, age, gender, body mass, exercise, stress, illness.

Common Cardiac Pathologies

• Arrhythmias: Irregular heart rhythms (bradycardia, tachycardia, fibrillation).

• Ectopic Pacemakers: Abnormal pacemaker sites controlling heart rhythm.

• Heart Block: Impaired conduction between atria and ventricles, often due to AV node damage.

• Artificial Pacemakers: Devices used to restore normal rhythm in cases of conduction system failure.

Summary Table: Heart Valves

Summary Table: Cardiac Conduction System

Additional info: Some details and terminology have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.