Cardiology and the Cardiovascular System

Definition and Overview

• Cardiology is the study of the heart and its functions in health and disease.

• The cardiovascular system consists of the heart and blood vessels, responsible for circulating blood throughout the body.

• The heart acts as a muscular pump, moving approximately 3,600 gallons of blood daily through about 60,000 miles of vessels.

• Located in the mediastinum (the central compartment of the thoracic cavity), between the lungs, above the diaphragm, and between ribs 2 and 7.

Pericardium and Layers of the Heart Wall

Pericardium

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

• Two main layers:

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

  • Serous Pericardium: Thin, inner membrane with two layers:

    • Visceral Layer (Epicardium): Directly covers the heart surface.

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

Layers of the Heart Wall

• Epicardium: Outer layer, also the visceral layer of the serous pericardium.

• Myocardium: Middle, muscular layer responsible for contraction and pumping action.

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

Chambers and Major Structures of the Heart

Chambers

• Four chambers:

  • Atria (Right and Left): Upper chambers that receive blood.

  • Ventricles (Right and Left): Lower chambers that pump blood out of the heart.

• Auricles: Small, ear-like extensions of the atria that increase atrial volume.

• Sulci: Grooves on the heart surface marking boundaries between chambers:

  • Coronary Sulcus: Separates atria from ventricles.

  • Anterior Interventricular Sulcus: Marks the boundary between the two ventricles on the anterior surface.

  • Posterior Interventricular Sulcus: Marks the boundary on the posterior surface.

Right Atrium

• Receives deoxygenated blood from three vessels:

  • Superior Vena Cava: Drains blood from the upper body.

  • Inferior Vena Cava: Drains blood from the lower body.

  • Coronary Sinus: Drains blood from the heart's own circulation.

• Interatrial Septum: Wall between the right and left atria.

• Fossa Ovalis: Remnant of the fetal foramen ovale, a depression in the interatrial septum.

• Tricuspid Valve: Guards the opening to the right ventricle; has three cusps.

• Chordae Tendineae and Papillary Muscles: Prevent valve prolapse during ventricular contraction.

Right Ventricle

• Contains trabeculae carneae (muscular ridges).

• Separated from the left ventricle by the interventricular septum.

• Septal Defects: Abnormal openings in the septum, can cause mixing of oxygenated and deoxygenated blood.

• Pumps blood through the pulmonary semilunar valve into the pulmonary trunk and then to the lungs via right and left pulmonary arteries.

Left Atrium

• Receives oxygenated blood from the lungs via four pulmonary veins.

• Blood passes through the bicuspid (mitral) valve into the left ventricle.

Left Ventricle

• Forms the apex of the heart.

• Has the thickest muscular wall to generate high pressure for systemic circulation.

• Contains trabeculae carneae.

• Pumps blood through the aortic semilunar valve into the ascending aorta, arch of aorta, descending aorta, thoracic aorta, and abdominal aorta to systemic arteries.

Heart Valves and the Fibrous Skeleton

Heart Valves

• Ensure unidirectional blood flow through the heart.

• Valves open due to blood pressure and close to prevent backflow; chordae tendineae and papillary muscles stabilize the valves.

Atrioventricular (AV) Valves

• Tricuspid Valve (right): Three cusps.

• Bicuspid (Mitral) Valve (left): Two cusps.

• Valvular Heart Disease (VHD): Dysfunction of valves, often due to carditis (inflammation), such as from rheumatic fever.

• Regurgitation: Backflow of blood due to valve failure.

Semilunar Valves

• Pulmonary Valve: Three cusps, between right ventricle and pulmonary trunk.

• Aortic Valve: Three cusps, between left ventricle and aorta.

Fibrous Skeleton

• Dense connective tissue framework that supports the valves and electrically isolates the atria from the ventricles.

Coronary Circulation

Arterial Supply

• Coronary arteries supply oxygenated blood to the heart muscle.

• Left Coronary Artery branches into:

  • Circumflex artery

  • Marginal artery

  • Anterior interventricular artery

• Right Coronary Artery branches into:

  • Marginal branches

  • Posterior interventricular artery

• Anastomoses: Connections between arteries that provide alternate routes for blood flow.

Venous Drainage

• Coronary veins collect deoxygenated blood from the myocardium.

• Major veins include:

  • Great cardiac vein

  • Posterior cardiac vein

  • Small cardiac vein

  • Middle cardiac vein

• All drain into the coronary sinus, which empties into the right atrium.

Coronary Artery Disease (CAD) and Related Conditions

• Coronary Artery Disease (CAD): Narrowing or blockage of coronary arteries, usually due to atherosclerosis.

• Coronary Ischemia: Reduced blood flow to the myocardium.

• Angina Pectoris: Chest pain due to temporary ischemia.

• Myocardial Infarction (MI): Heart attack; death of heart muscle due to prolonged ischemia.

• Coronary Thrombosis: Blood clot formation in a coronary artery.

• Embolus: A traveling blood clot that can block vessels.

• Enzymes for Diagnosis: Cardiac enzymes (e.g., troponin, CK-MB) are measured to diagnose MI.

• Treatment: May include medications, angioplasty, or coronary artery bypass surgery.

Cardiac Muscle and the Conduction System

Cardiac Muscle Structure

• Cardiac muscle cells are striated, branched, and interconnected by intercalated discs.

• Desmosomes provide mechanical strength; gap junctions allow electrical signals to pass rapidly between cells.

The Conduction System

• Composed of autorhythmic cells that generate and conduct electrical impulses.

• Pacemaker cells initiate the heartbeat.

• Main components:

  • Sinoatrial (SA) Node: Located in right atrium; sets the pace (80–100 bpm, modulated by hormones and neurotransmitters to 60–80 bpm); establishes sinus rhythm.

  • Atrioventricular (AV) Node: Located at the junction of atria and ventricles; intrinsic rate 40–60 bpm; delays impulse to allow ventricular filling.

  • Internodal Pathways: Conduct impulses from SA to AV node.

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

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

  • Purkinje Fibers: Large diameter fibers that rapidly distribute the impulse to ventricular muscle.

Cardiac Physiology

Action Potential in Cardiac Muscle

• Resting membrane potential (RMP) is about -90 mV in cardiac muscle cells.

• Phases:

  1. Rapid Depolarization: Na+ influx.

  2. Plateau: Slow Ca2+ channels remain open, prolonging depolarization.

  3. Repolarization: K+ efflux restores RMP.

  4. Refractory Period: Time during which the cell cannot be re-excited.

Electrocardiogram (ECG/EKG)

• Records the electrical activity of the heart.

• Key components:

  • P Wave: Atrial depolarization.

  • QRS Complex: Ventricular depolarization (and atrial repolarization, which is masked).

  • T Wave: Ventricular repolarization.

  • Analysis: Used to detect arrhythmias and other cardiac abnormalities.

Cardiac Cycle

• The sequence of events in one heartbeat, including contraction (systole) and relaxation (diastole) of atria and ventricles.

• Steps:

  1. Passive ventricular (and atrial) filling

  2. Atrial contraction

  3. Ventricular contraction and blood ejection

  4. End diastolic volume (EDV)

  5. Stroke volume (SV): Amount of blood ejected per beat (typically 60% of EDV)

  6. End systolic volume (ESV): Blood remaining after contraction (typically 40% of EDV)

Heart Sounds

• Heard via auscultation with a stethoscope.

• Four heart sounds (only S1 and S2 are normally audible):

  • S1 (LUBB): Closure of AV valves (bicuspid and tricuspid).

  • S2 (DUBB): Closure of semilunar valves (aortic and pulmonary).

  • S3 and S4: Usually not heard in healthy adults.

• Heart Murmur: Abnormal sound due to valve disorders (e.g., mitral stenosis, aortic stenosis, mitral valve prolapse).

Cardiac Output and Regulation

Formulas

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

• Formula:

• Cardiac Reserve: Difference between resting and maximal cardiac output.

Factors Affecting Stroke Volume

• Preload: Degree of stretch of cardiac muscle before contraction; depends on filling time (Frank-Starling Principle: "the more the heart fills, the more it pumps").

• Contractility: Strength of contraction at a given preload.

  • Positive inotropic agents: Increase contractility (e.g., sympathetic stimulation, norepinephrine).

  • Negative inotropic agents: Decrease contractility (e.g., parasympathetic stimulation, acetylcholine).

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

Regulation of Heart Rate

• Autonomic Nervous System (ANS): Cardiovascular center in the medulla oblongata receives sensory input from baroreceptors and chemoreceptors, analyzes it, and sends motor responses via the cardiac plexus.

  • Cardioacceleratory Center: Sympathetic stimulation increases heart rate.

  • Cardioinhibitory Center: Parasympathetic stimulation decreases heart rate.

  • SA and AV nodes, as well as ventricular cardiocytes, are innervated by both systems.

• Hormones: Stimulate beta-1 receptors (e.g., epinephrine, norepinephrine).

• Ions: Calcium (Ca2+), sodium (Na+), and potassium (K+) levels affect heart function.

• Drugs:

  • Beta blockers: Decrease heart rate and contractility.

  • Calcium channel blockers: Reduce contractility and heart rate.

Circulatory Pathways

• Systemic Circulation: Delivers oxygenated blood to all body tissues and returns deoxygenated blood to the heart.

• Pulmonary Circulation: Carries deoxygenated blood to the lungs for oxygenation and returns oxygenated blood to the heart.

• Coronary Circulation: Supplies blood to the heart muscle itself.

Summary Table: Heart Valves

Example: Cardiac Output Calculation

• If stroke volume (SV) is 70 mL/beat and heart rate (HR) is 75 beats/min:

• Thus, the heart pumps 5.25 liters of blood per minute.

Additional info: The above notes expand on the outline by providing definitions, explanations, and examples for each structure and concept. Where the original outline listed only terms, academic context and details have been added for clarity and completeness.