Chapter 18: The Cardiovascular System – The Heart

Cardiovascular System: The Heart

Introduction

The heart is a muscular organ responsible for pumping blood throughout the body via the blood vessels. This process ensures the delivery of nutrients and oxygen to tissues and the removal of waste products. The heart is centrally located in the thoracic cavity, within the mediastinum, and is roughly the size of a human fist.

Coverings of the Heart

Pericardium

• Pericardium: A double-walled sac that encloses the heart, providing protection and anchorage.

• Fibrous Pericardium: The tough, outer layer that anchors the heart to surrounding structures (diaphragm, sternum, great vessels).

• Serous Pericardium: A double-layered membrane deep to the fibrous pericardium.

  • Parietal Layer: Fused to the fibrous pericardium.

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

• Pericardial Cavity: Space between the serous layers, filled with serous fluid to reduce friction during heart contractions.

• Pericarditis: Inflammation of the pericardium, often due to infection, which can hinder fluid production and cause friction.

Structure of the Heart Wall

• Epicardium: The outermost layer, also known as the visceral pericardium.

• Myocardium: The thick, middle layer composed of cardiac muscle; responsible for contraction.

• Endocardium: The innermost layer, a smooth endothelium lining that reduces friction and is continuous with blood vessel endothelium.

• Fibrous Skeleton: Connective tissue network within the myocardium that supports cardiac muscle cells.

Heart Chambers and Associated Great Vessels

• Four Chambers:

  • 2 Atria (superior, receiving chambers)

  • 2 Ventricles (inferior, pumping chambers)

• Septa: Internal partitions separating the chambers:

  • Interatrial Septum (between atria)

  • Interventricular Septum (between ventricles)

• Atria: Thin-walled, minimal contraction; external auricles increase volume.

• Ventricles: Thick-walled, especially the left ventricle; contain trabeculae carneae and papillary muscles.

• Major Vessels:

  • Right Atrium: Receives blood from the superior vena cava, inferior vena cava, and coronary sinus.

  • Left Atrium: Receives blood from four pulmonary veins.

  • Right Ventricle: Pumps blood into the pulmonary arteries (to lungs).

  • Left Ventricle: Pumps blood into the aorta (to body).

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

Pathways of Blood Through the Heart

The heart functions as two side-by-side pumps, each serving a separate circuit:

• Pulmonary Circuit: Right side pumps deoxygenated blood to the lungs for gas exchange.

• Systemic Circuit: Left side pumps oxygenated blood to the body tissues.

• Sequence of Blood Flow:

  1. Superior/Inferior vena cava → Right atrium

  2. Right atrium → Tricuspid valve → Right ventricle

  3. Right ventricle → Pulmonary semilunar valve → Pulmonary trunk → Pulmonary arteries → Lungs

  4. Lungs → Pulmonary veins → Left atrium

  5. Left atrium → Bicuspid (mitral) valve → Left ventricle

  6. Left ventricle → Aortic semilunar valve → Aorta → Body

• Wall Thickness: Left ventricle is thicker due to higher resistance in systemic circuit.

Structure and Function of Heart Valves

• Purpose: Ensure one-way blood flow through the heart.

• Atrioventricular (AV) Valves:

  • Tricuspid Valve: Right AV valve with three cusps.

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

  • Chordae Tendineae: Collagen cords anchoring valve cusps to papillary muscles.

• Semilunar Valves:

  • Pulmonary Semilunar Valve: At base of pulmonary trunk.

  • Aortic Semilunar Valve: At base of aorta.

  • Each has three pocket-like cusps.

• Valve Disorders:

  • Valvular Stenosis: Stiffening/narrowing of valves, requiring increased force to pump blood.

  • Incompetent Valve: Fails to close completely, causing backflow.

Coronary Circulation

• Function: Supplies blood to the heart muscle (myocardium).

• Coronary Arteries: Right and left arteries branch to supply the heart; provide alternative routes (collateral circulation).

• Coronary Sinus: Cardiac veins drain deoxygenated blood from myocardium into the right atrium.

• Clinical Note: Blockage of coronary arteries can cause tissue death (myocardial infarction).

Intrinsic Conduction System and Electrocardiogram (ECG)

• Intrinsic Conduction System: Specialized cardiac cells (autorhythmic) initiate and distribute electrical impulses for coordinated contraction.

• Key Components:

  1. Sinoatrial (SA) Node: Pacemaker; initiates impulses (~75/min).

  2. Atrioventricular (AV) Node: Delays impulse (0.1 sec) to allow atrial contraction.

  3. AV Bundle (Bundle of His): Conducts impulses through interventricular septum.

  4. Right and Left Bundle Branches: Carry impulses down septum.

  5. Purkinje Fibers: Spread impulse through ventricles, causing contraction from apex upward.

• Electrocardiogram (ECG/EKG): Records electrical activity of the heart.

  • P wave: Atrial depolarization (SA node activity).

  • QRS complex: Ventricular depolarization (and atrial repolarization, not visible).

  • T wave: Ventricular repolarization.

Principal Events of the Cardiac Cycle

• Systole: Contraction phase (blood ejected).

• Diastole: Relaxation phase (chambers refill).

• Phases:

  1. Ventricular Filling (mid-to-late diastole): Blood flows passively into ventricles; atria contract to complete filling (end diastolic volume).

  2. Ventricular Systole: Ventricles contract, AV valves close (isovolumetric contraction), then semilunar valves open for ejection.

  3. Isovolumetric Relaxation (early diastole): Ventricles relax, semilunar valves close, AV valves reopen as atrial pressure exceeds ventricular pressure.

Heart Sounds and Clinical Significance

• Normal Sounds: Two main sounds per heartbeat:

  • "Lub": Closure of AV valves.

  • "Dup": Closure of semilunar valves.

• Heart Murmurs: Abnormal sounds due to turbulent blood flow, often from valve disorders (incompetence or stenosis).

Cardiac Output and Its Regulation

• Definition: Volume of blood pumped by each ventricle per minute.

• Formula:

  • Cardiac Output (CO): $CO = HR \times SV$

  • Where $HR$ = heart rate (beats/min), $SV$ = stroke volume (mL/beat)

• Factors Affecting Stroke Volume (SV):

  • Preload: Degree of stretch of cardiac muscle before contraction; increased preload increases SV.

  • Contractility: Force of contraction at a given preload; increased by sympathetic stimulation and certain hormones (e.g., epinephrine, thyroxine).

  • Afterload: Pressure that must be overcome to eject blood; increased afterload (e.g., in hypertension) reduces SV.

• Factors Affecting Heart Rate (HR):

  • Autonomic Nervous System: Sympathetic increases HR and contractility (norepinephrine); parasympathetic decreases HR (acetylcholine).

  • Hormones: Epinephrine and thyroxine increase HR.

  • Ions: Calcium, sodium, and potassium imbalances can affect HR and contractility.

  • Other: Age, gender, exercise, and body temperature.

Risk Factors in Heart Disease

• Congestive Heart Failure (CHF): Inefficient pumping leads to inadequate circulation.

• Coronary Atherosclerosis: Fatty buildup in coronary vessels impairs blood supply to heart muscle.

• Persistent High Blood Pressure: Increases workload on the heart.

• Multiple Myocardial Infarcts: Repeated heart attacks reduce contractile function due to scar tissue.

• Dilated Cardiomyopathy: Enlarged heart with stretched, weakened ventricles.

Summary Table: Heart Valves

Valve	Location	Function	Number of Cusps
Tricuspid (Right AV)	Between right atrium and right ventricle	Prevents backflow into right atrium	3
Bicuspid/Mitral (Left AV)	Between left atrium and left ventricle	Prevents backflow into left atrium	2
Pulmonary Semilunar	At base of pulmonary trunk	Prevents backflow into right ventricle	3
Aortic Semilunar	At base of aorta	Prevents backflow into left ventricle	3

Example: Cardiac Output Calculation

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

• $CO = 75 \times 70 = 5250$ mL/min (or 5.25 L/min)

Additional info: The notes above include expanded explanations and context for each objective, as well as a summary table for heart valves and a sample calculation for cardiac output.