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 transport of nutrients, oxygen, and waste products to and from body cells. The heart is centrally located in the thoracic cavity, within the mediastinum, and is roughly the size of a person's fist.

Coverings of the Heart

Pericardium

• Pericardium: A double-walled sac that encloses the heart.

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

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

  • Parietal pericardium: Fused to the fibrous pericardium.

  • Visceral pericardium (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 (audible as a rustling sound).

Structure of the Heart Wall

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

• Myocardium: Middle, thickest layer composed of cardiac muscle; responsible for contraction. Contains the fibrous skeleton (connective tissue fibers that support the muscle cells).

• Endocardium: Inner layer, a smooth sheet of endothelium lining the heart chambers and continuous with blood vessel endothelium.

Heart Chambers and Associated Great Vessels

• The heart has four chambers:

  • Two atria (right and left): Superior, thin-walled receiving chambers.

  • Two ventricles (right and left): Inferior, thick-walled pumping chambers.

• Chambers are separated by septa:

  • Interatrial septum: Between atria; contains the fossa ovalis (remnant of fetal foramen ovale).

  • Interventricular septum: Between ventricles.

• Auricles: External extensions of the atria that increase volume.

• Major vessels entering/exiting the heart:

  • Right atrium: Receives blood from the superior vena cava (above diaphragm), inferior vena cava (below diaphragm), and coronary sinus (from myocardium).

  • Left atrium: Receives oxygenated blood from four pulmonary veins (from lungs).

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

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

• Trabeculae carneae: Irregular muscle ridges on internal ventricular walls.

• Papillary muscles: Projecting muscle bundles involved in valve function.

Pathways of Blood Through the Heart

The heart functions as two side-by-side pumps:

• Pulmonary circuit (right side): Receives deoxygenated blood from the body and pumps it to the lungs for gas exchange.

• Systemic circuit (left side): Receives oxygenated blood from the lungs and pumps it to the body tissues.

• The left ventricle has a thicker wall due to the higher resistance of the systemic circuit.

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

Structure and Function of Heart Valves

• Four heart valves ensure unidirectional blood flow:

  • Atrioventricular (AV) valves:

    • Tricuspid valve: Right AV valve, three cusps.

    • Bicuspid (mitral) valve: Left AV valve, two cusps.

    • Chordae tendineae: Collagen cords anchoring valve flaps 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.

• Valves open and close in response to pressure differences.

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

Coronary Circulation

• The heart's muscle cells receive blood via the right and left coronary arteries.

• Coronary arteries branch extensively, providing alternative routes (collateral circulation).

• Blockage (occlusion) can cause tissue death (myocardial infarction).

• Cardiac veins collect deoxygenated blood from myocardium and drain into the coronary sinus, which empties into the right atrium.

Intrinsic Conduction System and Electrocardiogram (ECG)

• Intrinsic conduction system: Specialized cardiac cells (autorhythmic) initiate and distribute electrical impulses, ensuring coordinated contraction.

• Key components:

  • Sinoatrial (SA) node: Pacemaker, initiates impulses (~75/min).

  • Atrioventricular (AV) node: Delays impulse (0.1 sec) to allow atrial contraction.

  • Atrioventricular (AV) bundle (bundle of His): Conducts impulses through interventricular septum.

  • Right and left bundle branches: Carry impulses down septum.

  • Purkinje fibers: Spread impulse through ventricles, causing contraction from apex upward.

• Electrical activity is based on ion flow (Na+, K+, Ca2+).

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

  • P wave: Atrial depolarization (SA node).

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

  • T wave: Ventricular repolarization.

Principal Events of the Cardiac Cycle

• Systole: Contraction phase.

• Diastole: Relaxation phase.

• Cardiac cycle: All events in one heartbeat.

• 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 and blood is ejected.

  3. Isovolumetric relaxation (early diastole): Ventricles relax, semilunar valves close, atria refill, AV valves reopen.

Heart Sounds and Clinical Significance

• Two normal heart sounds per beat:

  • "Lub": Closure of AV valves.

  • "Dup": Closure of semilunar valves.

• Abnormal sounds (murmurs) indicate turbulent blood flow, often due to valve problems (incompetence or stenosis).

Cardiac Output and Its Regulation

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

• Formula: $CO = HR \times SV$

  • $CO$ = cardiac output (mL/min)

  • $HR$ = heart rate (beats/min)

  • $SV$ = stroke volume (mL/beat)

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

• Contractility: Force of contraction at a given preload; increased by hormones (thyroxine, epinephrine).

• Afterload: Pressure that must be overcome to eject blood; increased in hypertension, reducing SV.

Regulation of Heart Rate

• Autonomic nervous system:

  • Sympathetic: Releases norepinephrine, increases HR and contractility.

  • Parasympathetic: Releases acetylcholine, decreases HR.

• Hormones: Epinephrine (adrenaline) and thyroxine increase HR.

• Ions:

  • Low Ca2+: Depresses heart.

  • High Ca2+: Increases irritability, risk of spastic contraction.

  • High Na+: Inhibits Ca2+ transport, blocks contraction.

  • High K+: Interferes with depolarization, may cause heart block/arrest.

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

Risk Factors in Heart Disease

• Congestive heart failure: Inefficient pumping, inadequate circulation.

• Coronary atherosclerosis: Fatty buildup in coronary vessels, impairs oxygen delivery.

• Persistent high blood pressure: Increases cardiac workload.

• Multiple myocardial infarcts: Dead heart cells replaced by scar tissue, reducing efficiency.

• Dilated cardiomyopathy: Heart enlargement, decreased contractility.

Example Table: Comparison of Heart Valves

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

Additional info:

• Heart murmurs are more common in children and some adults but may not always indicate disease.

• Cardiac output in a healthy adult at rest is typically about 5 L/min.