Chapter 18: The Cardiovascular System – The Heart

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Cardiac Anatomy and Circulation

Pulmonary and Systemic Circuits

The heart is central to two major circulatory pathways: the pulmonary circuit and the systemic circuit. These circuits ensure oxygenation of blood and distribution of nutrients throughout the body.

Pulmonary circuit: Transports oxygen-poor blood from the right side of the heart to the lungs for oxygenation, then returns oxygen-rich blood to the left side of the heart.
Systemic circuit: Distributes oxygen-rich blood from the left side of the heart to body tissues, then returns oxygen-poor blood to the right side.
Color coding: Blue indicates oxygen-poor blood; red indicates oxygen-rich blood. Most veins are blue, most arteries are red, with exceptions in pulmonary circulation.
Arteries: Carry blood away from the heart.
Veins: Carry blood toward the heart.

Location and Orientation of the Heart

The heart is located in the thoracic cavity, within the mediastinum, between the lungs. It spans from the second to the fifth intercostal space and is slightly left of center.

Base: Superior portion, attached to large vessels; connection point to the body.
Apex: Inferior, pointed tip formed by the ventricles; points toward the left hip and allows free movement.

Protective Coverings of the Heart

The Pericardium

The heart is enclosed in a double-layered sac called the pericardium, which protects and anchors the heart.

Fibrous pericardium: Superficial, tough layer providing protection and anchoring.
Serous pericardium: Thin inner layer, further divided into:
- Parietal pericardium: Lines the inner surface of the fibrous pericardium.
- Visceral pericardium (epicardium): Adheres directly to the heart muscle.
Pericardial cavity: Space between parietal and visceral layers, containing lubricating fluid for frictionless movement.

Heart Wall Structure

Layers of the Heart Wall

The heart wall consists of three distinct layers, each with specific structure and function.

Layer	Description	Composition	Function
Epicardium	Outer layer, same as visceral serous pericardium	Simple squamous epithelium	Protects and lubricates heart surface
Myocardium	Middle, thickest layer	Cardiac muscle cells	Responsible for heart’s pumping action
Endocardium	Inner lining of chambers and valves	Simple squamous epithelium	Provides smooth lining continuous with blood vessels

Myocardium is the bulk of the heart, responsible for contraction. Endocardium lines chambers and valves, facilitating smooth blood flow.

Internal Cardiac Anatomy

Muscular Structures and Landmarks

Pectinate muscles: Raised ridges in atria, especially right atrium.
Fossa ovalis: Depression in right atrium, remnant of fetal foramen ovale.
Trabeculae carneae: Raised ridges in ventricles.
Papillary muscles: Projections in ventricles anchoring chordae tendineae (fibrous cords attached to AV valve cusps).
Interventricular septum: Wall separating left and right ventricles.

Major Vessels Opening into the Atria

Right atrium: Receives blood from superior vena cava, inferior vena cava, and coronary sinus.
Left atrium: Receives blood from pulmonary veins.

Cardiac Valves

Valve Type	Location	Alternate Names	Function/Notes
Right AV valve	Between right atrium & ventricle	Tricuspid valve	3 cusps; controls blood flow into right ventricle
Left AV valve	Between left atrium & ventricle	Bicuspid/Mitral valve	2 cusps; large due to left ventricle’s force
Pulmonary semilunar valve	Between right ventricle & pulmonary trunk	Pulmonary valve	Controls blood flow to lungs
Aortic semilunar valve	Between left ventricle & aorta	Aortic valve	Controls blood flow to systemic circulation

Coronary Circulation

Coronary Arteries and Cardiac Veins

The heart muscle is supplied by coronary arteries and drained by cardiac veins. This is the shortest circulation loop, providing oxygen and nutrients directly to the myocardium.

Coronary Artery	Areas Supplied	Important Branches
Left Coronary Artery	Left atrium, anterior septum, anterior ventricles, posterior left ventricle	Anterior interventricular artery, Circumflex artery
Right Coronary Artery	Right atrium, most of right ventricle	Right marginal artery, Posterior interventricular artery

Anterior interventricular artery: Runs along interventricular septum.
Circumflex artery: Circles left atrium posteriorly.
Right marginal artery: Supplies lateral right ventricle.
Posterior interventricular artery: Supplies posterior heart wall.

Cardiac Muscle Structure and Function

Cardiac Myocytes and Intercalated Discs

Cardiac muscle cells (myocytes) are interconnected by intercalated discs, which contain specialized junctions for mechanical and electrical coupling.

Desmosomes: Strong rivets holding cells together under mechanical stress.
Gap junctions: Channels allowing rapid electrical and ionic communication.
Functional syncytium: Cardiac cells contract in unison, producing coordinated heartbeats.

Pacemaker Cells and Calcium’s Role

Pacemaker cells: Specialized cells generating electrical impulses, enabling autonomous heart rhythm.
Calcium ions (Ca2+): Cardiac contraction relies on both extracellular and intracellular calcium.
Depolarization opens calcium channels, allowing extracellular Ca2+ influx, which triggers release of stored Ca2+ from the sarcoplasmic reticulum.
Cardiac muscle relies exclusively on aerobic respiration, with abundant mitochondria to meet high oxygen demands.

Intrinsic Cardiac Conduction System

Components and Pathway

The heart’s electrical impulse originates and propagates through a specialized conduction system, ensuring coordinated contractions.

Sinoatrial (SA) node: Located in upper right atrium; initiates impulse.
Atrioventricular (AV) node: Between atria above ventricles; receives impulse.
AV bundle (bundle of His): Transmits impulse to bundle branches.
Right and left bundle branches: Carry impulse toward respective ventricles.
Subendocardial conducting network (Purkinje fibers): Innervates ventricular muscle fibers.

This sequence ensures atrial contraction precedes ventricular contraction, optimizing blood flow and valve function.

Extrinsic Control and ECG Analysis

Extrinsic Control of Heart Rate

Autonomic nervous system: Sympathetic stimulation increases heart rate and contractility; parasympathetic input decreases heart rate.
Other factors: Age, gender, health status, chemicals, and medications.

Electrocardiogram (ECG)

An ECG records the heart’s electrical activity, mapping distinct events in the cardiac cycle.

ECG Component	Description	Cardiac Event
P wave	Small upward deflection	Atrial depolarization (SA node activity)
QRS complex	Large, sharp waveform	Ventricular depolarization; atrial repolarization masked
T wave	Medium upward wave	Ventricular repolarization
PR interval	Start of P to start of QRS	Atrial to ventricular conduction time
ST segment	End of QRS to start of T	Duration of ventricular depolarization
QT interval	Start of QRS to end of T	Total ventricular depolarization and repolarization

Deviations in timing or shape can indicate cardiac pathology.

The Cardiac Cycle

Phases and Events

The cardiac cycle describes the sequence of events in one heartbeat, including blood flow, pressure changes, and valve actions.

Systole: Contraction phase; chambers reach smallest volume.
Diastole: Relaxation phase; chambers expand to largest volume.

Phase 1: Ventricular Filling

Occurs during mid to late diastole; ventricles relaxed, atria contract.
AV valves open; semilunar valves closed.
Ventricles fill to End Diastolic Volume (EDV).

Phase 2: Ventricular Systole

Includes isovolumetric contraction and ventricular ejection.
AV valves close; semilunar valves open when pressure exceeds vessel pressure.
Blood ejected; End Systolic Volume (ESV) remains.

Phase 3: Isovolumetric Relaxation

Ventricles relax; pressure drops.
Semilunar valves close; AV valves open when atrial pressure exceeds ventricular pressure.
Cycle repeats.

Term	Definition	Importance
EDV	Volume of blood in ventricles at end of diastole	Maximum ventricular filling
ESV	Volume remaining after systole	Residual volume after contraction

Cardiac Output and Regulation

Cardiac Output Equation

Cardiac output (CO) is the volume of blood pumped by each ventricle per minute. It is determined by heart rate (HR) and stroke volume (SV).

Stroke volume (SV): Volume of blood ejected per beat.
Heart rate (HR): Beats per minute.

Formula:

Where:

Example: If EDV = 120 mL, ESV = 50 mL, HR = 75 bpm:

Factors Affecting Stroke Volume

Preload: Degree of stretch before contraction; increased by venous return and exercise.
Contractility: Strength of contraction independent of preload; increased by sympathetic stimulation.
Afterload: Pressure ventricles must overcome; increased by arterial blood pressure (hypertension).

Changes in any factor affect SV and thus CO. If SV decreases, HR may increase to compensate.

Summary Table: Factors Affecting Cardiac Output

Factor	Effect on SV	Effect on CO
Preload	Increases SV	Increases CO
Contractility	Increases SV	Increases CO
Afterload	Decreases SV	Decreases CO (unless HR compensates)

External Controllers of Heart Rate

Autonomic nervous system: Sympathetic increases HR and contractility; parasympathetic decreases HR.
Other factors: Age, gender, health, chemicals, medications.

Example:

During exercise, increased venous return raises preload, enhancing stroke volume and cardiac output.

Additional info: Parasympathetic input (via the vagus nerve) slows heart rate, while sympathetic input (via norepinephrine) increases both heart rate and contractility.