Chapter 18: The Heart – Structure, Function, and Cardiac Physiology

Chapter 18: The Heart – Structure, Function, and Cardiac Physiology

(18.1) Heart Chambers and Circulatory Pathways

The heart is a muscular organ with four chambers that pumps blood through two major circuits: the pulmonary and systemic circuits. Understanding the structure and function of these chambers is essential for grasping cardiac physiology.

• Four Chambers: Right atrium, right ventricle, left atrium, left ventricle

• Pulmonary Circuit: Carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium

• Systemic Circuit: Delivers oxygenated blood from the left ventricle to the body and returns deoxygenated blood to the right atrium

• Associated Structures:

  • Base and apex of the heart

  • Pericardium (fibrous and serous layers)

  • Myocardium (muscular layer)

  • Epicardium (visceral pericardium)

  • Pericardial cavity (contains fluid to reduce friction)

  • Cardiac tamponade (compression of the heart due to fluid accumulation)

Example: Blood enters the right atrium from the body, passes to the right ventricle, and is pumped to the lungs. Oxygenated blood returns to the left atrium, moves to the left ventricle, and is pumped to the body.

(18.2) Heart Valves and Unidirectional Blood Flow

Heart valves ensure that blood flows in one direction through the heart, preventing backflow and maintaining efficient circulation.

• Atrioventricular (AV) Valves:

  • Tricuspid valve (right side)

  • Mitral (bicuspid) valve (left side)

  • Chordae tendineae anchor the valves to papillary muscles

• Semilunar (SL) Valves:

  • Aortic valve

  • Pulmonary valve

• Valve Disorders:

  • Incompetent valves (do not close properly)

  • Mitral valve prolapse (leaflets bulge into atrium)

Example: The mitral valve prevents backflow of blood from the left ventricle to the left atrium during ventricular contraction.

(18.3) Blood Flow Pathways and Coronary Circulation

Blood flows from atria to ventricles and then to either the lungs or the rest of the body. The coronary circulation supplies the heart muscle itself with oxygen and nutrients.

• Pathway of Blood Flow: Right atrium → right ventricle → pulmonary artery → lungs → left atrium → left ventricle → aorta → body

• Coronary Arteries:

  • Left and right coronary arteries

  • Branches: Great, middle, small cardiac veins; coronary sinus

• Coronary Circulation Disorders:

  • Angina pectoris (chest pain due to reduced blood flow)

  • Myocardial infarction (heart attack due to blocked artery)

Example: Blockage of the left coronary artery can lead to myocardial infarction affecting the left ventricle.

(18.4) Cardiac Muscle Structure and Intercalated Discs

Cardiac muscle fibers are connected by intercalated discs, which allow the heart to function as a coordinated unit (functional syncytium).

• Intercalated Discs: Specialized connections containing gap junctions and desmosomes

• Functional Syncytium: Allows rapid transmission of electrical impulses

• Comparison: Cardiac muscle vs. skeletal muscle

  • Cardiac muscle: branched, single nucleus, involuntary, intercalated discs

  • Skeletal muscle: multinucleated, voluntary, no intercalated discs

Example: Intercalated discs enable the heart to contract in a coordinated manner during each heartbeat.

(18.5) Pacemaker Cells and Cardiac Conduction System

Pacemaker cells generate action potentials that initiate the heartbeat and coordinate contraction through the cardiac conduction system.

• Pacemaker Cells: Located in the sinoatrial (SA) node; set the pace of the heartbeat

• Conduction Pathway:

  • SA node → Bachmann's bundle → atrioventricular (AV) node → AV bundle (bundle of His) → right and left bundle branches → Purkinje fibers

• Action Potential Phases:

  • Depolarization: rapid influx of Na+

  • Plateau: Ca2+ influx maintains depolarization

  • Repolarization: K+ efflux restores resting potential

• Disorders:

  • Arrhythmias (irregular heart rhythms)

  • Fibrillation (uncoordinated contraction)

  • Heart block (impaired conduction)

  • Asystole (absence of contraction)

Example: The SA node initiates each heartbeat, and conduction through the AV node ensures proper timing of ventricular contraction.

(18.6) The Cardiac Cycle: Mechanical Events and Blood Flow

The cardiac cycle describes the sequence of mechanical events and pressure changes that occur during one heartbeat, including systole and diastole.

• Phases of the Cardiac Cycle:

  • Early diastole: isovolumetric relaxation

  • Mid-late diastole: ventricular filling

  • Systole: isovolumetric contraction and ventricular ejection

• Pressure and Volume Changes: Blood moves in response to pressure gradients between chambers

Example: During ventricular systole, the pressure in the ventricles rises, causing the semilunar valves to open and blood to be ejected into the arteries.

(18.7) Regulation of Stroke Volume and Heart Rate

Stroke volume and heart rate are key determinants of cardiac output, which is the amount of blood pumped by the heart per minute. These parameters are regulated by intrinsic and extrinsic factors.

• Stroke Volume (SV): The volume of blood ejected by a ventricle in one contraction

• Heart Rate (HR): Number of heartbeats per minute

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

• Regulatory Factors:

  • Preload: degree of stretch of cardiac muscle before contraction

  • Afterload: pressure the heart must overcome to eject blood

  • Contractility: strength of contraction (affected by inotropic agents)

  • Chronotropic factors: affect heart rate (e.g., tachycardia, bradycardia)

• Disorders:

  • Heart palpitation

  • Congestive heart failure

  • Pulmonary congestion

  • Peripheral congestion

  • Cor pulmonale (right-sided heart failure due to lung disease)

  • Cardiac fibrosis (scarring of heart tissue)

Example: Increased preload and contractility can raise stroke volume, thereby increasing cardiac output.

Key Table: Heart Valves and Their Functions

Valve	Location	Function	Common Disorders
Tricuspid (AV)	Between right atrium and right ventricle	Prevents backflow into right atrium	Incompetence, stenosis
Mitral (AV)	Between left atrium and left ventricle	Prevents backflow into left atrium	Mitral valve prolapse, stenosis
Pulmonary (SL)	Between right ventricle and pulmonary artery	Prevents backflow into right ventricle	Stenosis
Aortic (SL)	Between left ventricle and aorta	Prevents backflow into left ventricle	Stenosis, regurgitation

Key Equation: Cardiac Output

Cardiac output is calculated as:

$CO = SV \times HR$

Where:

• CO = Cardiac Output (mL/min)

• SV = Stroke Volume (mL/beat)

• HR = Heart Rate (beats/min)

Summary Table: Cardiac Cycle Phases

Phase	Event	Valve Status
Early Diastole	Isovolumetric relaxation	All valves closed
Mid-Late Diastole	Ventricular filling	AV valves open, SL valves closed
Systole	Ventricular contraction and ejection	AV valves closed, SL valves open

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for college-level Anatomy & Physiology students.