Cardiac Anatomy and Physiology: Structure, Function, and Regulation of the Heart

Chapter 18: The Heart – Structure, Function, and Regulation

(18.1) Heart Chambers and Circulation

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 each chamber is essential for grasping cardiac physiology.

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

• Pulmonary Circuit: Carries deoxygenated blood from the right side of the heart to the lungs

• Systemic Circuit: Delivers oxygenated blood from the left side of the heart to the body

• Key Structures:

  • Base and apex of the heart

  • Pericardium (fibrous and serous layers)

  • Myocardium (muscular layer)

  • Epicardium (visceral pericardium)

• Example: Cardiac tamponade is a clinical condition where fluid accumulates in the pericardial cavity, restricting heart movement.

(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: Separate atria from ventricles

  • Tricuspid valve (right side)

  • Mitral (bicuspid) valve (left side)

  • Chordae tendineae anchor valves to papillary muscles

• Semilunar (SL) Valves: Located at the exits of the ventricles

  • Aortic valve (left ventricle to aorta)

  • Pulmonary valve (right ventricle to pulmonary artery)

• Valve Disorders:

  • Incompetent valves (e.g., mitral valve prolapse) allow backflow

• Example: Mitral valve prolapse is a common cause of heart murmurs.

(18.3) Blood Flow Pathways and Coronary Circulation

Blood flows from atria to ventricles, then to the lungs or systemic circulation. The heart's own blood supply is provided by the coronary arteries and veins.

• Pathway of Blood Flow:

  1. Right atrium → right ventricle → pulmonary artery → lungs

  2. Lungs → pulmonary veins → left atrium → left ventricle → aorta → body

• Coronary Circulation:

  • Coronary arteries (left and right) supply oxygenated blood to myocardium

  • Coronary veins (great, middle, small, coronary sinus) return deoxygenated blood

• Disorders:

  • Angina pectoris: chest pain due to reduced blood flow

  • Myocardial infarction: heart attack due to blocked coronary artery

• Example: Coronary sinus collects blood from the myocardium and drains into the right atrium.

(18.4) Cardiac Muscle and Intercalated Discs

Cardiac muscle fibers are connected by intercalated discs, forming a functional syncytium that allows coordinated contraction.

• Intercalated Discs: Specialized junctions containing desmosomes and gap junctions

• Functional Syncytium: Allows rapid transmission of electrical impulses

• Comparison: Cardiac muscle is striated like skeletal muscle but contracts involuntarily and rhythmically

• Example: Desmosomes provide mechanical strength; gap junctions allow ion flow between cells.

(18.5) Pacemaker Cells and Cardiac Conduction

Pacemaker cells generate action potentials that initiate and regulate the heartbeat. The cardiac conduction system ensures coordinated contraction.

• Pacemaker Cells: Located in the sinoatrial (SA) node; set the heart rate

• Conduction Pathway:

  1. Sinoatrial (SA) node

  2. Bachmann's bundle

  3. Atrioventricular (AV) node

  4. AV bundle (bundle of His)

  5. Right and left bundle branches

  6. Purkinje fibers

• Action Potential Phases:

  • Depolarization: rapid influx of Na+

  • Plateau: Ca2+ influx maintains depolarization

  • Repolarization: K+ efflux restores resting potential

• Absolute Refractory Period: Prevents tetanic contractions

• Disorders: Arrhythmias, fibrillation, heart block, asystole

• Example: Electrocardiogram (ECG) records electrical activity of the heart.

(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: ventricular relaxation

  • Mid-late diastole: ventricular filling

  • Systole: ventricular contraction and ejection

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

• Example: Ventricular ejection occurs when ventricular pressure exceeds arterial pressure.

(18.7) Regulation of Stroke Volume and Heart Rate

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

• Stroke Volume (SV): Amount of blood ejected by a ventricle per beat

• Heart Rate (HR): Number of beats 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 palpitations, congestive heart failure, pulmonary congestion, peripheral congestion, cor pulmonale, cardiac fibrosis

• Example: Congestive heart failure results from the heart's inability to pump sufficient blood.

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	Regurgitation
Mitral (AV)	Between left atrium and left ventricle	Prevents backflow into left atrium	Mitral valve prolapse
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

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	Pressure Change	Volume Change
Early Diastole	Ventricular relaxation	Decreases	Minimal
Mid-Late Diastole	Ventricular filling	Low	Increases
Systole	Ventricular contraction/ejection	Increases	Decreases

Additional info: Some details, such as the full conduction pathway and the phases of the cardiac cycle, were expanded for clarity and completeness.