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Chapter 20: The Heart – Structure, Function, and Physiology

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The Heart and the Cardiovascular System

Overview of Heart Function

The heart is a muscular organ at the center of the cardiovascular system, responsible for circulating blood throughout the body. It ensures the delivery of oxygen and nutrients to tissues and the removal of carbon dioxide and metabolic wastes.

  • Primary Role: Circulates oxygen and nutrients; removes waste products.

  • Activity: Beats about 100,000 times daily, pumping approximately 2000 gallons of blood.

Circuits of the Heart

  • Pulmonary Circuit: Carries oxygen-poor blood from the right ventricle to the lungs for oxygenation, then returns oxygen-rich blood to the left atrium.

  • Systemic Circuit: Delivers oxygen-rich blood from the left ventricle to the body and returns oxygen-poor blood to the right atrium.

Order of Blood Flow in Pulmonary Circuit: RIGHT VENTRICLE → PULMONARY ARTERIES → LUNGS → PULMONARY VEINS → LEFT ATRIUM

Order of Blood Flow in Systemic Circuit: LEFT VENTRICLE → ARTERIES TO BODY → VEINS BACK TO HEART → RIGHT ATRIUM

Blood Vessels

  • Arteries: Carry blood away from the heart.

  • Veins: Return blood to the heart.

  • Capillaries: Thin-walled vessels for exchange of gases, nutrients, and wastes.

  • Great Vessels: Largest arteries and veins directly connected to the heart.

Mnemonic: Arteries go Away from the heart; Veins Visit the heart.

Heart Structure

Heart Chambers

The heart contains four chambers, each with a specific role in blood circulation:

  • Right Atrium: Receives oxygen-poor blood from the systemic circuit.

  • Right Ventricle: Pumps blood into the pulmonary circuit.

  • Left Atrium: Receives oxygen-rich blood from the pulmonary veins.

  • Left Ventricle: Pumps oxygen-rich blood into the systemic circuit.

Location and Position

  • Located in the thoracic cavity, within the mediastinum, anterior to the sternum.

  • The base is at the superior end, slightly left of the midline; the apex is the inferior, pointed tip.

Heart Wall Layers

  • Endocardium (inner): Simple squamous epithelium lining chambers and valves.

  • Myocardium (middle): Thick layer of cardiac muscle responsible for contraction.

  • Epicardium (outer): Serous membrane (visceral pericardium) covering the heart.

Pericardium

  • Fibrous Pericardium: Outer, tough connective tissue stabilizing the heart.

  • Serous Pericardium: Two layers (parietal and visceral/epicardium) with pericardial fluid in between, reducing friction.

  • Pericarditis: Inflammation of pericardial surfaces, often due to infection.

External Landmarks

  • Auricle: Expandable extension of each atrium.

  • Coronary Sulcus: Groove separating atria from ventricles.

  • Anterior/Posterior Interventricular Sulci: Grooves marking boundaries between ventricles, containing blood vessels and fat.

Connective Tissues and Cardiac Skeleton

  • Cardiac muscle cells are wrapped in elastic connective tissue for support and elasticity.

  • Cardiac Skeleton: Dense connective tissue framework supporting valves and great vessels, and electrically isolating atria from ventricles.

Heart Chambers and Septa

  • Interatrial Septum: Separates right and left atria.

  • Interventricular Septum: Separates right and left ventricles; thicker than interatrial septum.

Heart Valves

  • Atrioventricular (AV) Valves: Between atria and ventricles; include tricuspid (right) and mitral/bicuspid (left) valves.

  • Semilunar Valves: Between ventricles and great arteries; include pulmonary and aortic valves.

Valve Structure and Function

  • AV valves prevent backflow into atria during ventricular contraction; attached to chordae tendineae and papillary muscles.

  • Semilunar valves prevent arterial backflow into ventricles after contraction.

Special Features

  • Foramen Ovale: Fetal opening between atria; closes after birth to become fossa ovalis.

  • Pectinate Muscles: Muscular ridges in right atrium and auricle.

  • Trabeculae Carneae: Muscular ridges in ventricles.

Left vs. Right Ventricle

  • Left ventricle has thicker walls and generates higher pressure to pump blood through systemic circuit.

  • Right ventricle pumps at lower pressure to the lungs.

Coronary Circulation

Coronary Arteries

  • Right Coronary Artery: Supplies right atrium, right ventricle, and parts of the conduction system.

  • Left Coronary Artery: Supplies left atrium, left ventricle, and interventricular septum; branches into the circumflex artery.

Coronary Veins

  • Great Cardiac Vein: Drains anterior ventricles.

  • Anterior Cardiac Veins: Drain right ventricle.

Cardiac Muscle and Electrical Activity

Autorhythmicity

  • Cardiac muscle can contract spontaneously without external nervous stimulation.

  • Essential for coordinated atrial and ventricular contractions.

  • Measured by an electrocardiogram (ECG).

Types of Cardiac Muscle Cells

  • Autorhythmic Cells: Generate and conduct electrical impulses (pacemaker and conducting cells).

  • Contractile Cells: Produce force for blood ejection; activated by electrical impulses.

Conducting System

  • Action potentials originate in the conducting system and spread to contractile cells.

  • Atria contract first, followed by ventricles.

Key Components

  • Sinoatrial (SA) Node: Main pacemaker; initiates heartbeat.

  • Atrioventricular (AV) Node: Delays impulse before passing to ventricles.

  • Internodal Pathways: Transmit impulse from SA to AV node.

  • AV Bundle (Bundle of His), Bundle Branches, Purkinje Fibers: Distribute impulse through ventricles.

Depolarization and Heart Rhythm

  • Depolarization: Electrical activation due to sodium and calcium influx.

  • SA node sets the basic heart rhythm (sinus rhythm).

  • AV node limits maximum ventricular rate to ~230 bpm.

Impulse Conduction Pathway

  1. Impulse originates at SA node.

  2. Spreads through atria.

  3. Pauses at AV node.

  4. Travels down interventricular septum to apex.

  5. Spreads through ventricular myocardium to base.

Disturbances in Rhythm

  • Bradycardia: Slower than normal heart rate.

  • Tachycardia: Faster than normal heart rate.

Electrocardiogram (ECG)

  • Records electrical activity of the heart via body surface electrodes.

  • Assesses pacemaker, conducting, and contractile cell function.

Wave

Represents

P wave

Atrial depolarization

QRS complex

Ventricular depolarization (Q: septal, R: main, S: Purkinje fibers)

T wave

Ventricular repolarization

Arrhythmia

  • Any irregularity in heart rhythm.

  • Causes: Myocardial damage, pacemaker/conducting cell injury, drugs, electrolyte imbalance.

Intercalated Discs

  • Specialized connections between cardiac cells for force transmission and rapid impulse propagation.

Action Potential in Cardiac Cells

  1. Depolarization

  2. Plateau

  3. Repolarization

Includes a refractory period to prevent tetanus.

Energy for Contraction

  • Relies on mitochondria for ATP production from fatty acids and glucose.

  • Oxygen stored in myoglobin.

The Cardiac Cycle

Systole and Diastole

  • Systole: Contraction phase; blood is ejected, pressure rises.

  • Diastole: Relaxation phase; chambers fill, pressure falls.

Phases of the Cardiac Cycle

  1. Atrial Systole: Atria contract, topping off ventricular filling (EDV).

  2. Atrial Diastole: Atria relax and refill.

  3. Ventricular Systole: Ventricles contract, ejecting blood (ESV remains).

  4. Ventricular Diastole: Ventricles relax, AV valves open, passive filling occurs.

End-Diastolic Volume (EDV): Maximum volume in ventricle at end of diastole. End-Systolic Volume (ESV): Volume remaining after systole.

Heart Failure

  • Occurs when ventricles cannot pump effectively.

  • Atrial damage is less critical due to passive blood flow into ventricles.

Heart Sounds

  • S1 ("Lubb"): Closure of AV valves at start of ventricular systole.

  • S2 ("Dupp"): Closure of semilunar valves at start of ventricular diastole.

  • S3: Blood flow into ventricles (faint in adults).

  • S4: Atrial contraction (faint in adults).

Cardiac Output and Regulation

Cardiac Output (CO)

Cardiac output is the volume of blood pumped by the left ventricle per minute.

  • Formula:

  • CO: Cardiac Output (mL/min)

  • HR: Heart Rate (beats/min)

  • SV: Stroke Volume (mL/beat)

Stroke Volume (SV)

  • Amount of blood ejected per contraction.

Ejection Fraction (EF)

  • Percentage of EDV ejected per beat.

  • Normal: 55-70%. EF < 40% suggests heart failure.

Regulation of Heart Rate

  • Autonomic Nervous System (ANS):

    • Sympathetic: Increases HR (norepinephrine).

    • Parasympathetic: Decreases HR (acetylcholine).

  • Hormones: Various hormones can modulate HR.

  • Normal Resting HR: 60-100 bpm.

Summary Table: Heart Valves and Blood Flow

Valve

Location

Function

Tricuspid (Right AV)

Between right atrium and ventricle

Prevents backflow into right atrium

Pulmonary Semilunar

Between right ventricle and pulmonary trunk

Prevents backflow into right ventricle

Mitral/Bicuspid (Left AV)

Between left atrium and ventricle

Prevents backflow into left atrium

Aortic Semilunar

Between left ventricle and aorta

Prevents backflow into left ventricle

Example: Cardiac Cycle in Action

During exercise, sympathetic stimulation increases heart rate and stroke volume, raising cardiac output to meet the body's increased oxygen demand.

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