The Heart

LECTURE 4: THE HEART

Course Information

  • Course Title: Biology 220 - A&P II

  • Instructor: Dr. Dolan

  • Chapter: 20

  • Copyright: © 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved

Introduction to the Heart

  • Components of the Cardiovascular System:

    • Heart

    • Blood

    • Blood Vessels

  • Heart Function:

    • Beats approximately 100,000 times each day.

    • Pumps about 8000 liters of blood per day.

20.1 Anatomy of the Heart

Circuits of Blood Flow

  • Pulmonary Circuit:

    • Carries blood to and from the gas exchange surfaces of the lungs.

  • Systemic Circuit:

    • Carries blood to and from the rest of the body.

  • Sequence of Circuits:

    • Each circuit begins and ends at the heart, with blood traveling through them in sequence.

Types of Blood Vessels

  • Arteries:

    • Carry blood away from the heart.

  • Veins:

    • Return blood to the heart.

  • Capillaries (Exchange Vessels):

    • Interconnect the smallest arteries and veins.

    • Facilitate the exchange of dissolved gases, nutrients, and wastes between blood and surrounding tissues.

Chambers of the Heart

  • Right Atrium:

    • Receives blood from the systemic circuit.

  • Right Ventricle:

    • Pumps blood into the pulmonary circuit.

  • Left Atrium:

    • Receives blood from the pulmonary circuit.

  • Left Ventricle:

    • Pumps blood into the systemic circuit.

Heart Structure

  • Positioning:

    • Great vessels connect at the base (superior).

    • Pointed tip is called the apex (inferior).

    • Located between two pleural cavities in the mediastinum.

Pericardium

  • Components:

    • Surrounds the heart.

    • Outer Fibrous Pericardium:

    • Inner Serous Pericardium:

    • Outer parietal layer.

    • Inner visceral layer (epicardium).

  • Pericardial Cavity:

    • Located between parietal and visceral layers; contains pericardial fluid.

Pericarditis

  • Definition:

    • Inflammation of the pericardium often caused by pathogens.

  • Symptoms:

    • Inflammation leads to surfaces rubbing together, producing a distinctive scratching sound.

  • Complications:

    • Can cause cardiac tamponade due to restricted movement of the heart from excess fluid in the pericardial cavity.

Superficial Anatomy of the Heart

  • Atria:

    • Two thin-walled chambers each with an expandable outer auricle.

  • Sulci (Grooves):

    • Contain fat and blood vessels:

    • Coronary Sulcus: Marks the border between atria and ventricles.

    • Interventricular Sulci: Anterior and posterior sulci mark the boundary between left and right ventricles.

Heart Wall Layers

  1. Visceral Layer of Serous Pericardium (Epicardium):

    • Covers the surface of the heart.

    • Covered by the parietal layer of the serous pericardium.

  2. Myocardium:

    • Cardiac muscle tissue that makes up the bulk of the heart wall.

  3. Endocardium:

    • Covers the inner surfaces of the heart, composed of simple squamous epithelium and areolar tissue.

Connective Tissues of the Heart

  • Function:

    • Support cardiac muscle fibers, blood vessels, and nerves of myocardium.

    • Distribute forces of contraction.

    • Add strength and prevent overexpansion of the heart.

    • Provide elasticity that helps return the heart to original size and shape after contraction.

Cardiac Skeleton

  • Definition:

    • Comprised of four dense bands of tough elastic tissue that encircle heart valves and bases of the pulmonary trunk and aorta.

    • Functions:

    • Stabilize positions of heart valves and ventricular muscle cells.

    • Electrically insulate ventricular cells from atrial cells.

Internal Anatomy

  • Chambers of Heart:

    • Separated by muscular partitions known as septa.

    • Interatrial Septum: Separates atria.

    • Interventricular Septum:

    • Much thicker than interatrial septum; separates ventricles.

  • Atrioventricular (AV) Valves:

    • Tricuspid and mitral valves that allow blood to flow in one direction between atria and ventricles.

  • Semilunar Valves:

    • Prevent backflow of blood into ventricles.

Right Atrium Blood Sources

  • Superior Vena Cava:

    • Carries blood from the head, neck, upper limbs, and chest.

  • Inferior Vena Cava:

    • Carries blood from the trunk, viscera, and lower limbs.

Foramen Ovale and Pectinate Muscles

  • Foramen Ovale:

    • An opening in the interatrial septum that connects the two atria in the fetal heart.

    • Closes at birth, forming the fossa ovalis.

  • Pectinate Muscles:

    • Prominent muscular ridges found on the anterior wall of the right atrium and inner surface of the auricle.

Blood Flow

  • Blood flows from right atrium to right ventricle via the tricuspid valve:

    • Tricuspid Valve:

    • Right atrioventricular valve with three cusps preventing backflow of blood.

    • Free edges of the valve attach to chordae tendineae from papillary muscles in the ventricle to prevent valve from opening backward.

Right Ventricle Features

  • Trabeculae Carneae:

    • Muscular ridges on the internal surface of ventricular walls.

  • Moderator Band:

    • Muscular ridge that delivers stimulus for contraction to papillary muscles.

Left Atrium and Ventricle

  • Left Atrium:

    • Receives blood from the left and right pulmonary veins and passes it to the left ventricle through the mitral valve (also known as the bicuspid valve).

  • Left Ventricle:

    • Does not have a moderator band; blood leaves through the aortic valve into the ascending aorta.

    • Aortic Sinuses:

    • Saclike expansions at the base of the ascending aorta that play roles in heart function.

    • Aorta:

    • Branches into the aortic arch and downward to the descending aorta.

Comparisons of Ventricles

  • Right Ventricle vs. Left Ventricle:

    • Holds and pumps the same amount of blood but has thinner walls, develops less pressure, and is more pouch-shaped than round.

Heart Valves

  • Function:

    • Prevent backflow of blood.

  • Atrioventricular Valves (AV):

    • Open when ventricles relax; blood pressure closes them during contraction, with papillary muscles contracting to tense chordae tendineae.

    • Prevent regurgitation into atria.

  • Semilunar Valves:

    • Open to allow blood flow into arteries; prevent backflow into ventricles when relaxed.

Heart Disease

  • Valvular Heart Disease (VHD):

    • Deterioration of valve function that may develop after carditis, possibly leading to complications from rheumatic fever.

Coronary Circulation

  • Definition:

    • Supplies blood to the heart muscle tissue.

    • Coronary Arteries:

    • Originate at aortic sinuses with elevated blood pressure maintaining flow.

  • Right Coronary Artery: Supplies:

    • Right atrium and portions of both ventricles, electrical conducting system.

    • Branches include marginal arteries and the posterior interventricular artery.

  • Left Coronary Artery: Supplies:

    • Left ventricle, left atrium, and interventricular septum.

    • Branches include the circumflex artery and anterior interventricular artery.

  • Arteriovenous Anastomoses:

    • Interconnect anterior and posterior interventricular arteries, maintaining a constant blood supply to cardiac muscle.

Cardiac Veins

  • Blood is drained from the cardiac tissues into:

    • Great cardiac vein (from the anterior interventricular artery).

    • Other veins include a posterior vein of the left ventricle, middle cardiac vein, and small cardiac vein.

    • Anterior cardiac veins drain directly into the right atrium.

Coronary Artery Disease (CAD)

  • Definition: Partial or complete blockage of coronary circulation.

    • Cardiac muscle cells require a constant supply of oxygen and nutrients; reduced blood flow impacts performance.

  • Coronary Ischemia: Occurs due to partial or complete blockage; implicates risk of myocardial infarction (MI).

  • Angina Pectoris:

    • An early symptom of CAD indicating temporary ischemia; discomfort may occur during exertion.

Myocardial Infarction (MI)

  • Definition:

    • If coronary circulation is blocked, cardiac muscle cells die due to lack of oxygen.

    • Results in dead tissue known as an infarct, often due to coronary thrombosis linked to atherosclerotic plaques.

  • Consequences: Depend on blockage site; widespread damage may lead to cessation of heartbeat in severe cases, though survival is possible with smaller obstructions.

  • Symptoms: Include intense, persistent pain and possible undiagnosed cases.

Treatments for CAD and MI

  • Prevention and Management:

    • 25% of MI patients die before receiving medical help; immediate action is crucial after symptoms.

    • Risk Factor Modification: includes smoking cessation, managing hypertension, dietary changes, stress reduction, and increasing physical activity.

  • Drug Treatments:

    • Examples include aspirin, beta-blockers, nitroglycerin, and clot-dissolving medications.

  • Noninvasive Surgery Options:

    • Atherectomy: Removal of plaque via a catheter.

    • Balloon Angioplasty: Inflating a balloon to compress plaque, often accompanied by stent insertion.

  • Coronary Artery Bypass Graft (CABG): Utilizing sections from other vessels to bypass obstructions in one or more coronary arteries.

20.2 The Conducting System

Overview of Heartbeat

  • Definition:

    • A single cardiac contraction; all heart chambers contract in a sequence—the atria first, then the ventricles.

  • Cardiac Muscle Cells:

    • Autorhythmic Cells: Control and coordinate heartbeat.

    • Contractile Cells: Responsible for contractions that propel blood.

Conducting System Components

  • Function:

    • Specialized cardiac muscle cells initiate and distribute electrical impulses prompting contraction.

  • Autorhythmicity:

    • Cardiac muscle can contract without neural or hormonal stimulus.

  • Key Components:

    • Pacemaker Cells: Located in:

    • Sinoatrial (SA) Node: Wall of right atrium.

    • Atrioventricular (AV) Node: Junction of atria and ventricles.

    • Conducting Cells: Include:

    • Internodal pathways in atria.

    • AV bundle, bundle branches, and Purkinje fibers in ventricles.

Pacemaker Potential

  • Definition:

    • Gradual depolarization of SA node cells.

  • Rates of Action Potentials:

    • SA node: 60–100 bpm.

    • AV node: 40–60 bpm.

  • Sinus Rhythm: Established by SA node depolarization; parasympathetic stimulation lowers heart rate.

Impulse Conduction through the Heart

  1. SA node activity begins atrial activation.

  2. Stimulus spreads across atria to AV node.

  3. Impulse is delayed for 100 msec at AV node to allow atrial contraction to begin.

  4. Impulse travels through AV bundle into right and left bundle branches in the interventricular septum.

  5. Purkinje fibers distribute impulses throughout the ventricular myocardium, completing atrial contraction and initiating ventricular contraction.

Disturbances in Heart Rhythm

  • Bradycardia: Abnormally slow heart rate.

  • Tachycardia: Abnormally fast heart rate.

  • Ectopic Pacemaker: Abnormal cells that generate high action potentials, disrupting normal contraction timing.

Electrocardiogram (ECG or EKG)

  • Definition:

    • A recording of electrical events in the heart; uses electrodes placed on the body.

  • ECG Features:

    • P Wave: Atrial depolarization.

    • QRS Complex: Ventricular depolarization; ventricles contract shortly after R wave.

    • T Wave: Ventricular repolarization.

  • Time Intervals:

    • P-R Interval: From start of atrial depolarization to start of QRS complex.

    • Q-T Interval: Time for ventricles to undergo a complete cycle of depolarization and repolarization.

Cardiac Contractile Cells

  • Resting Membrane Potentials:

    • Approximately –90 mV for ventricular cells; –80 mV for atrial cells.

  • Intercalated Discs:

    • Connect cardiac contractile cells, facilitating force transfer and action potential propagation.

Action Potential in Cardiac Contractile Cells

  1. Rapid Depolarization: Influx of sodium ions through fast sodium channels.

  2. Plateau Phase: Calcium ions enter through slow calcium channels, maintaining depolarization.

  3. Repolarization: Potassium ions exit the cell through slow potassium channels.

  • Refractory Periods:

    • Absolute Refractory Period (200 msec): No response possible.

    • Relative Refractory Period (50 msec): Response only to strong stimuli.

    • The action potential duration is about 30 times longer than in skeletal muscle fibers, preventing summation and tetany.

Calcium Ions and Cardiac Contractions

  1. Extracellular calcium enters during the plateau, providing 20% of contraction energy.

  2. Calcium entry triggers additional release from sarcoplasmic reticulum (SR).

  • Cardiac muscle is very sensitive to extracellular calcium changes.

Energy for Cardiac Contractions

  • Aerobic Energy Source:

    • Derived from mitochondrial breakdown of fatty acids and glucose.

    • Oxygen delivered via circulation; cardiac cells store oxygen in myoglobin.

20.3 The Cardiac Cycle

Definition

  • Cardiac Cycle: The duration from the start of one heartbeat to the start of the next, composed of alternating contraction (systole) and relaxation (diastole) periods.

Phases of the Cardiac Cycle

  1. Atrial Systole:

    • Atrial contraction forces blood into the relaxed ventricles.

  2. Ventricular Systole:

    • Ventricles contract, exerting pressure to close AV valves and later open semilunar valves for blood ejection.

  3. Atrial Diastole:

    • Follows atrial systole as the atria relax.

  4. Ventricular Diastole:

    • Ventricles relax, allowing passive filling with blood.

  • Blood pressure in each chamber rises during systole and falls during diastole.

Heart Sounds

  • Sounds:

    • Loud sounds from AV valve closure (first sound) and semilunar valve closure (second sound).

    • Softer sounds from blood flow into ventricles and atrial contraction.

    • Heart Murmurs: Result from regurgitation through valves.

20.4 Cardiac Output

Definition

  • Cardiac Output (CO): Volume of blood pumped by the left ventricle in one minute, calculated as: CO = HR × SV where:

    • CO = cardiac output (mL/min)

    • HR = heart rate (beats/min)

    • SV = stroke volume (mL/beat)

Stroke Volume (SV)

  • Calculation: SV = E D V - E S V where:

    • End-Diastolic Volume (EDV): Amount of blood in each ventricle at the end of diastole.

    • End-Systolic Volume (ESV): Amount of blood remaining after contraction.

  • Ejection Fraction: Percentage of EDV ejected during contraction.

Factors Affecting Heart Rate

  • Influences On:

    • Autonomic nervous system (sympathetic vs. parasympathetic influences).

    • Circulating hormones.

Autonomic Innervation

  • Composition:

    • Cardiac plexus innervates the heart; carried by vagus nerves.

  • Controllers of Heart Rate:

    • Cardioacceleratory center (sympathetic increases HR).

    • Cardioinhibitory center (parasympathetic decreases HR).

Cardiac Reflexes

  • Monitoring Mechanisms:

    • Blood pressure (via baroreceptors).

    • Oxygen and carbon dioxide levels (via chemoreceptors).

  • Anticipated Adjustments:

    • Adjustments based on these inputs to modify cardiac activity to meet bodily needs.

Hormonal Effects on Heart Rate

  • Increasing Hormones:

    • Epinephrine, norepinephrine, and thyroid hormone can elevate heart rate.

Factors Affecting Stroke Volume

  • EDV and ESV shifts affect SV and are influenced by:

    • Filling Time: Duration of diastole.

    • Venous Return: The volume of blood returning to the heart.

Preload and Frank-Starling Principle

  • Preload: Degree of ventricular stretching during diastole, directly related to EDV and influences contraction force.

  • Frank-Starling Principle: Longer stretch (higher EDV) correlates with increased stroke volume, up to physical limits imposed by myocardial connective tissue and the pericardium.

Afterload and Contractility

  1. Afterload: Pressure required to open semilunar valves; any factor affecting resistance will inversely affect stroke volume.

  2. Contractility: Strength of contractions influenced by autonomic activity and hormones—sympathetic stimulation increases force of contractions, while parasympathetic decreases it.

Summary of Cardiac Output Control

  • Factors affecting:

    • Heart Rate Control: Autonomic nervous system, hormones, and physical factors.

    • Stroke Volume Control: Factors affecting EDV (filling times, venous return) and ESV (preload, contractility, and afterload).

Concluding Remarks

  • Cardiac Reserves: The difference between resting and maximal cardiac outputs. The cardiovascular system adjusts to ensure adequate circulation during varying physical activities and emergencies. Key to survival and optimal body function.