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Overview and Anatomy of the Heart

• Physical Dimensions and Location

  • The heart is approximately the size of an individual's fist.

  • It is located in the chest cavity (mediastinum).

  • The heart pumps approximately $1000\,\text{gallons}$ of blood through the circulatory system every day.

• General Function

  • The primary physiological function of the heart is to distribute blood continuously throughout the body.

• Chamber Architecture

  • The heart is divided into $4$ distinct chambers: $2$ upper chambers and $2$ lower chambers.

  • Atria (Upper Chambers)

  • Consists of the Right Atrium and Left Atrium.

  • Characterized by very thin muscular walls.

  • The left and right atria are separated by the interatrial septum, which forms part of the cardiac myocardium.

  • Ventricles (Lower Chambers)

  • Consists of the Right Ventricle and Left Ventricle.

  • Characterized by thick muscular walls.

  • The left and right ventricles are separated by the interventricular septum.

• Functional Disparity Between Ventricles

  • Equal Volume Output: Both the left ventricle and the right ventricle pump the exact same volume of blood per contraction.

  • Wall Thickness & Pumping Force:

  • The left ventricle has a significantly thicker myocardial wall than the right ventricle.

  • The right ventricle only pumps deoxygenated blood a short distance to the lungs.

  • The left ventricle must pump oxygenated blood to the entire rest of the body.

  • Because of systemic vascular resistance, the left ventricle must exert at least $5\,\text{times}$ more force than the right ventricle.

Heart Wall Layers and Valve Structure

• Layers of the Cardiac Wall

  • Endocardium

  • The innermost layer of the heart.

  • Composed of smooth epithelial tissue.

  • Primary function is to provide a smooth surface that prevents blood clot formation inside the chambers.

  • Myocardium

  • The middle layer of the heart wall.

  • Composed of specialized cardiac muscle responsible for contraction and pumping action.

  • Epicardium

  • The outermost protective layer of the heart wall.

• Heart Valves and Biological Purpose

  • Purpose of Valves

  • Primary function of all heart valves is to enforce unidirectional blood flow and prevent backflow (regurgitation).

  • Tricuspid Valve

  • An atrioventricular (AV) valve named for having $3$ cusps (flaps).

  • Located between the right atrium and the right ventricle.

  • Pulmonary Semilunar Valve (Pulmonary Valve)

  • A semilunar valve located between the right ventricle and the pulmonary artery.

  • Directs deoxygenated blood toward the lungs.

  • Bicuspid Valve (Mitral Valve)

  • An atrioventricular (AV) valve named for having $2$ cusps.

  • Located between the left atrium and the left ventricle.

  • Aortic Semilunar Valve (Aortic Valve)

  • A semilunar valve located between the left ventricle and the aorta.

  • Directs oxygenated blood out into systemic circulation.

Systematic Blood Flow Through the Cardiac Chambers

• Step-by-Step Pathway of Blood Flow

  1. Vena Cava Return: Deoxygenated blood returns from the systemic tissues via two main veins:

  • Superior Vena Cava: Drains deoxygenated blood from the upper body (head, neck, upper extremities).

  • Inferior Vena Cava: Drains deoxygenated blood from the lower body (trunk, lower extremities).

  1. Right Atrium: Receives deoxygenated blood from both the superior and inferior vena cava.

  2. Tricuspid Valve: Blood passes from the right atrium through the tricuspid valve.

  3. Right Ventricle: Receives deoxygenated blood from the right atrium.

  4. Pulmonary Semilunar Valve: Blood is pumped from the right ventricle through the pulmonary valve during ventricular contraction.

  5. Pulmonary Artery: Receives deoxygenated blood and branches off to carry it to both lungs.

  6. Lungs: Blood enters pulmonary capillaries surrounding alveoli, releases carbon dioxide, and picks up oxygen ($O_2$).

  7. Pulmonary Veins: Oxygenated blood leaves the lungs through approximately $4$ pulmonary veins.

  8. Left Atrium: Receives oxygenated blood from the pulmonary veins.

  9. Bicuspid (Mitral) Valve: Blood passes from the left atrium through the bicuspid valve.

  10. Left Ventricle: Receives oxygenated blood from the left atrium.

  11. Aortic Semilunar Valve: Blood is forcefully ejected from the left ventricle through the aortic valve.

  12. Aorta: Primary systemic artery that receives high-pressure oxygenated blood and branches off into systemic arteries.

  13. Systemic Circulation: Oxygenated blood is delivered to arterioles, capillaries, tissues, and cells throughout the entire body.

Coronary Blood Flow and Vascular Physiology

• Coronary Circulation

  • The heart muscle (myocardium) requires its own dedicated blood supply to function.

  • Oxygenated blood branches off the base of the aorta into the right and left coronary arteries.

  • Coronary arteries deliver oxygen and essential nutrients directly to myocardial cells to generate energy for contraction.

  • Metabolic waste products and deoxygenated blood return from cardiac tissue through coronary veins back into the right atrium.

• Arteries vs. Veins vs. Capillaries

  • Arteries

  • Carry blood away from the heart.

  • Typically carry oxygenated blood (with the unique exception of the pulmonary artery, which carries deoxygenated blood to the lungs).

  • Do not contain valves because blood flow is driven by high pressure and assisted by gravity depending on trajectory.

  • Veins

  • Carry blood toward the heart.

  • Typically carry deoxygenated blood (with the unique exception of the pulmonary veins, which carry oxygenated blood from the lungs).

  • Contain inner valves to ensure unidirectional flow and prevent backflow caused by gravity working against venous return (especially from lower extremities).

  • Capillaries

  • Microscopic blood vessels that link arterioles to venules.

  • Site of gas, nutrient, and waste exchange between blood and interstitial tissues.

  • Capillary Refill Assessment: Clinical test evaluated to verify adequate peripheral tissue perfusion and capillary blood flow.

Cardiac Conduction System and Pacemaker Dynamics

• Electrical Impulse Generation

  • The heart possesses intrinsic automaticity, allowing it to generate its own electrical impulses to trigger mechanical contractions.

• Primary and Backup Pacemaker Hierarchy

  • Sinoatrial (SA) Node

  • Location: Superior wall of the right atrium.

  • Function: Primary pacemaker of the heart.

  • Normal Intrinsic Rate: $60\text{ to }100\,\text{beats per minute}$.

  • Atrioventricular (AV) Node

  • Function: Secondary/backup pacemaker if the SA node fails.

  • Intrinsic Rate: $40\text{ to }60\,\text{beats per minute}$.

  • Clinical Consideration: This rate is life-sustaining, but prolonged reliance on the AV node is suboptimal.

  • Bundle of His and Purkinje Fibers

  • Location: Runs through the interventricular septum and spreads throughout the ventricular myocardium.

  • Function: Tertiary backup pacemaker that triggers ventricular muscle depolarization.

  • Intrinsic Rate: $20\text{ to }35\,\text{beats per minute}$ (also referenced as $30\text{ to }35\,\text{beats per minute}$).

  • Clinical Consideration: This rate is not long-term life-sustaining and requires immediate clinical intervention.

Mechanical Events of the Cardiac Cycle and Heart Sounds

• The Cardiac Cycle

  • Represents the complete sequence of mechanical events during a single heartbeat.

  • Sequential Contraction: Atria contract simultaneously first, followed immediately by simultaneous contraction of the ventricles.

  • Systole: Phase of chamber contraction and blood ejection.

  • Diastole: Phase of chamber relaxation and ventricular filling.

• Phonocardiography / Heart Sounds ("Lub-Dub")

  • Heart sounds are generated by the closing of cardiac valves, not by muscle contractions.

  • $S_1$ Heart Sound ("Lub")

  • Produced by the closure of the Atrioventricular (AV) valves (Tricuspid and Bicuspid/Mitral valves) at the onset of ventricular systole.

  • $S_2$ Heart Sound ("Dub")

  • Produced by the closure of the Semilunar valves (Aortic and Pulmonary valves) at the end of ventricular systole / onset of diastole.

Autonomic and Endocrine Regulation of Cardiac Function

• Central Nervous System Regulation

  • The medulla oblongata receives sensory inputs and regulates heart rate according to systemic physiological demands.

  • Sympathetic Nervous System (SNS)

  • Mediates the "fight-or-flight" physiological stress response (e.g., encountering a threat such as a bear).

  • Increases heart rate, myocardial contractility, and cardiac output.

• Hormones Regulating Cardiac Function

  • Epinephrine and Norepinephrine

  • Secreted by the adrenal medulla (inner region of adrenal glands located atop the kidneys).

  • Increase heart rate and cardiac force during sympathetic stimulation.

  • Aldosterone

  • Secreted by the adrenal cortex (outer region of adrenal glands).

  • Regulates electrolyte balance by promoting renal reabsorption of sodium ($Na^+$) into the bloodstream and excretion of potassium ($K^+$).

  • Water follows sodium passively; thus, aldosterone retains fluid in the intravascular space to raise blood volume and blood pressure.

  • Clinical Note: Serum potassium imbalances (hyperkalemia or hypokalemia) directly cause cardiac arrhythmias.

  • Atrial Natriuretic Peptide (ANP)

  • Secreted by atrial myocardial cells when elevated blood volume or pressure causes excessive stretching of the atrial walls.

  • Promotes renal excretion of sodium ($Na^+$) and water.

  • Inhibits aldosterone release.

  • Direct antagonist to aldosterone.

Hemodynamic Metrics: Ejection Fraction, Cardiac Output, and Stroke Volume

• Ejection Fraction (EF)

  • Quantitative measurement of left ventricular pumping efficiency.

  • Represents the percentage of end-diastolic blood volume ejected per contraction.

  • Normal Range: $55\%\text{ to }70\%$ per beat.

• Cardiac Output (CO)

  • Total volume of blood pumped by a ventricle per unit of time ($1\,\text{minute}$).

  • Normal Resting Range: $4\text{ to }8\,\text{L/min}$ in healthy adults.

• Stroke Volume (SV)

  • The actual volume of blood pumped out of a ventricle with each individual contraction.

  • Normal Range: $50\text{ to }75\,\text{mL/beat}$.

  • Approximately $60\text{ to }70\,\text{mL}$ of blood enters the aorta with each ventricular contraction.

Preload, Afterload, and Myocardial Contractility

• Preload

  • Definition: The initial stretching of cardiac muscle fibers (myocytes) prior to contraction, dictated by end-diastolic volume in the left ventricle.

  • Factors Increasing Preload:

  • Systemic Hypertension.

  • Aortic valve disease.

  • Hypervolemia (fluid overload).

  • Factors Decreasing Preload:

  • Rapid heart rates (decreased diastolic filling time).

  • Hypovolemia (low fluid volume).

  • Hemorrhage.

  • Dehydration.

  • Pharmacologic agents such as diuretics (which promote urinary fluid excretion).

• Afterload

  • Definition: The vascular resistance or pressure that the ventricles must overcome to open semilunar valves and eject blood during systole.

  • Specific Ventricular Resistances:

  • Right Ventricle works against pulmonary arterial pressure.

  • Left Ventricle works against systemic vascular resistance.

  • Factors Increasing Afterload:

  • Systemic hypertension.

  • Elevated peripheral vascular resistance.

  • Consequence of Increased Afterload: Leads to elevated cardiac workload, forcing myocardial cells to overwork and resulting in ventricular hypertrophy.

• Myocardial Contractility

  • Definition: The intrinsic strength and speed of myocardial contraction for a given preload.

• Cellular Adaptations: Hypertrophy vs. Atrophy

  • Hypertrophy: Enlargement and thickening of muscle cells/tissue resulting from sustained overwork or high pressure.

  • Atrophy: Shrinkage or wasting away of cellular tissue size.

Academic and Clinical Administrative Announcements

• Live NCLEX/Program Review Event

  • Dates: November $2\text{nd}$ and November $3\text{rd}$.

  • Daily Schedule: $9:00\,\text{AM}\text{ to }4:00\,\text{PM}$.

  • Attendance Policy: Mandatory on-campus attendance.

  • Provider: Elsevier representative.

  • Content Scope: Comprehensive program review, study strategy refinement, question breakdown techniques, and refresher on core curriculum.

  • Credit Value: $8\,\text{hours}$ awarded for each completed day ($16\,\text{hours}$ total).