Chapter 19: The Heart and Cardiovascular Function – Structured Study Notes

The Heart and Cardiovascular Function

Orientation and Structure of the Heart

The heart is a muscular organ located in the mediastinum, slightly left of the midline, and is responsible for pumping blood throughout the body. Its structure and position are essential for understanding its function and clinical relevance.

• Base: Superior surface where great veins and arteries attach; located posterior to the sternum at the third costal cartilage, centered 1.2 cm left of midline.

• Apex: Inferior, pointed tip reaching the fifth intercostal space, about 7.5 cm left of midline.

• Borders:

  • Superior border: Formed by the base.

  • Right border: Formed by the right atrium.

  • Left border: Formed by the left ventricle and part of the left atrium.

  • Inferior border: Mainly the inferior wall of the right ventricle.

Heart Wall and Tissues

The heart wall consists of three distinct layers, each with specialized functions and structures.

• Epicardium (Visceral Pericardium): Outer serous membrane covering the heart, composed of mesothelium and areolar tissue.

• Myocardium: Middle muscular layer forming atria and ventricles; contains concentric layers of cardiac muscle tissue, blood vessels, and nerves.

• Endocardium: Inner layer covering heart surfaces and valves; made of simple squamous epithelium (endothelium) and areolar tissue, continuous with endothelium of great vessels.

The Pericardium and Pericardial Cavity

The pericardium is a double-layered serous membrane that surrounds and protects the heart, providing lubrication and stability.

• Visceral Pericardium: Attached directly to the heart (epicardium).

• Parietal Pericardium: Lines the outer wall of the pericardial cavity, reinforced by a dense fibrous layer.

• Pericardial Sac: Formed by the parietal pericardium and dense fibrous layer; stabilizes the heart's position.

• Pericardial Cavity: Space between visceral and parietal pericardium, containing 10–15 mL of pericardial fluid to reduce friction.

• Clinical Conditions:

  • Pericarditis: Inflammation of pericardial surfaces, causing friction and audible scratching sounds.

  • Cardiac Tamponade: Accumulation of fluid restricting heart movement, often due to injury or acute pericarditis.

Cardiac Muscle Tissue

Cardiac muscle tissue is specialized for continuous, rhythmic contraction and is unique compared to skeletal muscle.

• Cell Characteristics: Smaller size, single central nucleus, branching interconnections.

• Intercalated Discs: Specialized junctions containing desmosomes (anchor cells) and gap junctions (allow ion flow and electrical connectivity).

• Functional Syncytium: Cardiac muscle acts as a single coordinated unit due to intercalated discs.

• Metabolism: Highly aerobic, rich in mitochondria and myoglobin, extensive capillary network.

Surface Anatomy of the Heart

The heart's external features include chambers, vessels, and grooves marking boundaries.

• Anterior Surface: All four chambers visible; auricles are expandable pouches; ligamentum arteriosum connects pulmonary trunk to aortic arch.

• Sulci:

  • Coronary Sulcus: Border between atria and ventricles, contains fat and blood vessels.

  • Anterior Interventricular Sulcus: Boundary between left and right ventricles.

• Posterior Surface: Connections to pulmonary veins, coronary veins, and venae cavae; coronary sinus drains myocardium.

Coronary Circulation

The heart receives a constant supply of oxygen and nutrients via coronary arteries and veins.

• Coronary Arteries:

  • Right Coronary Artery: Supplies right atrium, parts of both ventricles, and conducting system; branches into marginal arteries and posterior interventricular artery.

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

  • Anastomoses: Connections between arteries maintain blood flow despite pressure changes.

• Coronary Veins:

  • Great Cardiac Vein: Drains area supplied by anterior interventricular artery.

  • Anterior Cardiac Veins: Drain anterior surface of right ventricle.

  • Coronary Sinus: Collects blood from myocardium, empties into right atrium.

  • Posterior, Small, and Middle Cardiac Veins: Drain various regions, empty into coronary sinus.

• Elastic Rebound: Aortic wall recoil maintains continuous blood flow during relaxation.

Internal Anatomy of the Heart

The heart contains four chambers separated by septa and valves that ensure unidirectional blood flow.

• Atria:

  • Right Atrium: Receives deoxygenated blood from venae cavae and coronary sinus; contains fossa ovalis and pectinate muscles.

  • Left Atrium: Receives oxygenated blood from pulmonary veins.

• Ventricles:

  • Right Ventricle: Receives blood from right atrium via tricuspid valve; pumps blood to pulmonary trunk via pulmonary valve.

  • Left Ventricle: Receives blood from left atrium via bicuspid (mitral) valve; pumps blood to aorta via aortic valve; thicker wall for higher pressure.

  • Trabeculae Carneae: Muscular ridges on inner surfaces.

• Comparison: Left ventricle is thicker and generates more pressure than right ventricle.

Heart Valves

Valves prevent backflow and ensure proper blood flow through the heart.

• Atrioventricular (AV) Valves: Tricuspid (right) and bicuspid/mitral (left); open during atrial contraction, closed during ventricular contraction.

• Semilunar Valves: Aortic and pulmonary; open during ventricular contraction, closed during relaxation.

• Valve Structure: Cusps attached to chordae tendineae and papillary muscles; prevent regurgitation.

• Cardiac Skeleton: Dense connective tissue stabilizes valves and electrically isolates atria from ventricles.

• Valve Disorders: Valvular heart disease (VHD) may require prosthetic replacement.

Arteriosclerosis and Coronary Artery Disease

Arteriosclerosis is the thickening and hardening of artery walls, leading to cardiovascular complications.

• Types: Most commonly atherosclerosis (lipid deposits in tunica media).

• Risk Factors: Age, gender, high cholesterol, high blood pressure, smoking.

• Effects: Can cause coronary artery disease (CAD), strokes, and ischemia.

• Treatments: Surgical replacement, balloon angioplasty, stent insertion.

The Cardiac Cycle

The cardiac cycle is the sequence of events from one heartbeat to the next, including contraction (systole) and relaxation (diastole).

• Phases:

  1. All chambers relaxed; ventricles partially filled.

  2. Atrial systole (100 msec): Atria contract, filling ventricles.

  3. Atrial diastole (270 msec): Atria relax.

  4. Ventricular systole – first phase: AV valves close (isovolumetric contraction).

  5. Ventricular systole – second phase: Semilunar valves open, blood ejected.

  6. Ventricular diastole – early: Semilunar valves close.

  7. Isovolumetric relaxation: All valves closed.

  8. Ventricular diastole – late: AV valves open, ventricles fill passively.

• Pressure Changes: Opening and closing of valves produce characteristic pressure patterns (e.g., dicrotic notch).

• Heart Sounds:

  • S1 (lubb): AV valves close.

  • S2 (dupp): Semilunar valves close.

  • S3, S4: Rarely heard in adults.

Cardiac Output and Conducting System

Cardiac output is the volume of blood pumped by the left ventricle per minute, determined by heart rate and stroke volume.

• Formula:

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

  • Stroke Volume (SV): $SV = EDV - ESV$

• Conducting System: Specialized cardiac muscle cells initiate and distribute contraction stimulus (automaticity).

• Components:

  1. Sinoatrial (SA) node: Primary pacemaker.

  2. Internodal pathways: Distribute signal to atria.

  3. Atrioventricular (AV) node: Secondary pacemaker.

  4. AV bundle and bundle branches: Electrical connection between atria and ventricles.

  5. Purkinje fibers: Trigger ventricular contraction.

Cardiac Muscle Cell Contractions

Cardiac muscle contraction differs from skeletal muscle, ensuring coordinated and sustained heartbeats.

• Action Potential Stages:

  1. Rapid depolarization: Fast sodium channels open.

  2. Plateau: Slow calcium channels open, maintaining near 0 mV.

  3. Repolarization: Calcium channels close, potassium channels open.

• Refractory Period: Prevents tetanic contractions, vital for heart function.

Autonomic Effects on Heart Rate

The heart rate is regulated by autonomic nervous system influences and pacemaker cell activity.

• Pacemaker Potential: Gradual depolarization in SA and AV node cells; SA node sets the pace.

• Parasympathetic Stimulation: Acetylcholine opens K+ channels, slows depolarization, decreases heart rate.

• Sympathetic Stimulation: Norepinephrine opens ion channels, increases depolarization, increases heart rate.

• Resting Heart Rate: 60–100 bpm; bradycardia (<60 bpm), tachycardia (>100 bpm).

• Cardiac Centers: Medulla oblongata contains cardioacceleratory (sympathetic) and cardioinhibitory (parasympathetic) centers.

Factors Affecting Stroke Volume

Stroke volume is influenced by factors such as heart contractility, preload, and afterload.

• Heart Failure: Inability of the heart to meet tissue demands.

• Calculation Example: If ESV = 40 mL and EDV = 125 mL, $SV = 125 - 40 = 85$ mL.

• Rapid Heart Rate: May reduce filling time, decreasing stroke volume and cardiac output.

Electrocardiogram (ECG/EKG)

An ECG records the electrical activity of the heart, useful for diagnosing arrhythmias and conduction abnormalities.

• P Wave: Atrial depolarization.

• QRS Complex: Ventricular depolarization; masks atrial repolarization.

• T Wave: Ventricular repolarization.

• P–R Interval: Start of atrial to start of ventricular depolarization; >200 msec may indicate AV node damage.

• Q–T Interval: Time for ventricles to complete a cycle; affected by electrolyte disturbances and medications.

Cardiac Arrhythmias

Arrhythmias are abnormal heart rhythms that may affect cardiac efficiency.

• PACs: Premature atrial contractions, often benign.

• PAT: Paroxysmal atrial tachycardia, rapid atrial activity.

• Atrial Fibrillation: Atria quiver, may not affect ventricular rate.

• PVCs: Premature ventricular contractions, common and usually harmless.

• Ventricular Tachycardia (VT): Four or more PVCs in succession, may indicate serious issues.

• Ventricular Fibrillation (VF): Ventricles quiver, no blood pumped; rapidly fatal.

Cardiovascular Regulation

Regulation of cardiovascular function involves neural and hormonal mechanisms to maintain adequate blood flow and pressure.

• Cardiac Output: Must generate sufficient pressure for systemic circulation.

• Blood Pressure: Higher in arteries, lower in veins; influenced by resistance and vessel diameter.

• Peripheral Resistance: Opposition to blood flow, affected by vessel length, diameter, viscosity, and turbulence.

• Venous Return: Blood arriving at right atrium per minute, equal to cardiac output.

Factors Affecting Peripheral Resistance

Peripheral resistance is determined by vascular resistance, viscosity, and turbulence.

• Vascular Resistance: Friction between blood and vessel walls; affected by vessel length and diameter.

• Formula: Resistance varies inversely with the fourth power of radius: $R = \frac{1}{r^4}$

• Viscosity: Thickness of blood; normally stable, but affected by hematocrit and plasma composition.

• Turbulence: Eddies and swirls caused by high flow rates or irregular surfaces; increases resistance.

Factors Affecting Blood Flow

Blood flow is directly proportional to pressure and inversely proportional to resistance.

• Pressure Gradient: Difference in pressure from one end of vessel to another; largest from aorta to capillaries.

• Changes in Vessel Diameter: Decreasing diameter increases resistance and decreases flow.

• Arterial Pressure: Systolic (peak during contraction), diastolic (minimum during relaxation), pulse pressure (difference), mean arterial pressure (MAP).

• MAP Formula: $MAP = DP + \frac{1}{3}(SP - DP)$

Capillary Exchange

Capillary exchange involves diffusion, osmosis, and filtration to move substances between blood and interstitial fluid.

• Capillary Hydrostatic Pressure (CHP): Drives filtration of water and small molecules out of capillaries.

• Blood Colloid Osmotic Pressure (BCOP): Draws water into capillaries due to plasma proteins.

• Net Filtration Pressure (NFP): $NFP = CHP - BCOP$

• Filtration: Occurs at arterial end; CHP > BCOP.

• Reabsorption: Occurs at venous end; CHP < BCOP.

• Edema: Excess filtration or reduced reabsorption leads to fluid accumulation in tissues.

Cardiovascular Regulatory Mechanisms

Homeostatic mechanisms ensure tissue perfusion matches metabolic demands.

• Autoregulation: Local changes in blood flow regulated by precapillary sphincters and vasodilators.

• Central Regulation: Neural (cardioacceleratory and vasomotor centers) and endocrine (hormones like NE, ADH, angiotensin II, EPO, aldosterone) mechanisms.

• Baroreceptor Reflexes: Respond to changes in blood pressure; receptors in carotid sinuses, aortic sinuses, right atrium.

• Endocrine Responses: Short-term (epinephrine, norepinephrine) and long-term (ADH, angiotensin II, EPO, aldosterone, natriuretic peptides) regulation.

• Chemoreceptor Reflexes: Respond to changes in CO2, O2, and pH; receptors in carotid bodies, aortic bodies, medulla oblongata.

Summary Table: Major Heart Valves and Their Functions

Valve	Location	Function	Number of Cusps
Tricuspid (Right AV)	Between right atrium and right ventricle	Prevents backflow into right atrium	3
Bicuspid/Mitral (Left AV)	Between left atrium and left ventricle	Prevents backflow into left atrium	2
Pulmonary Semilunar	Between right ventricle and pulmonary trunk	Prevents backflow into right ventricle	3
Aortic Semilunar	Between left ventricle and aorta	Prevents backflow into left ventricle	3

Summary Table: Phases of the Cardiac Cycle

Phase	Event	Valve Status
Atrial Systole	Atria contract, ventricles fill	AV valves open, semilunar valves closed
Ventricular Systole (1st phase)	Ventricles contract, AV valves close	AV valves closed, semilunar valves closed
Ventricular Systole (2nd phase)	Semilunar valves open, blood ejected	AV valves closed, semilunar valves open
Ventricular Diastole (early)	Semilunar valves close, ventricles relax	AV valves closed, semilunar valves closed
Ventricular Diastole (late)	AV valves open, ventricles fill	AV valves open, semilunar valves closed

Example: Calculation of Stroke Volume

• If End-Diastolic Volume (EDV) = 125 mL and End-Systolic Volume (ESV) = 40 mL:

• Stroke Volume (SV): $SV = EDV - ESV = 125 - 40 = 85$ mL

Additional info:

• Cardiac output can be rapidly adjusted to meet metabolic demands by altering heart rate and stroke volume.

• Capillary exchange is essential for nutrient and waste transport; imbalances can lead to edema or dehydration.

• Regulation of blood pressure involves both short-term (neural, hormonal) and long-term (renal, endocrine) mechanisms.