Heart - Study Notes (From Prof Notes)
The Heart
Overview of the Heart
Definition and Function: The heart is an integral component of the cardiovascular system.
Pumps blood throughout the body.
Supplies nutrients and oxygen.
Removes waste products, thereby maintaining homeostasis.
Cardiovascular System Components:
Pump: The heart.
Routes (Pathways): Blood vessels.
Vehicle: Blood.
Circulatory Pathways
Pulmonary & Systemic Circuits: The heart functions as two pumps side by side.
Right Side: Receives deoxygenated blood from tissues and pumps it to the lungs for oxygenation and CO₂ removal.
Left Side: Receives oxygenated blood from the lungs and pumps it throughout the body.
Color Coding:
Blue indicates oxygen-poor blood.
Red indicates oxygen-rich blood.
Blood Flow Directions: Arrows indicate the direction of blood flow in diagrams.
Anatomy of the Heart
Position: Located in the mediastinum, superior to the diaphragm, anterior to the vertebral column, and posterior to the sternum.
Ranges from the second rib to the fifth intercostal space.
Size is roughly that of a fist, cone-shaped, weighing less than 1 pound.
Approximately two-thirds of the heart is to the left of the midsternal line.
Right side rests on the diaphragm, partially obscured by the lungs.
Covering of the Heart
Pericardium: A double-walled sac enclosing the heart.
Fibrous Pericardium: Superficial protective layer, loosely attached to the heart, made of dense connective tissue.
Functions: Protection, anchoring the heart, preventing overfilling with blood.
Serous Pericardium: Deep two-layered structure forming a closed sac.
Parietal Layer: Inner layer facing the heart.
Visceral Layer: Part of the heart wall; continuation of the parietal layer.
Pericardial Cavity: Space between the parietal and visceral layers filled with serous fluid for lubrication, decreasing friction.
Conditions:
Pericarditis: Inflammation of the pericardium, causing friction rub during auscultation.
Cardiac Tamponade: Accumulation of excess fluid in the pericardial space compressing heart function.
Layers of the Heart Wall
Three Layers:
Epicardium: Outer layer; synonymous with the visceral layer of serous pericardium.
Myocardium: Middle layer, composed mainly of cardiac muscle in circular or spiral bundles; links heart structures and aids contraction.
Endocardium: Innermost layer lining the heart chambers and covering the valves; continuous with the endothelium of great vessels.
Gross Anatomy of the Heart
Anterior and Posterior Surfaces: Heart anatomy depicted affects function,
Base: The upper part where great vessels attach; directed toward the right shoulder.
Apex: Pointing inferiorly toward the left hip; serves as an anatomical landmark palpated between the fifth and sixth ribs beneath the left nipple.
Heart Chambers and Surface Features
Chambers: Composed of four main chambers:
Right Atrium: Receives deoxygenated blood from superior/inferior vena cavae and coronary sinus.
Left Atrium: Receives oxygenated blood from four pulmonary veins.
Right Ventricle: Pumps blood to the pulmonary trunk.
Left Ventricle: Pumps blood into the aorta.
Surface Grooves: Visual indicators of heart structure:
Coronary Sulcus: Encircles the atrioventricular junction.
Anterior Interventricular Sulcus: Marks the anterior septum of ventricles.
Great Blood Vessels
Three Veins to Right Atrium:
Superior Vena Cava: Returns blood from areas above the diaphragm.
Inferior Vena Cava: Returns blood from areas below the diaphragm.
Coronary Sinus: Collects blood from cardiac veins.
Pulmonary Veins: Four veins returning blood to the left atrium, two from each lung.
Major Arteries:
Pulmonary Trunk: Carries blood from the right ventricle to the lungs.
Aorta: The largest artery, distributing oxygenated blood from the left ventricle.
Heart Valves
Function: Ensure unidirectional blood flow by responding to pressure changes.
Types of Valves:
Atrioventricular (AV) Valves: Located between atria and ventricles.
Right AV Valve (Tricuspid Valve): Three cusps.
Left AV Valve (Mitral/Bicuspid Valve): Two cusps, resembles a bishop's miter
Semilunar (SL) Valves: Located between ventricles and major arteries, prevents backflow.
Pulmonary SL Valve: Between right ventricle and pulmonary trunk.
Aortic SL Valve: Between left ventricle and aorta.
Mechanism of Action:
When atrial pressure exceeds ventricular pressure, AV valves open.
When ventricular contraction occurs, pressure forces AV valves closed, aided by chordae tendineae and papillary muscles.
Blood Pumping and Output
Equal Volumes: Blood is pumped equally to pulmonary and systemic circuits, with the cardiac output (CO) defined as the volume of blood expelled by each ventricle per minute.
Environmental Factors: Functional differences between right and left ventricles:
The Right Ventricle: Thinner walls, pumps at a lower pressure to the lungs.
The Left Ventricle: Thicker walls, pumps blood throughout the body under higher pressure.
Coronary Circulation
Nutrient Supply: Heart muscles receive blood via coronary circulation, separate from its pumping activity.
Coronary Arteries:
Left Coronary Artery: Supplies the left atrium and left ventricle.
Right Coronary Artery: Supplies the right atrium and most of the right ventricle.
Coronary Veins: Return deoxygenated blood to the right atrium through the coronary sinus.
Cardiac Muscle Histology
Structure: Cardiac muscle cells are striated, branched, and interconnected.
Intercalated Discs: Junctions between cardiac cells:
Desmosomes: Provide mechanical stability and prevent detachment during contraction.
Gap Junctions: Allow ion passage and facilitate synchronized contractions.
Contrast with Skeletal Muscle
Contraction Differences:
Cardiac muscle exhibits a functional syncytium; all cardiomyocytes contract as a unit, while skeletal muscle cells contract independently.
Increased Calcium Influx: Extracellular calcium triggers additional calcium release from the sarcoplasmic reticulum (SR) in cardiac muscle but not in skeletal muscle.
Tetany Prevention: Cardiac muscle has a longer refractory period, preventing tetanic contractions critical for effective heart pumping.
Pacemaker Activity
Role of Pacemaker Cells: Lead to spontaneous depolarization and action potentials;
Characterized by unstable resting membrane potentials (pacemaker potentials).
Action Potential Phases:
Pacemaker Potential: Slow Na⁺ influx.
Depolarization: Rapid Ca²⁺ influx leads to action potential spike.
Repolarization: K⁺ efflux returns membrane to resting potential.
Conduction System
Components:
Sinoatrial Node (SA Node): Located in the right atrial wall, primary pacemaker generating impulses approximately 75 times per minute.
Atrioventricular Node (AV Node): Delays impulse transmission for about 0.1 seconds to allow for atrial contraction.
Bundled Pathway: Impulses travel via the bundle of His and Purkinje fibers to facilitate coordinated ventricular contractility.
ECG Interpretation
Electrocardiogram Basics: An ECG records the electrical activity of the heart.
Waves Representing Heart Activity:
P Wave: Atrial depolarization.
QRS Complex: Ventricular depolarization.
T Wave: Ventricular repolarization.
Intervals and Segments: QT interval measures the duration of ventricular action potential.
Abnormal ECG patterns may indicate heart disease or problems with conduction systems.
Cardiac Cycle Dynamics
Phases:
Atrial Systole: Atria contract, pushing blood into ventricles.
Ventricular Systole: Ventricles contract, closing AV valves and forcing blood through pulmonary and aortic valves.
Diastole: Heart relaxes, valves close, and chambers fill.
Points of Blood Flow: Blood flows from areas of high pressure to low pressure; valve states determine flow direction.
Heart Sounds and Murmurs
Auscultation: Use of stethoscope to hear heart sounds associated with valve closure.
First Sound (Lub): Closure of AV valves.
Second Sound (Dup): Closure of SL valves.
Murmurs: Abnormal sounds suggesting valve issues:
Insufficient valves allow backflow (swishing sound).
Stenotic valves restrict blood flow (clicking sound).
Cardiac Output Regulation
Formula: Cardiac Output (CO) is equal to Heart Rate (HR) times Stroke Volume (SV).
CO = HR × SV; average cardiac output for males is around 5.25 L/min at a resting heart rate of 75 beats/min.
Stroke Volume Factors: Determined by preload, contractility, and afterload.
Preload, Contractility, and Afterload
Preload: The degree of stretch on cardiac muscle before it contracts; directly relates to ventricular filling.
Contractility: Amount of force produced by muscle contraction, independent from preload, regulated by inotropic agents.
Afterload: The pressure against which the heart must work; high afterload can reduce stroke volume and increase end-systolic volume.
Heart Rate Regulation
Influencing Factors: Autonomic nervous system dictates heart rate through sympathetic impulses (increasing rate) and parasympathetic impulses (decreasing rate).
Chemical Influences: Hormones like norepinephrine, epinephrine, and thyroxine can raise heart rate; ion concentrations influence cardiac function and stability.