Heart - Study Notes (From Slides)
The Heart (Topic 4.2)
4.2.1 Overview
Chapter 18 of uOttawa resources
Presentation is for educational purposes only.
Circulatory Systems
Pulmonary Circuit:
Right side of the heart receives deoxygenated blood from tissues.
Pumps blood to the lungs via the Pulmonary Arteries.
Purpose: to discard CO2 and acquire O2.
Oxygenated blood returns to the left side of the heart.
Systemic Circuit:
Left side of the heart receives oxygenated blood from the lungs.
Pumps blood to the body tissues via the Systemic Arteries.
Deoxygenated blood is returned to the right side of the heart.
Functions as a transport system composed of two side-by-side pumps.
Anatomy of the Heart
Apical Impulse:
Palpated between the 5th & 6th ribs, below the left nipple.
Base:
The posterior surface leans towards the right shoulder.
Apex:
Points towards the left hip.
Located in the mediastinum between the 2nd rib and the 5th intercostal space.
On the superior surface of the diaphragm.
2/3 of the heart lies to the left of the midsternal line.
Anterior to the vertebral column, posterior to the sternum.
Covering of the Heart
Fibrous Pericardium:
Protects and anchors the heart.
Prevents overfilling of the heart.
Pericardium:
A double-walled fibro-serous sac.
Serous Pericardium:
Composed of parietal and visceral epicardium layers.
Separated by the fluid-filled pericardial cavity.
Layers of the Heart Wall
Epicardium:
Visceral layer of the serous pericardium.
Myocardium:
Composed mainly of circular or spiral bundles of contractile cardiac muscle cells, accounting for the bulk of the heart wall.
Cardiac Skeleton:
A non-excitable, dense network of collagen and elastic fibers.
Functions:
Anchors cardiac muscle.
Supports great vessels and valves.
Limits the spread of action potentials to specific pathways.
Endocardium:
Innermost layer that consists of endothelium and connective tissue.
Continuous with the endothelial lining of blood vessels.
Lines heart chambers and covers the cardiac skeleton of valves.
Gross Anatomy of the Heart
Anterior View
Major structures:
Brachiocephalic trunk, superior vena cava, right pulmonary artery, pulmonary trunk, left common carotid artery, left subclavian artery, aortic arch, ligamentum arteriosum, left pulmonary artery, left pulmonary veins, auricle of left atrium, right pulmonary veins, right atrium, right coronary artery, anterior cardiac vein, right ventricle, right marginal artery, small cardiac vein, inferior vena cava, circumflex artery, left coronary artery, left ventricle, great cardiac vein, anterior interventricular artery.
Posterior View
Structures similar to the anterior view including:
Left pulmonary artery, left pulmonary veins, auricle of left atrium, left atrium, right pulmonary artery, right pulmonary veins, right atrium, inferior vena cava, posterior vein of left ventricle, left ventricle, coronary sinus, right coronary artery, posterior interventricular artery, and middle cardiac vein.
The Chambers of the Heart
Atria (receiving chambers):
Right atrium receives deoxygenated blood from:
Superior vena cava (from above diaphragm).
Inferior vena cava (from below diaphragm).
Coronary sinus (returns blood from coronary veins).
Left atrium receives oxygenated blood from the lungs via pulmonary veins.
Ventricles (discharging chambers):
Right ventricle pumps blood into the Pulmonary Trunk.
Left ventricle pumps blood into the Aorta.
Known as pumps of the heart, ventricles have thicker walls, especially the left ventricle.
Internal walls feature muscle bundles.
Heart Valves
Blood flow through the heart is unidirectional, enforced by four heart valves that respond to pressure changes.
Two Major Types of Valves:
Atrioventricular (AV) Valves:
Tricuspid Valve:
Located between the right atrium and right ventricle.
Mitral Valve (Bicuspid):
Positioned between the left atrium and left ventricle.
Semilunar Valves:
Located between the ventricles and major arteries.
Pulmonary Valve:
Between right ventricle and pulmonary trunk.
Aortic Valve:
Between left ventricle and aorta.
Mechanism of Valves
Atrioventricular (AV) Valves:
Chordae tendineae anchor the AV valve cusps to papillary muscles to hold the valve flaps in a closed position and prevent them from everting into the atria.
Semilunar (SL) Valves:
Designed to prevent backflow from arteries into ventricles during ventricular relaxation, closing when there is a decrease in intraventricular pressure.
Blood Pumping by the Heart
The heart pumps equal volumes of blood into the pulmonary and systemic circuits, but the ventricles have unequal workloads.
Pulmonary Circuit (right ventricle):
Shorter, low-pressure circulation.
Systemic Circuit (left ventricle):
Longer with 5x more resistance to flow.
Anatomy: Right vs Left Ventricles
Right Ventricle:
Thinner wall than the left ventricle.
Crescent shape, wraps around the left ventricle.
Left Ventricle:
Thicker wall than the right ventricle.
Round shape.
Blood Supply to the Heart
The heart has various anastomoses which provide its blood supply.
Although the heart is only about 1/200 of body weight, it requires approximately 1/20 of the blood supply.
Coronary Arteries
Arise from the base of the aorta and encircle the heart in the coronary sulcus.
Key arteries include:
Right coronary artery, left coronary artery (and branches such as anterior interventricular artery and circumflex artery).
Coronary Veins
Return deoxygenated blood from the heart muscles to the right atrium.
Microscopic Anatomy
Intercalated Discs:
Connecting junctions between cardiac cells containing:
Desmosomes: hold cells together, preventing separation during contraction.
Gap Junctions: allow ions to pass from cell to cell, electrically coupling adjacent cells, forming a functional syncytium.
The intercellular space is filled with connective tissue matrix (endomysium) which contains numerous capillaries and connects cardiac muscle to the cardiac skeleton.
Differences Between Cardiac and Skeletal Muscle
Similarities:
Muscle contraction follows depolarizing action potentials.
Depolarization wave travels down T tubules, leading to sarcoplasmic reticulum (SR) release of Ca2+.
Excitation-contraction coupling: Ca2+ binds to troponin causing filament sliding.
Differences:
Some cardiac muscle cells are self-excitable (automaticity).
Two cell types:
Contractile Cells (99%): responsible for contraction.
Pacemaker Cells (1%): noncontractile, spontaneously depolarize, initiating depolarization of the entire heart.
Cardiac Pacemaker Cells and Action Potentials
Action Potential Initiation:
Characterized by unstable resting membrane potentials, known as pacemaker potentials or prepotentials.
Setting the Basic Rhythm
The heart contracts without nervous system stimulation but can alter rhythm through the autonomic nervous system.
Coordinated Heartbeat:
Coordinated via gap junctions and an intrinsic conduction system composed of autorhythmic cells that initiate and distribute impulses for depolarization and contraction.
Nodal Structures
Sinoatrial (SA) Node:
The pacemaker located in the right atrial wall below the entrance of the superior vena cava.
Generates impulses 75 times/min and modulates the heart rate from its inherent 100 beats/min via extrinsic factors.
Atrioventricular (AV) Node:
Located in the inferior interatrial septum above the tricuspid valve.
Delays impulses by approximately 0.1 seconds, allowing atrial contraction prior to ventricular contraction.
Has an inherent rate of 50 beats/min without SA node input.
The Bundles and Conducting Network
Atrioventricular (AV) Bundle (Bundle of His):
Only electrical connection between atria and ventricles, splitting into two bundle branches that continue down the interventricular septum.
Subendocardial Conducting Network (Purkinje Fibers):
Conducts impulses from the apex of the heart up through the ventricular walls.
Depolarization takes about 0.22 seconds from SA node initiation to complete ventricular contraction.
Homeostatic Imbalances in the Conducting System
Defects in the conduction system may lead to:
Arrhythmias: irregular heart rhythms.
Fibrillation: rapid, irregular twitching contractions that impair blood circulation and may lead to brain death.
Ectopic Focus: abnormal pacemaker activity when the SA node is defective.
Heart Block: partial or total failure of impulse conduction from atria to ventricles, resulting in a slower heart rate.
Modifying the Basic Rhythm
The autonomic nervous system modifies the heartbeat via cardiac centers in the medulla oblongata.
Cardioacceleratory Center:
Activates sympathetic nervous system, increasing heart rate and contractility.
Stimulates the SA and AV nodes and heart muscle.
Cardioinhibitory Center:
Activates parasympathetic nervous system via the vagus nerve to decrease heart rate.
Depolarization in Contractile Cells
Rising Phase:
Fast voltage-gated Na+ channels open causing a rapid influx of Na+, raising membrane potential from -90 mV to +30 mV.
Plateau Phase:
Slow Ca2+ channels open at +30 mV, prolonging depolarization and creating a plateau in the AP.
Repolarization Phase:
After around 200 ms, slow Ca2+ channels close, while voltage-gated K+ channels open, allowing K+ efflux and bringing the membrane potential back to resting levels (RMP).
Electrocardiogram (ECG)
The ECG is a graphic recording of heart electrical activity, consisting of various leads.
Key Features:
P Wave: Represents atrial depolarization.
QRS Complex: Represents ventricular depolarization and atrial repolarization.
T Wave: Represents ventricular repolarization.
Intervals:
P-R interval, S-T segment, and Q-T interval detail the timing of electrical events.
Homeostatic Imbalances in ECG
ECG changes may indicate heart disease or conduction system issues, e.g., elevated S-T segment could suggest ischemia, and prolonged Q-T interval can indicate a risk for arrhythmias.
Mechanical Events of the Heart
Systole: Myocardial contraction phase.
Diastole: Myocardial relaxation phase.
Cardiac Cycle: Sequence of events during one complete heartbeat involving atrial and ventricular systole and diastole.
Atrial contraction lasts ~0.1 seconds, ventricular systole lasts ~0.3 seconds, quiescent phase lasts ~0.4 seconds, with one complete heartbeat taking ~0.8 seconds.
Ventricular Filling and Systole
Early Diastole:
AV valves open, SL valves close; blood fills ventricles under low pressure.
Atrial contraction pushes the last 20% of blood into ventricles triggering the subsequent depolarization (QRS) leading to ventricular systole.
Ventricular Systole:
Triggered by ventricular depolarization; pressures rise in ventricles, closing AV valves and remaining valves are closed until pressure exceeds arterial pressure.
Isovolumetric Relaxation:
Following depolarization, ventricles relax; pressure decreases leading to closure of semilunar valves which creates the dicrotic notch seen on pressure graphs.
Pressure Changes in the Heart
Blood flow occurs from high to low pressure, regulated by valve opening and closing.
Atrioventricular (AV) valves close when ventricular pressure exceeds atrial pressure; semilunar (SL) valves open when ventricular pressure exceeds arterial pressures.
Auscultation of Heart Sounds
Normal Heart Sounds:
First Sound (Lub): Caused by closure of AV valves at the start of ventricular systole.
Second Sound (Dup): Caused by the closure of SL valves at the start of ventricular diastole.
Heart Murmurs: Abnormal sounds caused by blood hitting obstructions due to incompetent or stenotic valves.
Cardiac Output (CO)
CO is the amount of blood pumped by each ventricle in one minute measured in milliliters (ml/min).
Calculated using the formula:
Where HR is the heart rate (beats/min) and SV is the stroke volume (ml/beat).
Normal resting values:
HR ~75 beats/min, SV ~70 ml/beat, leading to a normal CO of 5250 ml/min.
Maximum CO can be significantly higher (up to 35 L/min in athletes).
Regulation of Cardiac Output
Factors affecting CO include stroke volume and heart rate.
Stroke Volume (SV) Regulation
Preload:
Degree of cardiac myocyte stretch prior to contraction affecting the stroke volume through Frank-Starling law.
Relationship: increased preload results in increased stroke volume.
Contractility:
Effectiveness of contraction strength at a given muscle length independent of stretch and EDV. Increased contractility leads to decreased ESV.
Factors that increase contractility include sympathetic nervous activation and positive inotropic agents (e.g., norepinephrine).
Afterload:
Pressure that ventricles must overcome to eject blood. Afterload can be affected by chronic hypertension, leading to increased ESV and reduced SV.
Autonomic Nervous System Regulation of Heart Rate
If stroke volume decreases due to decreased blood volume or heart weakness, CO can be maintained by increasing HR and contractility.
Positive and Negative Chronotropic Factors:
Positive: Increase heart rate (due to emotional or physical stress).
Negative: Decrease heart rate via the parasympathetic nervous system.
Chemical Regulation of Heart Rate
Hormones and ion concentrations in extracellular fluid influence heart functions.
Congenital Heart Defects
Ventricular Septal Defect:
Superior part of the septum does not form, causing blood mixing between ventricles.
Coarctation of the Aorta:
The aorta is narrowed, increasing left ventricle workload.
Tetralogy of Fallot:
Includes four defects leading to mixed blood flow.
Summary of Blood Flow Tracing
Pathway:
From the right atrium through the tricuspid valve -> right ventricle -> pulmonary valve -> pulmonary trunk -> right & left pulmonary arteries -> lungs -> pulmonary veins -> left atrium -> mitral valve -> left ventricle -> aortic valve -> aorta -> systemic arteries -> body tissues -> systemic veins -> superior/inferior vena cava returning to the right atrium.