97
The Heart
Objectives
• Describe the organization of the cardiovascular system.
• Describe the location and general features of the heart, including the pericardium.
• Discuss the differences between nodal cells and conducting cells, and describe the
components and functions of the conducting system of the heart.
• Identify the electrical events associated with a normal electrocardiogram.
• Explain the events of the cardiac cycle including atrial and ventricular systole and
diastole, and relate the heart sounds to specific events in the cycle.
• Define cardiac output, heart rate, and stroke volume and describe the factors that
influence these variables.
• Explain how adjustments in stroke volume and cardiac output are coordinated at
different levels of activity.
The cardiovascular system is divided into two circuits
• Pulmonary circuit
• blood to and from the lungs
• System circuit
• blood to and from the rest of the body
• Vessels carry the blood through the circuits
• Arteries carry blood away from the heart
• Veins carry blood to the heart
• Capillaries permit exchange
Anatomy of the Heart
The pericardia
• Visceral pericardium or epicardium
• Parietal pericardium
• Pericardial fluid between the above two
• Superficial fibrous pericardium is anchored to mediastinum
Superficial anatomy of the heart
• The heart consists of four chambers
• Two atria and two ventricles
• Major blood vessels of the heart include
• Inferior and superior vena cavae
• Aorta and pulmonary trunk
The heart wall
• Components of the heart wall include
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• Epicardium – visceral pericardium
• Myocardium – cardiac muscle
• Endocardium – simple squamous epithelium
Internal anatomy and organization
• Atria
• Thin-walled chambers that receive blood from the veins (VC & PV)
• Pump blood into ventricles
• Ventricles
• Thick-walled chambers separated from the atria by AV valves
• Pump blood into arteries (PA & Aorta)
• Chordae tendineae
• Tendinous fibers attached to the AV valves and papillary muscles
Blood flow through the heart
• Right atrium
• Tricuspid valve
• Right ventricle
• Pulmonary valve – pulmonary trunk – pulmonary arteries
• Pulmonary circuit – pulmonary veins
• Left atrium
• Bicuspid valve(mitral valve)
• Left ventricle
• Aortic valve
• Aorta and systemic circuit – vena cava – right atrium
Heart chambers and valves
• Structural Differences in heart chambers
• The left side of the heart is more muscular than the right side
• Functions of valves
• AV valves prevent backflow of blood from the ventricles to the atria
• Semilunar valves prevent backflow into the ventricles from the pulmonary trunk
and aorta
Blood supply to the heart
• Coronary arteries include the right and left coronary arteries, marginal arteries,
anterior interventricular artery (anterior descending), and the circumflex artery
• Coronary veins include the great cardiac vein, anterior and posterior cardiac veins,
the middle cardiac vein, and the small cardiac vein, and the coronary sinus
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Cardiac physiology
• Two classes of cardiac muscle cells
• Specialized muscle cells of the conducting system (control heartbeat)
• Contractile cells (produces powerful contractions that propels blood)
The conducting system
• The conducting system includes
• Sinoatrial (SA) node
• Atrioventricular (AV) node
• Conducting cells
• Atrial conducting cells are found in internodal pathways
• Ventricular conducting cells consist of the AV bundle, bundle branches, and
Purkinje fibers
Impulse conduction through the heart
• SA node begins the action potential
• Stimulus spreads to the AV node
• Impulse is delayed at AV node
• Impulse then travels through ventricular conducting cells (AV bundle, bundle
branches, and Purkinje fibers)
• Then distributed by Purkinje fibers
The electrocardiogram (ECG)
• A recording of the electrical events occurring during the cardiac cycle
• The P wave accompanies the depolarization of the ventricles
• The QRS complex appears as the ventricles depolarize (masks atrial repolarization)
• The T wave indicates ventricular repolarization
Contractile cells
• Resting membrane potential of approximately –90mV
• Action potential
• Rapid depolarization
• A plateau phase unique to cardiac muscle
• Repolarization
• Refractory period follows the action potential
Calcium ion and cardiac contraction
• Cardiac action potentials cause an increase in Ca2+ around myofibrils
• Ca2+ enters the cell membranes during the plateau phase
• Additional Ca2+ is released from reserves in the sarcoplasmic reticulum
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The cardiac cycle
• The period between the start of one heartbeat and the beginning of the next
• During a cardiac cycle
• Each heart chamber goes through systole and diastole
• Correct pressure relationships are dependent on careful timing of contractions
Pressure and volume changes: atrial systole
• Rising atrial pressure pushes blood into the ventricle
• Atrial systole
• The end-diastolic volume (EDV) of blood is in the ventricles
Pressure and volume changes: ventricular systole
• Isovolumetric contraction of the ventricles: ventricles are contracting but there is no
blood exiting chamber
• Ventricular pressure increases forcing blood through the semilunar valves
Pressure and volume changes: ventricular diastole
• The period of isovolumetric relaxation when all heart valves are closed
• Atrial pressure forces the AV valves open
Heart sounds
• Auscultation – listening to heart sound via stethoscope
• Four heart sounds
• S1 – “lubb” caused by the closing of the AV valves
• S2 – “dupp” caused by the closing of the semilunar valves
• S3 – a faint sound associated with blood flowing into the ventricles
• S4 – another faint sound associated with atrial contraction
Cardiodynamics – stroke volume and cardiac output
• Cardiac output – the amount of blood pumped by each ventricle in one minute
• Cardiac output equals heart rate times stroke volume
CO
cardiac output
(ml/min)
=
Hr
heart rate
(beats/min)
X
SV
stroke volume
(ml/beat)
Normal Resting Values
6000 ml/min or 6 l/min = 75/beats/min X 80 ml/beat
Moderate Exercise Values
13.44 l/min = 120/beats/min X 112 ml/beeat
Heavy exercise can increase CO to 18-30 l/min
Elite athletes can increase CO ≈ 700% to 40 l/min!!!!
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SV ⇑ from ⇑ in End-Diastolic Volume (120ml resting)
venous pressure ⇑ ventricular volume
This stretches the LV myocardium = ⇑ force of contraction Starling’s
*Why does RHR ⇓ for athletes?
The proportion of blood ejected from the LV compared to total volume is
termed ejection fraction (65%)
Summary: regulation of heart rate and stroke volume
• Sympathetic stimulation increases heart rate
• Parasympathetic stimulation decreases heart rate
• Circulating hormones, specifically E, NE, and T3, accelerate heart rate
• Increased venous return increases heart rate
• EDV is determined by available filling time and rate of venous return
• ESV is determined by preload, degree of contractility, and afterload
Heart Disease mostly from poor coronary circulation
Ischemia decreased O2 supply
Angina Pectoris chest pain from ischemia w/o death to myocardium.
Caused by: stress (Ψ or phys.), atherosclerosis, ↑BP, nicotine…
Myocardial Infarction (MI) Heart attack – #1 cause of death in USA
Necrosis of myocardium.
Congenital Heart Disease #1 cause of full term newborn fatality
Atrial Septal Defect (ASD)
Foramen ovale remains open after birth
Ventricular Septal Defect (VSD)
Hole in interventricular septum
Patent Ductus Arteriosus (PDA)
Patent fetal Ductus Arteriosus
Tetrolagy of Fallot
VSD, aorta arising from RV or VSD, pulmonic RV hypertrophy
Largest contributor to “blue baby”
Transposition of the Great Vessels
Aorta arises from the RV
Pulmonary artery arises from the LV
Other Heart Diseases
Rheumatic heart disease, congestive heart disease, arrhythmic heart disease
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