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Chapter 19

The Circulatory System:

The Heart

Lecture PowerPoint

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Circulatory System: The Heart

• Gross anatomy of the heart

• Overview of cardiovascular system

• Cardiac conduction system and cardiac muscle

• Electrical and contractile activity of heart

• Blood flow, heart sounds, and cardiac cycle

• Cardiac output

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Circulatory System: The Heart

• Circulatory system – heart, blood vessels and blood

• Cardiovascular system – heart, arteries, veins and capillaries

Two major divisions:

• Pulmonary circuit - right side of heart– carries blood to lungs for gas exchange

• Systemic circuit - left side of heart– supplies blood to all organs of the body

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Cardiovascular System Circuit

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Position, Size, and Shape

• Located in mediastinum, between lungs

• Base - broad superior portion of heart

• Apex - inferior end, tilts to the left, tapers to point

• 3.5 in. wide at base, 5 in. from base to apex and 2.5 in. anterior to posterior; weighs 10 oz

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Heart Position

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Pericardium

• Sac which surrounds heart allows heart to beat without friction, room to expand and anchors the heart to surrounding structure.

• Parietal pericardium– outer, tough, fibrous layer of CT

• Pericardial cavity – filled with pericardial fluid

• Visceral pericardium (a.k.a. epicardium of heart wall)– inner, thin, smooth, moist serous layer – covers heart surface

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Pericardium and Heart Wall

Pericardial cavity contains 5-30 ml of pericardial fluid

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Heart Wall• Epicardium:outer (a.k.a. visceral pericardium)

– serous membrane covers heart

• Myocardium: middle– thick muscular layer (cardiac muscle)– fibrous skeleton - network of collagenous and

elastic fibers• provides structural support and attachment for

cardiac muscle• electrical nonconductor, important in coordinating

contractile activity

• Endocardium: inner – lines inner heart

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Heart Chambers• 4 chambers

– right and left atria • two superior,

posterior chambers• receive blood

returning to heart

– right and left ventricles

• two inferior chambers

• pump blood into arteries

• Atrioventricular (coronary) sulcus- groove separates atria, ventricles• Anterior and posterior interventricular sulci - grooves separate

ventricles (next slide)

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External Anatomy - Anterior

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External Anatomy - Posterior

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Heart Chambers - Internal

• Interatrial septum– wall that separates atria

• Pectinate muscles– internal ridges of myocardium in right atrium

and both auricles

• Interventricular septum– wall that separates ventricles

• Trabeculae carneae– internal ridges in both ventricles

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Internal Anatomy - Anterior

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Heart Valvesprevent backflow of blood

• Atrioventricular (AV) valves– right AV valve has 3 cusps (tricuspid valve)– left AV valve has 2 cusps (mitral, bicuspid

valve)– chordae tendineae - cords connect AV valves

to papillary muscles (on floor of ventricles)

• Semilunar valves - control flow into great arteries( pulmonary artery/aorta)– Pulmonary valve: right ventricle into

pulmonary trunk– Aortic valve: from left ventricle into aorta

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Heart Valves

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Heart Valves

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AV Valve Mechanics

• Ventricles relax– pressure drops– Semilunar (pulm valve/aortic valve) valves close– AV (tricuspid/bicuspid) valves open– blood flows from atria to ventricles

• Ventricles contract– AV valves close– pressure rises– semilunar valves open– blood flows into great vessels( pulmonary artery and

aorta)

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Operation of Atrioventricular Valves

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Operation of Semilunar Valves

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Blood Flow Through Heart

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Coronary Circulation(Blood vessels of Heart wall)

• Left coronary artery (LCA)– anterior interventricular branch

• supplies blood to interventricular septum and anterior walls of ventricles

– circumflex branch• passes around left side of heart in coronary sulcus,

supplies left atrium and posterior wall of left ventricle

• Right coronary artery (RCA)– right marginal branch

• supplies lateral R atrium and ventricle

– posterior interventricular branch• supplies posterior walls of ventricles

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Angina and Heart Attack

• Angina pectoris – partial obstruction of coronary blood flow can

cause chest pain – pain caused by ischemia, often activity

dependent

• Myocardial infarction – complete obstruction causes death of cardiac

cells in affected area– pain or pressure in chest that often radiates

down left arm

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Venous Drainage of Heart(venous drainage refers to the route by which blood

leaves an organ)

• 20% drains directly into right atrium and ventricle via small thebesian veins

• 80% returns to right atrium via:– great cardiac vein

• blood from anterior interventricular sulcus

– middle cardiac vein • from posterior sulcus

– left marginal vein– coronary sinus

• collects blood and empties into right atrium

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Coronary Vessels - Anterior

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Coronary Vessels - Posterior

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Nerve Supply to Heart

• Sympathetic nerves from – upper thoracic spinal cord, through

sympathetic chain to cardiac nerves– directly to ventricular myocardium– can raise heart rate to 230 bpm

• Parasympathetic nerves– right vagus nerve to SA node– left vagus nerve to AV node– vagal tone – normally slows heart rate to

70 - 80 bpm

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Cardiac Conduction System(controls the route and timing of electrical conduction so all 4 chambers

are in sync with one another)

• Properties– myogenic - heartbeat originates within heart– autorhythmic – regular, spontaneous

depolarization

• Components– next slide

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Cardiac Conduction System

• SA node: (pacemaker) myocytes in RA which initiates heartbeat and sets heart rate

• AV node: electrical gateway to ventricles

• AV bundle: way out for signals to leave AV node

• Right and left bundle branches: divisions of AV bundle that enter interventricular septum

• Purkinje fibers: distribute electrical excitation

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Cardiac Conduction System

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Structure of Cardiac Muscle• Short, branched cells, one nucleus• Sarcoplasmic reticulum is less developed

than in skeletal muscle, large T-tubules – admit more Ca2+ from ECF

• Intercalated discs join myocytes end to end– mechanical junctions (2 types): tightly join

myocytes1. fascia adherens: actin anchored to plasma

membrane; transmembrane proteins link one cell to the next

2. desmosomes

– electrical junctions - gap junctions which forms channels that allow ions to flow from the cytoplasm from one cell directly into the next

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Structure of Cardiac Muscle Cell

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Cardiac Rhythm

• Systole – ventricular contraction

• Diastole - ventricular relaxation• Sinus rhythm: (normal heartbeat, triggered by the SA

node)

– set by SA node at 60 – 100 bpm– adult at rest is 70 to 80 bpm

• Premature ventricular contraction (PVC): (the following can cause other parts of the conduction system to fire before the SA node does, setting off an extra heartbeat called a PVC)

– Causes: hypoxia, electrolyte imbalance, stimulants, stress, etc.

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Cardiac Rhythm• Ectopic foci – any region of spontaneous firing

other than the SA node (example: if the SA node is damaged)

– nodal rhythm - set by AV node, 40 to 50 bpm • Sufficient to sustain life

– intrinsic ventricular rhythm - 20 to 40 bpm • If neither SA node or AV node are functional• Provides too little flow to the brain to be survivable.

– An artificial pacemaker may be implanted

• Arrhythmia - abnormal cardiac rhythm– One cause of an arrhythmia is:

heart block (failure of conduction system)• bundle branch block• total heart block (damage to AV node)

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Pacemaker Physiology(depolarization of SA Node)

• SA node - no stable resting membrane potential• Pacemaker potential

– gradual depolarization from -60 mV, slow influx of Na+

• Action potential – occurs at threshold of -40 mV– depolarizing phase to 0 mV

• fast Ca2+ channels open, (Ca2+ in)

– repolarizing phase• K+ channels open, (K+ out)• at -60 mV K+ channels close, pacemaker potential starts over

• Each depolarization creates one heartbeat– Each depolarization of the SA node sets off one heartbeat– SA node at rest fires at 0.8 sec, about 75 bpm

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SA Node Potentials

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Impulse Conduction to Myocardium

• SA node signal travels at 1 m/sec through atria• AV node slows signal to 0.05 m/sec

– thin myocytes with fewer gap junctions– delays signal 100 msec, allows ventricles to fill

• AV bundle and purkinje fibers– speeds signal along at 4 m/sec to ventricles

• Ventricular systole begins at apex, progresses up– spiral arrangement of myocytes twists ventricles

slightly

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Contraction of Myocardium• Myocytes have stable resting potential of -90

mV• Depolarization (very brief)

– stimulus opens voltage regulated Na+ gates, (Na+ rushes in) membrane depolarizes rapidly

– action potential peaks at +30 mV – Na+ gates close quickly

• Plateau - 200 to 250 msec, sustains contraction– slow Ca2+ channels open, Ca2+ binds to fast Ca2+

channels on SR, releases Ca2+ into cytosol:

contraction

• Repolarization - Ca2+ channels close, K+ channels open, rapid K+ out returns to resting potential

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Action Potential of Myocyte

1) Na+ gates open

2) Rapid depolarization

3) Na+ gates close

4) Slow Ca2+ channels open

5) Ca2+ channels close, K+ channels open

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Electrocardiogram (ECG)• Composite of all action potentials of nodal and

myocardial cells detected, amplified and recorded by electrodes on arms, legs and chest

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ECG

• P wave– SA node fires, atrial depolarization– atrial systole

• QRS complex– ventricular depolarization– (atrial repolarization and diastole - signal

obscured)

• ST segment - ventricular systole

• T wave– ventricular repolarization

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Normal Electrocardiogram (ECG)

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1) atrial depolarization begins

2) atrial depolarization complete (atria contracted)

3) ventricles begin to depolarize at apex; atria repolarize (atria relaxed)

4) ventricular depolarization complete (ventricles contracted)

5) ventricles begin to repolarize at apex

6) ventricular repolarization complete (ventricles relaxed)

Electrical Activity of Myocardium

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Diagnostic Value of ECG

• Invaluable for diagnosing abnormalities in conduction pathways, MI, heart enlargement and electrolyte and hormone imbalances

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ECGs, Normal and Abnormal

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Heart Sounds

• Auscultation - listening to sounds made by body

• First heart sound (S1), louder and longer “lubb”, occurs with closure of AV (tricuspid/bicuspid) valves

• Second heart sound (S2), softer and sharper “dupp” occurs with closure of semilunar (pulmonary/aortic) valves

• S3 - rarely heard in people > 30

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Phases of Cardiac Cycle(The pressure changes that occur, and how the pressure changes, and

valves govern the flow of blood. Less than 1 second)

• Ventricular filling

• Isovolumetric contraction

• Ventricular ejection

• Isovolumetric relaxation

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Ventricular Filling - 3 phases

1. Rapid ventricular filling • AV valves first open

2. Diastasis • sustained lower pressure, venous return

3. Atrial systole • filling completed

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Isovolumetric Contraction of Ventricles

• Atria repolarize and relax, and remain in diastole for the rest of the cardiac cycle

• Ventricles depolarize

• QRS complex appears in ECG

• Ventricles contract

• Rising pressure closes AV valves - heart sound S1 occurs

• No ejection of blood yet (no change in volume)

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Ventricular Ejection

• Rising pressure opens semilunar valves

• Rapid ejection of blood (at first)

• Reduced ejection of blood (less pressure)

*Stroke volume: amount of blood ejected, 70 ml at rest

*End-systolic volume: amount of blood left in heart

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Ventricles- Isovolumetric Relaxation

• T wave appears in ECG

• Semilunar valves close (sound S2 occurs)

• AV valves remain closed

• Ventricles expand but do not fill (no change in volume)

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Major Events of Cardiac Cycle

• Ventricular filling

• Isovolumetric contraction

• Ventricular ejection

• Isovolumetric relaxation

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Events of the Cardiac Cycle

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Cardiac Output (CO)

• Amount ejected by ventricle in 1 minute

• Cardiac Output = Heart Rate x Stroke Volume– about 4 to 6 L/min at rest– vigorous exercise CO to 21 L/min for fit

person and up to 35 L/min for world class athlete

• Cardiac reserve: difference between a persons maximum and resting CO with fitness, with disease

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Heart Rate

• Pulse = surge of pressure in artery– infants have HR of 120 bpm or more– young adult females avg. 72 - 80 bpm– young adult males avg. 64 to 72 bpm– HR rises again in the elderly

• Tachycardia: resting adult HR above 100– stress, anxiety, drugs, heart disease or

body temp.

• Bradycardia: resting adult HR < 60– in sleep and endurance trained athletes

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Sympathetic Nervous System

• Cardioacceleratory center– stimulates sympathetic cardiac nerves to SA

node, AV node and myocardium– these nerves secrete norepinephrine, which

binds to -adrenergic receptors in the heart(positive chronotropic effect)

– CO peaks at HR of 160 to 180 bpm– Sympathetic n.s. can HR up to 230 bpm,

(limited by refractory period of SA node), but SV and CO (less filling time)

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Parasympathetic Nervous System

• Cardioinhibitory center stimulates vagus nerves

• right vagus nerve - SA node• left vagus nerve - AV node

– secretes ACH (acetylcholine) which binds to muscarinic receptors

• nodal cells hyperpolarized, HR slows

– vagal tone: background firing rate holds HR to sinus rhythm of 70 to 80 bpm

• severed vagus nerves (intrinsic rate-100bpm)• maximum vagal stimulation HR as low as 20 bpm

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Inputs to Cardiac Center

• Higher brain centers affect HR– cerebral cortex, limbic system, hypothalamus

• sensory or emotional stimuli (rollercoaster, IRS audit)

• Proprioceptors – inform cardiac center about changes in

activity, HR before metabolic demands arise

• Baroreceptors signal cardiac center– aorta and internal carotid arteries

• pressure , signal rate drops, cardiac center HR• if pressure , signal rate rises, cardiac center HR

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Inputs to Cardiac Center

• Chemoreceptors– sensitive to blood pH, CO2 and oxygen

– aortic arch, carotid arteries and medulla oblongata

– primarily respiratory control, may influence HR

CO2 (hypercapnia) causes H+ levels, may create acidosis (pH < 7.35)

– Hypercapnia and acidosis stimulates cardiac center to HR