Download Chapter 19 - Heart

The Heart
Chapter 19 & 20
• Circulatory System
– heart, blood vessels and blood
• Cardiovascular System
– heart, arteries, veins and capillaries
• 2 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
•
•
•
•
Heart Anatomy
Cardiac Cycle
Myocardial Physiology
Cardiac Conduction System and
Innervation
• Coronary Circulation
• Blood Pressure
Cardiovascular System Circuit
• Cardiovascular
System consists of:
– heart, arteries, veins
and capillaries
• 2 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
Heart Shape and Position
• Heart is located in
the mediastinum,
between lungs
• Base of the heart is
the broad superior
portion of heart
• Apex of the heart is
the inferior end, tilts
to the left, tapers to
point
Heart Position
Pericardium
• Pericardium consists of the membranes around the
heart that allows the heart to beat with minimal friction,
provides room for the heart to expand but resists
excessive expansion.
• Pericardial Sac (also called the parietal pericardium) is
the membrane that loosely surrounds the heart.
Pericardial sac has 2 layers:
– Fibrous layer: outer, tough, fibrous layer of CT
– Serous layer: inner, thin, smooth, moist membrane
• Pericardial Cavity is the space between heart and
pericardial sac that is filled with pericardial fluid that
acts as a lubricant.
• Visceral Pericardium (also called the epicardium) is
the thin, smooth, moist serous layer that covers heart
surface.
Figure 19.3
Pericardium
and
Heart Wall
3 Layers of the Heart Wall
• Epicardium (visceral pericardium)
– serous membrane covers heart
• Myocardium
– thick muscular layer
– fibrous skeleton - network of collagen fibers and
elastic fibers
• provides structural support
• attachment for cardiac muscle
• nonconductor important in coordinating contractile
activity
• Endocardium
– smooth inner lining
– equivalent to the lining of blood vessels
Figure 19.3
3 layers of the
Heart Wall
Bacterial Endocarditis
Gross pathology of subacute bacterial endocarditis involving mitral valve. Left ventricle of heart has
been opened to show mitral valve fibrin vegetations due to infection with Haemophilus
parainfluenzae. Autopsy. CDC/Dr. Edwin P. Ewing, Jr. http://pathmicro.med.sc.edu/ghaffar/bord-hemo.htm
Cardiac Cycle
• One complete contraction (systole) and
relaxation (diastole) of all 4 chambers of the
heart followed by a brief period of inactivity:
Cardiac Cycle Animation
http://msjensen.cehd.umn.edu/1135/Links/Animations/Flas
h/0028-swf_the_cardiac_cy.swf
Heart Sounds
• Heart sounds as heard through a stethoscope
are due to turbulent flow of blood as heart
valves close. Auscultation - listening to sounds
made by body.
• First heart sound (S1) is louder and longer,
sounds like “LUBB”, occurs when the AV
valves close.
• Second heart sound (S2), is softer and
sharper, sounds like “DUPP”, occurs when the
semilunar valves close.
• S3 – rarely audible in people older than 30 –
known as triple rhythm or gallop.
Heart Sounds
• S1 and S2 sounds of normal rhythm can be
simulated by drumming two fingers on a table.
• If there is a third heart sound (S3) it can be
simulated by drumming three fingers on a table,
and it is described as a “gallop”.
• The cause of each heart sound is not known with
certainty, but they are correlated with specific
phases of the cardiac cycle.
http://egeneralmedical.com/egeneralmedical/listohearmur.html
http://www.3m.com/us/healthcare/professionals/littmann/jhtml/
sounds/normal_first_and_second_heart_sounds.jhtml
Phases of Cardiac Cycle
1)
2)
3)
4)
5)
Quiescent period
Ventricular filling
Isovolumetric contraction
Ventricular ejection
Isovolumetric relaxation
1) Quiescent period
– all chambers
relaxed
– AV valves open
– blood flowing into
ventricles
2) Ventricular Filling
– SA node fires, atria
depolarize
– P wave appears on
ECG
– atria contract, force
additional blood into
ventricles
– ventricles now
contain end-diastolic
volume (EDV) of
about 130 ml of
blood
3) Isovolumetric Contraction of Ventricles
• Atria repolarize and
relax
• Ventricles depolarize
• QRS complex appears
in ECG
• Ventricles contract
• Rising pressure closes
AV valves - heart
sound S1 occurs
• No ejection of blood
(no change in volume)
4) Ventricular Ejection
• Rising pressure opens
semilunar valves
• Rapid ejection of blood
• Stroke volume: amount
ejected
– about 70 ml at rest
• End-systolic volume:
amount left in heart
• SV/EDV= ejection
fraction, at rest ~ 54%,
during vigorous exercise
as high as 90%,
diseased heart < 50%
• Both ventricles must
eject same amount of
blood
5) Isovolumetric Relaxation of Ventricles
• T wave appears in
ECG
• Ventricles repolarize
and relax (begin to
expand and fill)
• Semilunar valves close
(dicrotic notch of aortic
pressure curve) - heart
sound S2 occurs
• AV valves remain
closed (not shown)
• Ventricles expand but
do not fill so there is
no change in volume
(not shown)
Cardiac Output (CO)
• Amount ejected by each ventricle in 1 minute
• CO = Heart Rate (beats/min) x Stroke Volume (ml/beat)
• Resting values, CO = 75 beats/min x70 ml/beat =
5,250 ml/min, usually about 4 to 6 liters/min
• 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 person’s
maximum CO and resting CO
–  with fitness,  with disease
• Right Ventricle and Left Ventricle eject the same
amount of blood.
Preload
• Preload is the amount of tension in ventricular
myocardium before it contracts.
•  preload causes  contraction strength
– exercise  venous return, stretches myocardium
( preload) , myocytes generate more tension
during contraction,  CO matches  venous return
• Frank-Starling law of heart:
StrokeVolume  End Diastolic Volume
– ventricles eject as much blood as they receive;
The more they are stretched ( preload) the
harder they contract.
Unbalanced Ventricular Output
Unbalanced Ventricular Output
Heart Rate
•
•
•
•
•
•
Heart Rate (HR) can be measured from pulse.
Infants have HR of 120 beats per minute or more
Young adult females average 72 - 80 bpm
Young adult males average 64 to 72 bpm
HR rises again in the elderly
Indurance trained athletes have resting
heart rates around 40 bpm
– Miguel Indurain, 5 time Tour de France
winner (1991-1995), had a resting
heart rate of 28 bpm and a
cardiac output of 50 liters/minute
Heart Rate
• Tachycardia: persistent, resting adult HR > 100
– can be caused by stress, anxiety, drugs, heart
disease or  body temperature
• Bradycardia: persistent, resting adult HR < 60
– common during sleep and also in endurance
trained athletes
Sympathetic Nervous System
• Cardioacceleratory Center
– stimulates sympathetic nerves to SA node, AV
node and myocardium
– these nerves secrete norepinephrine, which
binds to -adrenergic receptors in the heart
– CO normally 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)
Parasympathetic Nervous System
• Cardioinhibitory Center
– stimulates vagus nerves
• right vagus nerve - SA node
• left vagus nerve - AV node
– secrete ACh (acetylcholine), binds to muscarinic
receptors
• opens K+ channels in nodal cells, hyperpolarized, fire
less frequently, HR slows down
– vagal tone: background firing rate holds HR to
sinus rhythm of 70 to 80 bpm
• severed vagus nerves results in the SA node firing at its
intrinsic rate of about 100bpm
• maximum vagal stimulation  HR as low as 20 bpm
Structure of Cardiac Muscle
• Short, thick, branched cells with central nuclei.
• Sarcoplasmic reticulum is less developed than in skeletal
muscle.
– cardiac cells must get more Ca2+ from extracellular fluid
during excitation
• T-tubules are invaginations of sarcolemma (muscle cell
plasma membrane) that carry depolarization into the cells.
• Intercalated Discs, join cells end to end.
– interdigitating folds -  surface area for strength and
communication
– mechanical junctions tightly join cells
• adherent junctions and desmosomes
– electrical junctions
• gap junctions form channels that allow ions to flow
directly from cell to cell to synchronize contractions
Intercalated Disk
Cardiac Muscle Histology
Metabolism of Cardiac Muscle
• Fatigue resistant aerobic respiration
• Cells have abundant myoglobin
– most tissues extract about 25% of the oxygen
from the blood as it circulates, but cardiac muscle
extracts about 65% of the oxygen
• Abundant, large mitochondria
• Organic fuels: fatty acids, glucose, glycogen,
ketones
Cardiac Conduction System
• Cardiac Conduction System is Myogenic - heartbeat
originates within heart
• Cardiac Conduction System is Autorhythmic –
heartbeats are regular and spontaneous
Cardiac Conduction System
• Cardiac Conduction System Components:
– Cardiac Conduction System is composed of Nodes and Bundles
of specialized cardiac cells.
– Sinoatrial (SA) Node is the pacemaker that initiates heartbeat and
sets heart rate.
– Atrioventricular (AV) node is an electrical relay to the ventricles
– AV bundle (bundle of His) is a pathway for signals from the AV
node to muscles of ventricles.
– Right and left bundle branches are divisions of the AV bundle that
enter the interventricular septum and descend to the apex.
– Purkinje Fibers project upward from apex and spread throughout
ventricular myocardium.
– Moderator Band is a muscular bundle of heart tissue crossing the
ventricular cavity from the interventricular septum to the base of
the paillary muscles of the right ventricle.
– fibrous skeleton of the heart is connective tissue that insulates
atria from ventricles.
Cardiac Conduction System
Impulse Conduction
• SA node signal travels at 1 m/sec across the atria
• AV node slows signal to 0.05 m/sec
– composed of thinner myocytes with fewer gap junctions
– delays signal 100 msec, allows ventricles to fill
• AV bundle and purkinje fibers speeds up signal to
ventricles at 4 m/sec
• Papillary muscles get the signal first, and their
contraction stabilizes the AV valves
• Ventricular systole begins at apex, progresses up
– spiral arrangement of myocytes twists ventricles slightly
during contraction which increases the efficiency of blood
ejection from the chambers
Cardiac Rhythm
• Sinus Rhythm is set by the SA node
– adult sinus rhythm at rest is 70 to 80 bpm
• Nodal Rhythm is set by the AV node
– adult nodal rhythm is 40 to 50 bpm but it is normally
overridden by the sinus rhythm
• Ectopic Foci are regions of spontaneous firing outside of the
SA node
– ectopic foci can be caused by hypoxia, caffeine, nicotine,
electrolyte imbalance, drugs
• Arrhythmia – any abnormal cardiac rhythm
– heart block: failure of conduction system due to disease
• bundle branch block (caused by damage to bundle
branches)
• total heart block (caused by damage to AV node)
Physiology of the SA Node
• SA node - no stable resting membrane potential
• Pacemaker potential
– gradual depolarization of SA cells from a slow inflow of Na+
into the SA cells
– membrane potential rises from -60 mV to -40 mV
• Action potential
– occurs at threshold of -40 mV
– depolarizing phase from -40 mV to 0 mV
• Caused by opening of fast Ca+2 channels (lets Ca+2 in)
– repolarizing phase
• Caused by opening of K+ channels (let K+ out)
• at -60 mV K+ channels close, pacemaker potential starts
over
• Each depolarization creates one heartbeat
– SA node at rest fires every 0.8 sec or about 75 bpm at rest
Membrane Potentials of SA Node Cells
Contraction of Myocardium
• Myocytes have stable resting potential of -90 mV
• Depolarization (very brief)
– stimulus opens voltage regulated Na+ channels and allows
Na+ to rush in which rapidly depolarizes the membrane
– action potential peaks at +30 mV
– Na+ channels close again quickly
• Plateau of 200 to 250 msec, sustains contraction
– slow Ca+2 channels open, Ca+2 binds to Ca+2 channels on
sarcoplasmic reticulum which releases Ca+2 into cytoplasm
and results in muscle contraction
• Repolarization - Ca+2 channels close, K+ channels
open, rapid K+ outflow returns cell to resting potential
Myocardial Contraction
and Action Potential
1) Voltage gated Na+ channels
open
2) Rapid depolarization due to
Na+ inflow
3) Na+ channels close
4) K+ channels and slow Ca+2
channels open. K+ flows out
restoring membrane
polarity, but Ca+2 flowing in
prolongs membrane
depolarization
5) Ca+2 channels close and
Ca+2 is pumped out
K+ channels still open and
re-polarizes as K+ flows out
Electrocardiogram (ECG)
• Composite of action potentials of nodal and myocardial cells
• Detected, amplified and recorded by electrodes on arms, legs
and chest
ECG
• P wave
– SA node fires, atrial depolarization
– atrial systole
• QRS complex
– AV node fires, ventricular
depolarization
– ventricular systole
– (atrial repolarization and diastole signal obscured)
• T wave
– ventricular repolarization
ECG and the Cardiac Cycle
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)
Diagnostic Value of ECG
• Valuable for diagnosing abnormalities in
conduction pathways, myocardial infarction
(MI), heart enlargement, electrolyte
imbalances and hormone imbalances.
Myocardial Infarction
• Sudden death of heart tissue caused by
interruption of blood flow from coronary artery
narrowing or occlusion.
• Dead myocardium is replaced with thin, noncontractile connective tissue.
• Anastomoses defend against interruption by
providing alternate blood pathways.
– circumflex artery and right coronary artery join and
form the posterior interventricular artery.
– anterior and posterior interventricular arteries join
at apex of heart.
Coronary Blood
Vessels
Coronary Vessel Atherosclerosis
Plaque
Artery Wall
Major Events of Cardiac Cycle
Chapter 20
Principles of Blood Flow
• Blood Flow: amount of blood flowing through
a tissue in a given time (ml/min)
• Perfusion: rate of blood flow per given mass
of tissue (ml/min/g)
• Important for delivery of nutrients and
oxygen, and removal of metabolic wastes
Variations in Blood Flow in the
Circulatory System
• From aorta to capillaries, blood pressure and
flow DECREASES because:
– friction between blood and vessels
– smaller radii of arterioles and capillaries
– farther from the heart, more vessels, greater total
cross sectional area
• From capillaries to vena cava, rate of flow
INCREASES because:
– large amount of blood is funneled back into fewer
vessels.
Blood Pressure Changes With Distance
More pulsatile
closer to heart
Blood Pressure
• Usually measured at brachial artery of arm
• Systolic pressure: BP during ventricular
systole
• Diastolic pressure: BP during ventricular
diastole
• Normal value, young adult: 120/75 mm Hg
Neural Blood Pressure Control:
Baroreflex
• Changes in BP detected by stretch receptors
(baroreceptors) in large arteries above heart in the:
– aortic arch
– aortic sinuses (behind aortic valve cusps)
– carotid sinus (base of each internal carotid artery)
• Autonomic negative feedback response
– baroreceptors send signals to brainstem
–  BP increases rate of signals to brainstem which inhibits
the sympathetic nerves to blood vessels causing
vasodilation (causes BP )
–  BP causes rate of signals to drop, excites vasomotor
center,  sympathetic nerves, causes vasoconstriction and
BP 
Baroreceptors
Baroreflex
Negative Feedback Response
Neural Control: Chemoreflex
• Chemoreceptors in aortic bodies and carotid
bodies are located in aortic arch, subclavian
arteries, external carotid arteries
• Chemoreceptors monitor pH
– drop in pH of blood and cerebrospinal fluid due to
increase in CO2 concentration in tissues triggers
an increase in blood pressure and respiration rate
– Carbon dioxide dissolves in water to form carbonic
acid: CO2 + H2O  H2CO3
– carbonic acid dissociates in water into H+ (acidic
hydrogen ions) and HCO3- (bicarbonate ions)
Chemoreceptors
Carotid body
Aortic body
Aortic body
Hormonal Control of BP and Flow
• Angiotensin II ( BP)
– potent vasoconstictor produced by liver, kidneys and lungs
• Aldosterone ( BP)
– Na+ and water retention by the kidneys increases blood volume
• Vasopressin (ADH) ( BP)
– causes water retention by the kidneys which increases blood volume
– also causes generalized vasoconstriction
• Atrial Natriuretic Factor ( BP)
– produced by atria of heart
–  urinary sodium excretion which lowers blood volume
– also causes generalized vasodilation
• Epinephrine and Norepinephrine ( and  BP)
– binds to -adrenergic receptors on smooth muscle of blood vessels
causing vasoconstriction which increases blood pressure
– binds to -adrenergic receptors on blood vessels in skeletal and cardiac
muscle causing vasodilation which decreases blood pressure
Abnormalities of Blood Pressure
• Hypertension
– chronic resting BP above 140/90
– can damage capillaries and weaken small arteries
causing aneurysms
• Hypotension is chronic low resting BP
– Blood pressure lower than 120/80 mm Hg is
considered normal. There is no specific number at
which day-to-day blood pressure is considered too
low, as long as no symptoms of trouble are present.
– Hypotension can be caused by blood loss or
dehydration
– Hypotension lowers renal function, causes dizziness
and impairs brain function
http://www.americanheart.org/presenter.jhtml?identifier=4623