Chapter 20 - The Heart
Location and Size of Heart
 Located in thoracic
cavity in mediastinum
 About same size as
closed fist
 base is the wider
anterior portion
 apex is tip or point
Pericardium: Heart Covering
Fibrous Pericardium
 Rests on and is
attached to diaphragm
 Tough, inelastic sac of
fibrous connective
tissue
 Continuous with blood
vessels entering,
leaving heart at base
 Protects, anchors
heart, prevents
overstretching
Parietal (Outer) Serous Pericardium
 Thin layer adhered to
inside of fibrous
pericardium
 Secretes serous
(watery) lubricating
fluid
Pericardial Cavity
 Contains pericardial
(serous) fluid
 lubricates surface of
parietal and visceral
serous pericardium
 decreases friction
Visceral (Inner) Serous Pericardium
 Adheres to Heart
 Forms epicardium
 Secretes watery
(serous) lubricating
fluid
Pericardium
Homeostatic Imbalances
Pericarditis
inflammation of pericardium
painful, rubbing of tissues
can damage myocardium
Cardiac tamponade
a buildup of pericardial fluid
bleeding into pericardial cavity
may result in cardiac failure
Heart wall - Three layers
 Epicardium (outer)
 visceral layer of
pericardium
 thin, transparent
 smooth, slippery
 Myocardium (middle) cardiac muscle
 Endocardium (inner)
 endothelium over connective
tissue
 smooth lining for inside of
heart, valves
 continuous w/ endothelium
of vessels
Chambers of the Heart
 External landmarks
 coronary sulcus
separates
atria/ventricles
 anterior/posterior
interventricular sulcus
separates right/left
ventricles
 Internally - 4
compartments
 R/L atrium w/ auricles
 R/L ventricles
 Interatrial septum
separates atria
 Interventricular
septum separates
ventricles
 Ventricular thickness varies depending on
function
 Right – pumps to lungs (pulmonary circulation)
 Left – pumps to the body (systemic circulation)
Valves of the Heart
 Function to prevent
backflow of blood
into/through heart
 Open, close in response
to changes in pressure
in heart
 Four valves
Valve Structure
 Dense connective
tissue covered by
endocardium
 AV valves
 chordae
tendineae - thin
fibrous cords
 connect valves to
papillary muscles
Valve Function
 Opening and closing a
passive process
 when pressure low, valves
open, flow occurs
 with ventricular
contraction, pressure
increases
 papillary muscles
contract, prevent valves
from pushing back into
atria
Atrioventricular (AV) valves
 Separate atria,
ventricles
 tricuspid valve -
right
 bicuspid (mitral)
valve - left
Semilunar valves
 In arteries that exit
heart to prevent blood
from re-entering heart
 pulmonary semilunar
valves
 aortic semilunar valves
 Pathologies
 incompetent – do not
close
 stenosis – stiff and do
not close
Blood Flow Through Heart
 Right atrium (RA) receives deoxygenated
blood from three
sources
 superior vena cava
(SVC)
 inferior vena cava (IVC)
 coronary sinus
 Right ventricle (RV)
 receives blood from RA
 pumps to lungs
 Pulmonary trunk - from RV
branches into pulmonary
arteries (PA)
 Pulmonary arteries
 from heart to lungs for gas
exchange
 right and left branches for
each lung
 blood gives up CO2 and picks
up O2
 Pulmonary veins (PV) oxygenated blood from
lungs to heart
Pulmonary Circulation
 Left atria
 receives blood from PV
 pumps to left ventricle
 Left ventricle (LV)
 sends blood to body via
ascending aorta
 aortic arch
 curls over heart
 three branches off of it
that feed superior
portion of body
 thoracic aorta
 abdominal aorta
Myocardial Blood Supply
 Myocardium has own
blood supply
 coronary vessels
 diffusion into tissue
impossible due to
thickness
 much overlap of vessels
and anastomoses (art-art
connections)
 Heart can survive on 1015% of normal arterial
blood flow
 Arteries
 left coronary artery divides
into anterior interventricular
artery and circumflex arteries
 anterior interventricular
artery supplies walls of both
ventricles and septum
 circumflex supplies LV and LA
 right coronary artery small
branches to RA, divides into
posterior interventricular and
marginal artery
 posterior interventricular
supplies walls of both
ventricles
 marginal branch supplies RV
 Coronary veins
 blood into muscle then
drains into coronary sinus
 supplied by great cardiac
vein (drains anterior of
heart) and middle cardiac
vein (drains posterior)
Coronary Circulation Pathologies
 Faulty coronary
circulation due to:
 blood clots
 fatty atherosclerotic
plaques
 smooth muscle spasms in
coronary arteries
 Problems
 ischemia
 hypoxia
Pathologies (cont.)
Angina pectoris - "strangled chest"
 pain w/ myocardial ischemia - referred pain!
 tight/squeezing sensation in chest
 labored breathing, weakness, dizziness,
perspiration, foreboding
 often during exertion - climbing stairs, etc
 silent myocardial ischemia
Pathologies (cont.)
 Myocardial infarction
(MI) - heart attack
 thrombus/embolus in
coronary artery
 tissue distal to
blockage dies
 if survival, muscle
replaced by scar tissue
 Long term results
 size of infarct, position
 pumping efficiency?
 conduction efficiency,
heart rhythm
Pathologies (cont.)
 Treatment
 clot-dissolving agents
 angioplasty
 Reperfusion damage
 re-establishing blood flow may damage tissue
 oxygen free radicals - electrically charged molecules
w/ unpaired electron
 radicals attack proteins (enzymes),
neurotransmitters, nucleic acids, plasma membranes
 further damage to previously undamaged tissue
or already damaged tissue
Myocardium (Cardiac Muscle)
 Cells are involuntary, striated, branched
 Fibers connected to others by intercalated discs
 gap junctions allow AP's to pass from fiber to fiber
 desmosomes
 “spot welds”
 prevent cardiac fibers from separating
Intercalated Discs
Normal Action Potential
Cardiac Muscle Action Potential
 Long absolute
refractory period
Pacemaker potentials
 Leaky membranes
 Spontaneously depolarize
Conduction System and Pacemakers
 Autorhythmic cells
 cardiac cells repeatedly fire
spontaneous action potentials
 autorhythmic cells: the
conduction system
 pacemakers
 SA node
 origin of cardiac excitation
 fires 60-100/min
 AV node
 conduction system
 AV bundle of His
 R and L bundle branches
 Purkinje fibers
Conduction System and Pacemakers
 Arrhythmias
 irregular rhythm
 abnormal atrial and ventricular contractions
 Fibrillation
 rapid, out of phase contractions
 squirming bag of worms
 Ectopic pacemakers (ectopic focus)
 abnormal pacemaker controlling the heart
 SA node damage, caffeine, nicotine, electrolyte
imbalances, hypoxia, toxic reactions to drugs
 Heart block
 AV node damage - severity determines outcome
 may slow conduction or block it
Conduction System and Pacemakers
SA node damage (MI)
 AV node can run things (40-50 bts/min)
 if AV node out AV bundle, bundle
branch/conduction fibers fire at 20-40
bts/min
Artificial pacemakers - can be activity
dependent
Atrial,Ventricular Excitation Timing
SA node to AV node - small delay
 about 0.05 sec from SA to AV, 0.1 sec to
get through AV node
 conduction slows
 allows atria time to finish contraction and better
fill the ventricles
 once to AV bundle, conduction rapid to rest
of ventricle
Extrinsic Control of Heart Rate
 Basic rhythm of heart set
by pacemaker system
 Central control from medulla
 sympathetic input
 parasympathetic input
Electrocardiogram
 Electrical activity
of the heart
 P wave
 QRS complex
 T wave
Cardiac Cycle
 Connection between
electrical and
mechanical events
 Systole
 Diastole
 Isovolumetric
contraction
 Isovolumetric
relaxation
Quiz!!!!!
 1. Superior vena cava
 2. Right atrium
 3. Tricuspid valve
 4. Right ventricle
 5. Papillary muscle
 6. Aorta (aortic
arch)
 7. Pulmonary trunk
 8. Left atrium
 9. Bicuspid valve
 10. Interventricular
septum
Cardiac Output
Amount of blood pumped by each
ventricle in 1 minute
CO = HR x SV
 HR
 heart rate
 70 bts/min
 SV
 stroke volume
 70 ml/min
 CO  5 L/min (70 bts x 70 ml)
Regulation of Stroke Volume
SV = EDV – ESV
 EDV
 End Diastolic Volume
 volume of blood in the heart after it fills
(following diastole)
 120 ml
 ESV
 End Systolic Volume
 volume of blood in the heart after contraction
(after systole)
 50 ml
 each beat ejects about 60% of blood in
ventricle
 Most important factors in regulating SV:
preload, contractility and afterload
 Preload
 degree of stretch of cardiac muscle cells before
contraction
 determined by EDV
 Contractility
 increase in contractile strength separate from
stretch and EDV
 determined by changes in Ca++ availability
 Afterload
 pressure that must be overcome for ventricles to
eject blood from heart
 determined by TPR
Preload
 Muscle mechanics
 length-tension relationship?
 fiber length determines # of cross bridges
 cross bridge # determines force
 increase/decrease fiber length increase/decrease
force generation
 Cardiac muscle
 how is cardiac fiber length determined/regulated?
 fiber length determined by filling of heart – EDV
 factors that effect EDV (anything that effects
blood return to heart) increase/decrease filling
 increase/decrease SV
 Preload – Frank-Starling Law of the
Heart
 length tension relationship of heart
 length = EDV
 tension = SV
Contractility
Increase in contractile strength
separate from stretch and EDV
Do not change fiber length but increase
contraction force?
 what determines force?
 how can we change this if we don’t change
length?
 Increase the number
of cross bridges by
increasing amount of
Ca++ inside the cell –
positive inotrope
 Sympathetic nervous
system opens channels
to allow Ca++ to enter
the cell
 Increase force of contraction without
changing fiber length
Afterload
Flow = P/R
If blood pressure is high (TPR), difficult
for heart to eject blood
More blood remains in ventricle with
each beat
Heart has to work harder to eject blood,
change the length/tension of the heart
Regulation of Heart Rate
 Intrinsic regulators
 pacemakers
 Bainbridge effect
 increase in EDV increases HR
 filling stretches SA node increasing depolarization and HR
 Extrinsic regulators
 autonomic nervous system
 sympathetic
 parasympathetic
 hormones – epinephrine, thyroxine
 ions – Na+, K+, Ca++
 body temperature
 age
 gender
 exercise