Download Heart

Circulatory System
major connection between external and internal environment:
everything going in or out of body must go through the circulatory
system to get to where its going
two major transport systems in body:
circulatory (cardiovascular) system
lymphatic system
circulatory system works in conjunction with lymphatic system
= an open system
circulatory system consists of “plumbing” and
“pumps”:
1. blood travels within a closed system of vessels; never leaves vessels
2. has muscular pump that helps to move it
is one of first organ systems to appear in developing embryo
heart is beating by 4th week
The Heart
about size and shape of closed fist
beats >100,000 x’s/day (~103,680b/d)
Histology of Heart
cardiac muscle fibers:
striated myofibrils are highly ordered
1 nucleus
branched cells
T tubules and SR less developed than skeletal mm
separated by intercalated discs
myocardium behaves as single unit
but atrial muscles separated from ventricular muscles by conducting tissue
sheath
atria contract separately from ventricles
mitochondria account for 25% of cardiac muscle cells
(compared to 2% of skeletal muscle cells)
Human Anatomy & Physiology: Cardiovascular System; Ziser, 2004
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greater dependence on oxygen than skeletal muscles
can’t build up much oxygen debt
more adaptable in nutrient use; can use:
glucose
fatty acids (preferred)
lactic acid
long contraction phase and long refractory period
almost as long as contraction phase
(~250ms vs 1-2 ms in skeletal muscle)
prevents tetanus
Conducting System
heart has some specialized fibers that are modified cardiac muscle cells
don’t contract; fire impulses that coordinate contraction of heart muscle
innervated by autonomic NS
consists of:
SA Node
intrinsic rhythm
70-75 beats/min
initiates stimulus that causes atria to contract
(but not ventricles directly due to separation)
AV Node
picks up stimulus from SA Node
if SA Node is not functioning it can act as a pacemaker
=ectopic pacekmaker (usually slower intrinsic rhythm)
AV Bundle (Bundle of His)
connected to AV Node
takes stimulus from AV Node to ventricles
Purkinje Fibers
takes impulse from AV Bundle out to cardiac mucscle fibers
of ventricles causing ventricles to contract
the heart conducting system generates a small electrical current that can be
picked up by an electrocardiograph
=electrocardiogram (ECG; EKG)
ECG is a record of the electrical activity of the conducting system
Human Anatomy & Physiology: Cardiovascular System; Ziser, 2004
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body is a good conductor of electricity (lots of salts)
potential changes at body’s surface are picked up by 12 leads
ECG is NOT a record of heart contractions
R
P
T
Q
S
P wave
=
passage of current through atria
from SA Node
atrial depolarization
QRS wave
=
passage of current through
ventricles from AV Node – AV Bundle – Purkinje
Fibers
ventricular depolarization
T wave
=
repolarization of ventricles
(atrial repolarization is masked by QRS)
by measuring intervals between these waves can get idea of how rapidly the
impulses are being conducted
amplitude of waves also gives info on condition of conducting system and
myocardium
Abnormalities of ECG’s = arrhythmias
1. bradycardia (<60 bpm)
decrease in body temperature
some drugs (eg digitalis)
overactive parasympathetic system
endurance athletes
2. tachycardia (>100 bpm)
increased body temperature fever
emergencies, stress activation of sympathetic NS
some drugs
may promote fibrillation
Human Anatomy & Physiology: Cardiovascular System; Ziser, 2004
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3. flutter
short bursts of 200-300 bpm
but coordinated
4. fibrillation
rapid, uncoordinated contractions of individual muscle cells
atrial fibrillation is OK
(since it only contributes 20% of blood to heart beat)
ventricular fibrillation is lethal
electrical shock used to defibrillate and recoordinate contractions
5. AV Node Block
normal P - Q interval = 0.12 – 0.20 seconds
changes indicate damage to AV Node
difficulty in signal getting past AV Node
1st º block:
>0.20 seconds
2nd º block:
AV Node damaged so only so wave passes
through ventricles only after every 2-4 P waves
rd
3 º block: (complete block)
no atrial waves can pass through
ventricle paced by different ectopic pacemaker
therefore beat abnormally slow
Cardiac Cycle
1 complete heartbeat (takes ~ 0.8 seconds)
consists of:
systole contraction of each chamber
diastole relaxation of each chamber
two atria contract simultaneously
as they relax, ventricles contract
ventricular systole (atrial diastole) = 0.3 sec
ventricular diastole
= 0.5 sec
relation of ECG to cardiac cycle
contraction and relaxation of ventricles produces characteristic heart sounds
lub-dub
lub = systolic sound
contraction of ventricles and closing of AV valves
dub =
diastolic sound
shorter, sharper sound
Human Anatomy & Physiology: Cardiovascular System; Ziser, 2004
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ventricles relax and SL valves close
abnormal sounds: “murmurs” defective valves
congenital
rheumatic (strep antibodies)
septal defects
relationship of cardiac cycle, ECG, heart sounds, pressure and volume
Cardiac Output
CO
=
=
=
Heart RateX
Stroke volume
75b/m
X
70ml/b
5250 ml/min (=5.25 l/min)
~ normal blood volume
A. Heart Rate:
innervated by autonomic branches to SA and AV nodes
control center in medulla (cardiac center)
receives sensory info from:
Baroreceptors (stretch)
in aorta and carotid sinus
increased stretch slower
Chemoreceptors
monitor carbon dioxide and pH
more CO2 or lower pH faster
Other Factors that Affect Heartrate:
a. hormones
epinephrin faster
acetylcholine slower
thyroxine faster
b. drugs
atropine (from belladonna)
anticholinergic drug faster
digitalis slows, but greater force
c. ions
low Calcium slower
high Calcium faster
Human Anatomy & Physiology: Cardiovascular System; Ziser, 2004
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spastic heart contractions
high Sodium blocks Ca++ slows
high Potassium faster
may cause cardiac arrest
d. temperature
heat increases heart rate
e. age
younger = faster, slows with age
f. gender
women = faster (72-80 bpm)
men = slower (64-72 bpm)
g. exercise
increases during exercise
also heart beats slower in physically fit
h. emotions
fear, anxiety, anger increase HR
depression, grief reduce HR
any marked, persistent changes in rate may signal cardiovascular disease
B. Stroke Volume:
healthy heart pumps ~60% of blood in it
normal SV = ~70 ml
SV
=
EDV (end diastolic vol) – ESV (end systolic vol)
affected by:
1. mean blood (arterial)pressure
back pressure
2. condition of heart tissue
eg. heart contractility
indicates amt of damage
3. venous return/ amount of stretch
Starling’s Law: within physiological limits the heart pumps
all the blood that returns to it without undue damming
of blood in veins.
intrinsic regulatory mechanisms permit adaptation of the
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heart to varying rates of venous return.
>stretch = >strength of contraction
4. blood viscosity
(>RBC’s, dehydration, blood proteins)
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