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7/29/2019 1st Lec on Heart Physiology by Dr. Roomi
1/21
HEART PHYSIOLOGY
BY
DR. MUDASSAR ALI ROOMI (MBBS, M. PHIL)
7/29/2019 1st Lec on Heart Physiology by Dr. Roomi
2/21
Physiologic anatomy of heart
Hollow muscular
pumping organ
Weight: 330 grams
Size: closed fist
Function:is to receive
blood from veins and
then pump it intoarteries
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Physiologic anatomy of heart
Heart is considered as two pumps
Left and right hearts each havingan atrium and a ventricle.
Each atriumis aweak primerpump for the ventricle, helping to
move blood into the ventricle.
The ventricles then supply themain pumping force that propelsthe blood either:
(1) through the pulmonary
circulation by the right ventricleor
(2) through the peripheral orsystemic or greater circulation bythe left ventricle
7/29/2019 1st Lec on Heart Physiology by Dr. Roomi
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7/29/2019 1st Lec on Heart Physiology by Dr. Roomi
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HISTOLOGY of Cardiac Muscle
At intercalated disc cellmembranes formcommunicating junctions (gapjunctions) that allow almosttotally free diffusion of ions.
action potentials travel easilyfrom one cardiac muscle cell tothe next, past the intercalateddiscs.
cardiac cells are sointerconnected that when one of
these cells becomes excited, theaction potential spreads to all ofthem, spreading from cell to cellthroughout the latticework.(Functional syncytium).
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The Heart Actually Is Composed Of Two
functional Syncytia
the atrial syncytium thatconstitutes the walls of the twoatria, and the ventricularsyncytium that constitutes thewalls of the two ventricles.
The atria are separated from the
ventricles by fibrous tissue. Normally, the electric impulses
are not conducted from the atrialsyncytium into the ventricularsyncytium directly through thisfibrous tissue. Instead, they are
conducted only by way of aspecialized conductive system ofconductive fibers .
7/29/2019 1st Lec on Heart Physiology by Dr. Roomi
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importance of having
two functional syncytia:
This division of the muscle
of the heart into two
functional syncytiums
allows the atria to contract
a short time ahead of
ventricular contraction,
which is important foreffectiveness of heart
pumping.
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Functions of atria
1-Conduit function : Conduct bloodfrom great veins into theventricles.
2-Reservior function: reservoir ofblood.
3-Pumping functions: Atria act asPrimer Pumps for the ventricles
4-Venous blood drainage
5-Pacemaker and conductivityfunction: genesis and conduction
of cardiac impulse6-Endocrine function: Atrialnatriuretic peptide (ANP) secretedfrom special endocrine cells ofatria at the time of fluid overload.MCQ
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Properties of cardiac muscle
1-Automaticity and rhythmicity
(autorhythmicity)
2-Excitability
3:Conductivity
4-Contractility
5- All or none Law6-Reractory period
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1-Automaticity and rhythmicity
Automaticity: ability of a cell to produce
electrical impulses spontaneously.
Rhythmicity: it means spontaneous
depolarization occurs at regular
intervals.
Sinus (Sinoatrial) Node or S-A node:
SA node is the pacemaker of heart.**
Pacemaker activity is myogenic and
not neurogenic.*****
Location of SA node: It is located in
the posterolateral wall of the rightatrium immediately below and
slightly lateral to the opening of the
superior vena cava.
The fibers of this node have almost
no contractile muscle filaments.**
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2:Conductivity:(Dromotropic effect)
Ability to propagate an electricalimpulse.
All heart muscle fibers can conductimpulses but conduction is morerapid through the special conductingtissue: SA node-inter-NODALpathways-AV node AV bundle-Rt. and Lt. bundle branchesPurkinje fibers.
Normally conduction occurs in onedirection and in synchronous way.
What is +ve andve dromotropiceffect????
Epinephrine +ve effect
Acetylcholineve effect
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VELOCITY OF CONDUCTION
(dromotropy): 0.3-0.5 m/sec in atrial and ventricular
muscle fibers.
1/250 of the velocity in large nerve
fibers and 1/10 of that in skeletal
muscle fibers. (slow!)
Velocity in purkinje fibers: 4m/sec
MCQ
Slowest velocity= AV node
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3-Contractility: (Inotropy)
Shortening of myocardial muscles in response tostimulus.
0.2 second in atrial muscle and 0.3 second in
ventricular muscle.
Contractility is increased by (+ve inotropic effect):
sympathetic stimulation, Norepinephrines,
catecholamines , Calcium ions, digitalis, caffeine etc.
decreased by (-ve inotropic effect): acetylcholine,beta blockers drugs and Ca channels blockers, K+
ions, acidosis.
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3-Contractility (cont..)
Frank-Starling mechanism ofthe heart cotraction: greaterthe heart muscle is stretched
during diastolic filling (moreinitial or end diastolic length),the greater is the force ofcontraction and the greater isthe quantity of blood pumpedinto the aorta (within
Physiologic limits). Frank starling Law is applied to
each individual skeletal musclefiber but on heart as a whole.
Cardiac function curve
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ventricular function curves is a
way ofexpressing the Frank-
Starling mechanism of the
heart.
Greater the venous return
greater will be cardiac
output.***
MECHANISM: (as actin myosinmove apart by stretching to an
optimum length contract
more powerfully.
Cardiac function curve
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4-Excitability- Bathmotropy
Excitability is the property to respond to stimuli.
Stimuli: nervous, chemical, mechanical, electrical.
This property enables the heart muscles to respond to
artificial pacemaker.*** The nerves, drugs, ions and ischemia affect the excitability
of cardiac muscles.
+ve bathmotropic effect: epinephrine, nor-epinephrine,
sympathetic stimulation, caffeine, theophylline
-ve bathmotropic effect: acetylcholine, parasympathetic
stimulation.
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5- All or none Law (cardiac muscle)
Heart muscle contracts to its maximum or
not at all in response to a threshold stimulus.
Obeyed by heart muscle as a whole because
heart is a functional syncitium. ***
skeletal muscle fibers show it individually
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6-RERACTORY PERIOD of cardiac muscle
DEFINITION: it is the intervalduring which a normal cardiacimpulse cannot re-excite analready excited area of cardiac
muscle. (0.25-0.30 sec) Absolute refractory period: It is
the period during which alreadyexcited cardiac muscle does notrespond to a second stimulus.(0.25 sec)**
Relative refractory period: It isthe period during which alreadyexcited cardiac muscle givesresponse to a powerful excitatorystimulus. (0.05 sec)**
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6-RERACTORY PERIOD of cardiac muscle (cont..)
The normal refractoryperiod of the ventricleis almost equal to theduration of plateau
phase of actionpotential. ****
The refractory period ofatrial muscle (0.15 sec)
is much shorter thanthat of the ventricles(0.25 to 0.30 sec).
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Heart muscle cannot be tetanized!!!
It is due to plateau in
action potential of
cardiac muscle because
plateau increases therefractory period.
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TETANIZATION SEEN IN SKELETAL
MUSCLE