1st Lec on Heart Physiology by Dr. Roomi

7/29/2019 1st Lec on Heart Physiology by Dr. Roomi

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HEART PHYSIOLOGY

BY

DR. MUDASSAR ALI ROOMI (MBBS, M. PHIL)


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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


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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 .


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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

1st Lec on Heart Physiology by Dr. Roomi

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