heart-physiology-by-dr-roomi

HEART PHYSIOLOGY

BY

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

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 into arteries

Physiologic anatomy of heart Heart is considered as two

pumps Left and right hearts each having

an atrium and a ventricle. Each atrium is a weak primer

pump for the ventricle, helping to move blood into the ventricle.

The ventricles then supply the main pumping force that propels the blood either:

(1) through the pulmonary circulation by the right ventricle or

(2) through the peripheral or systemic or greater circulation by the left ventricle

HISTOLOGY of Cardiac Muscle

At intercalated disc cell membranes form “communicating” junctions (gap junctions) that allow almost totally free diffusion of ions.

action potentials travel easily from one cardiac muscle cell to the next, past the intercalated discs.

cardiac cells are so interconnected that when one of these cells becomes excited, the action potential spreads to all of them, spreading from cell to cell throughout the latticework. (Functional syncytium).

The Heart Actually Is Composed Of Two functional Syncytia the atrial syncytium that

constitutes the walls of the two atria, and the ventricular syncytium that constitutes the walls of the two ventricles.

The atria are separated from the ventricles by fibrous tissue.

Normally, the electric impulses are not conducted from the atrial syncytium into the ventricular syncytium directly through this fibrous tissue. Instead, they are conducted only by way of a specialized conductive system of conductive fibers .

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 for effectiveness of heart pumping.

Functions of atria

1-Conduit function : Conduct blood from great veins into the ventricles.

2-Reservior function: reservoir of blood.

3-Pumping functions: Atria act as Primer Pumps for the ventricles

4-Venous blood drainage

5- Pacemaker and conductivity function: genesis and conduction of cardiac impulse

6-Endocrine function: Atrial natriuretic peptide (ANP) secreted from special endocrine cells of atria at the time of fluid overload. MCQ

Properties of cardiac muscle

1-Automaticity and rhythmicity (autorhythmicity)

2-Excitability

3:Conductivity

4-Contractility

5- All or none Law

6-Reractory period

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 right atrium immediately below and slightly lateral to the opening of the superior vena cava.

The fibers of this node have almost no contractile muscle filaments.**

2:Conductivity:(Dromotropic effect)

Ability to propagate an electrical impulse.

All heart muscle fibers can conduct impulses but conduction is more rapid through the special conducting tissue: SA node-inter-NODAL pathways-AV node AV bundle- Rt. and Lt. bundle branches Purkinje fibers.

Normally conduction occurs in one direction and in synchronous way.

What is +ve and –ve dromotropic effect????

Epinephrine +ve effect Acetylcholine –ve effect

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

3-Contractility: (Inotropy)

Shortening of myocardial muscles in response to stimulus.

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..)

Cardiac function curve

Frank-Starling mechanism of the heart cotraction: greater the heart muscle is stretched during diastolic filling (more initial or end diastolic length), the greater is the force of contraction and the greater is the quantity of blood pumped into the aorta (within Physiologic limits).

Frank starling Law is applied to each individual skeletal muscle fiber but on heart as a whole.

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Cardiac function curve

ventricular function curves is a way of expressing the Frank-Starling mechanism of the heart.

Greater the venous return greater will be cardiac output.***

MECHANISM: as actin myosin move apart by stretching to an optimum length contract more powerfully.

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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 interval during which a normal cardiac impulse cannot re-excite an already excited area of cardiac muscle. (0.25-0.30 sec) Absolute refractory period: It is

the period during which already excited cardiac muscle does not respond to a second stimulus. (0.25 sec)**

Relative refractory period: It is the period during which already excited cardiac muscle gives response to a powerful excitatory stimulus. (0.05 sec)**

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6-RERACTORY PERIOD of cardiac muscle (cont..)

The normal refractory period of the ventricle is almost equal to the duration of plateau phase of action potential. ****

The refractory period of atrial muscle (0.15 sec) is much shorter than that 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 the refractory period.

TETANIZATION SEEN IN SKELETAL MUSCLE