Sunday, February 27, 2011

Heart muscles,as a pump and functions of valves


4 chambers:
Right and left atria
Right and left ventricles

4 valves:
Biscuspid valve between lft atrium and lft ventricles
Trcuspid valve between rt atrium and rt ventricles
2 semilunar valves ; one between lft ventricle and aorta and the other is between rt ventricle and pulmonary arteries

3 types of muscle:
Atrial muscle
Ventricular muscle
Specialized muscle:Excitatory muscle and conductory muscle

-Cardiac muscles are striated muscle with actin and myosin filaments sliding along side of each other as skeletal muscle.

-Syncytium:
The cardiac muscles are interconnected with each other by intercalated discs. At intercalated discs the membrane of cells fuses forming gap junctions through which adjacents cells can communicate with each other. These intercalated discs allows action potentail of one muscle cell to pass to other cells. By this way the whole heart muscle acts as a single unit. This property of heart muscle is called Syncytium.
2 types: Atrial and ventricular syncytium


-Atrium and ventricles are not directly connected. They are separated by fibrous tissues which insulates atria and ventricles.
-The impulses from atria are conducted to ventricles though specialezed conductive tissues called A-V bundle.
-Because of this insulation between atria and ventricles , atria contracts a short time ahead of ventricles.


Action potential in cardiac muscles

RMP: -85 mV
Peak/spike potential: +20 mV
After spkie potential it remains depolarised for about 0.2 sec called plateau phase and then only repolarization occurs. Plateau is the characteristics of heart muscle. Because of plateau ventricular muscles remains in contraction for 15 times longer duration than skeletal muscles.


Causes of plateau in Cardiac muscles

1.The cause of action potential in cardiac muscle is by opening of 2 types of channels:
a. fast Na ions channels
b. slow Na- Ca ions channels ( not present in skeletal muscles)

Slow Na-Ca ion channels opens slowly and remains opened for longer time, so large number of Na, Ca ions enters causing prolonged duration of depolarization and this causes plateau phase. The Ca ions that have influxed during plateau causes cardiac muscle contraction. But for skeletal muscle contraction Ca ions comes from sarcoplasmic reticulum .

2.After depolarization K ion permeability through cardiac mucle membrane decreases by 5 folds. So+ve ions remains inside the cell casuing plateau.


Velocity of conduction in cardiac muscles

Atrial , ventricular: 0.3 - 0.5 m/sec
Purkinje fiber: 4 m/sec

Refractory period: ventricles 0.25 – 0.30 sec (till plateau) ; atrium 0.15 sec

Relative refractory period: 0.05 sec, strong stimuli required for premature beats formation



Excitation – contraction coupling

-It is the mechanism by which action potential causes myofibrils to contract.

-It is same as skeletal muscle.in skeletal muscle only one source of Ca ions: from sarcoplasmic reticulum

-When action potential reaches T tubules it causes releae of Ca ions from t tubules itself and cisternae of sarcoplasmic reticulum.

-The difference is that there are two Ca ions source: one is from cisternae of sarcoplasmic reticulum and the another is from T tubules itself. Because of the extra Ca from T tubules the cardiac muscles contraction is strong. The diameter of T tubules is bigger and it also contains large number of negative ions which binds Ca ions and releases when action potential reaches T tubules.


Duration of contraction:

contraction begins just a few milliseconds after action potential starts and continues till few milliseconds after end of action potential.

Atrial: 0.2 sec

Ventricle: 0.3 sec



The Cardiac Cycle

The events that occurs from beginning of heart beat to the beginning of next is called cardiac cycle.

Action potential is generated in SA node , it is located in the superior lateral wall of right atrium near the opening of superior venacava. Then it travels through atria and then though A-V bundle into ventricles. At A-V node there is delay in conduction of 0.1 sec, because of this atria contracts prior to ventricles and so pumps blood in ventricles and then ventricles contracts and pumps blood out.



Diastole and systole:

Diastole is the phase of cardiac cycle during which heart remains relaxed and is filled with blood .

Systole is the phase during which heart remains contracted and pumps blood out.


Relationship of heart sounds to heart pumping

First heart sound:
is due to closure of A-V valves when ventricle starts to contract

Second heart sounds:
closure of aortic and pulmonary valves at end of systole



Relationship between left ventricular volume and intraventricular pressure during diastole and systole.

Preload and afterload

Preload is the pressure with which the ventricle starts to contract and is the end diastolic volume

Afterload is the pressure/load against which the ventricle has to contract and is the systolic pressure.


Regulation of heart pumping

1.Intrinsic regulation of heart pumping (Frank starling mechanism):
Within physiologic limit, the greater the heart muscle stretched during blood filling, greater is the force of contraction and greater quantity of blood is pushed into aorta. This is known as Frank-starling mechanism.

2.Control of heart by autonomic nervous system:
-sympathetic stimulation causes increase heart rate and increased force of contraction
-parasympathetic/vagus stimulation decreases heart rate



Effect of K & Ca on heart function

Inc. K ions in ECF:
-heart becomes dilated, flaccid
-HR decreases
-slows/blocks conduction of impulses through A-V node
-decreases the RMP

Inc Ca ions: opposite to Inc K ions
-spastic contraction

Dec Ca ions:
-flaccidity


Effect of temperature on heart function

Dec temperature:
decrease heart rate ; is due to less ions are permaeble to heart muscles membrane at low temp

Moderate inc temp:
contractile strength increased but high temp causes fatigue of muscle by inc metabolism.

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