Events In Cardiac Cycle:

[dr_ravi]

EVENTS IN CARDIAC CYCLE:

sysotle=contraction diastole= relaxation.

1) Atrial contractionduration 0.1 sec)

During ventricular relaxation blood flows from atria to ventricles.
About 75% of blood flows to ventricles before atrial contraction.
When both atria contracts almost simultaneously and pupms remaining 25% of blood flows in respective ventricles (therefore even when if atrial fails to function it is unlikely to be noticed unless a person exercises).
Contraction of atria increses pressure inside atria to 4-6 mmHg in right atrium and abt. 7-8mmHg in left atrium.
The pressure rise in right atrium is reflected into the veins and this wave is recorded as 'a' wave (recorded from jugular vein with the help of a transducer)

2) Then there is a period of atrial relaxation for rest of caridac cycel abt duration 0.7 sec during which various ventricular events occur in a sequences as follows:

3)Ventricular contration (0.3 sec):

At the termination of atrial contration,the pressure of blood in ventricles rises (normally < 12mm Hg) .
Rising the ventricular pressure now exceeds the atrial pressure.
This causes closure of AV valves(tricuspid & mitral valves) which is a major component responsible for generating 1st heart sound.
Then there are following phases of ventricular contration:

a) Isovolumic or isometric contractions:

At beginning of this phase AV valve are closed but semilunar valves (pulmonary and aortic valves) are not yet opened.
Thus,ventricular chambers are sealed from both atria and arteries.
The ventricles starts contracting but volume of blood inside both ventricles remains the same hence this phase is called as isovolumic phase of contraction.
This phase lasts for abt.0.05 sec.
During this phase ventricles contract as a closed chamber and pressure inside the ventricles rises rapidly to a high value.
When pressure in left ventricle is slightly above 80 mmHg and right ventricular pressure slightly above 8 mmHg then the ventricular pressures push the semilunar valves open.
This causes ejection of blood from ventricles to the respective arteries in next phases.

b) Rapid ejection phase:

As soon as the semilunar valves open blood is rapidly ejected.
Abt.2/3rd of stroke volume is ejected in this rapid ejection phase.
Stroke volume: Volume of blood that is ejected by each ventricles with each beat.It is 70ml.
The duration of this phase is abt 0.11 sec.
The pressure inside the left ventricle rises to 120mmHg during this phase.
The end of rapid ejection phase occurs at abt the peak of ventricular and atrial contraction pressure.
The right ventricle ejection begins before that of left and continues even after the left ventricular ejection is complete.
As the both ventricles almost eject same volume of blood,the velocity of right ventricular ejection is less than that of left ventrcicle.

c) Reduced ejection phase:

During later 2/3rd of contratction rate of ejection declines.
During this phase of reduced ejection,rest one third stroke volume is ejected.
This phase lasts for abt 0.14 sec.
During the period of slow ejection ventricular pressure fall to a value slightly lower than that in Aorta but still blood continues to empty into Aorta because blood flowing out has built up momentum.
As this momentum decreases,kinetic energy of momentum is converted to pressure in the Aorta.
This causes aortic pressure to rise slightly above that of ventricle.

4) Ventricular relaxation: (0.5 sec)

It occurs in following phase:

a) Protodiastole:

At the end of ventricular systole ventricles start relaxing allowing rapid fall in the intraventricular pressure.
This is the period of protodiastole which lasts for 0.004 sec.
At the end of this phase elevated pressure in distended arteries (Aorta and Pulmonart artery) immediately pushes the blood back toward the ventricles which snaps the aortic and pulmonary semilunar valves closes.
This is the major component in generating 2nd heart sound (closure of semilunar valve).
It also causes dicrotic notch in the down slope of aortic pressure called Incisura.
Incisura indicates end of systole and the onset of diastole.

b) Isovolumic or isometri relaxation phase:

The ventricles continue to relax as closed chambers as semilunar valves are closed and AV valves are not yet open.
This causes rapid fall of pressure inside ventricles (from 80mmHg to abt 2 to 3 mmHg in left ventricle)
This phase lats for 0.06 sec.
Because the ventricular volume remains constant this phase is called isovolumic phase.
When ventricular pressures fall below atrial pressure the AV valves opens.

c) Rapid filling phase:

During ventricular contraction because AV valve are closed ,large amount of blood accumulates in Atria because Veins continue to empty the blood into them and this causes increase in the pressure inside the Atria.
High atrial pressure causes blood to flow rapid into ventricles.
Then pressures in both the chamber fall as ventricular relaxation continues.

d) Reduced filling phase or diastasis:

After the rapid filling phase,pressure in atria and ventricles rises slowly as blood continues to return to the heart.
This decreases the rate of blood flow from atria to ventricles causing slow filling of ventricles called diastasis.
During rapid filling and diastasis phase abt 75% of blood passes from atria to ventricles.
Then next cycles begins with atrial contraction.

[Soul]

good info thanks...but how ventricular contraction as well as relaxation occurs?

[dr_ravi]

hello thankx for question..Did you mean ventricular contraction and ventricular relaxation.
At first you must understand this :
SAN -> atria (systole) -> AVN -> bundle of His -> bundle branches – ventricles (systole)


Ventricular Diastole
At the end of ejection, ventricular volume is at its minimum and ventricular muscle fibers begin to relax.
1. Ventricular repolarization occurs (T wave), marking the end of contraction and beginning of relaxation.

2. The linear velocity of blood flow in the aorta falls (pressure in the aorta exceeds pressure in the ventricle and the pressure gradient is reversed), then flow is transiently reversed and blood returns toward the ventricle.

3. The aortic valve now closes . Momentum of blood returning to the ventricle from the aorta causes the aortic valve to bulge into the ventricle, then return to its normal position. This oscillation causes the incisura or dicrotic notch in the aortic pressure recording.

4. Ventricular pressure falls very rapidly, but volume does not change because no valves are open. This is the period of isovolumic relaxation, and its end is marked by the opening of the mitral valve .

5. Maximum atrial pressure has been achieved (also maximum atrial volume, causing the "v" wave. Atrial pressure exceeds ventricular pressure, the AV valve opens, and blood rapidly enters the relaxing ventricle (this is the rapid filling phase. Pressure in the atrium falls ("y" descent), but remains above that in the ventricle until a subsequent ventricular contraction occurs. There follows a period of reduced filling , then a period of further reduced filling referred to as diastasis .

[Soul]

dr_ravi thanx for ur reply.
Actually i was looking for molecular mechanisms of cardiac contaction and relaxation....such as how excitation reaches T-tubules, Ca++ enters and triggers release of Ca++ channels in SR--(Ca induced Ca release), which ultimately leads to cardiac contraction.
Again how Ca++ uptake occurs which leads to cardiac relaxation. such as role of SERCA and phospholamban.
Anyways, thanx for ur great effort, its quite helpful for quick revision.

[SUMAN-SAJAN]

nice post dr-ravi