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Cardiac Haemodynamics: the Diastole

 

Aim: What happens in the heart during diastole?.

 

A. Blood flow during the diastole:

1.

The diastole is the period during which the ventricles relax, rest and gets filled again with blood for the next systole.

2.

As with the systole, we can divide this period into three phases:

  1. the rapid filling phase
  2. the passive filling phase
  3. the active filling phase.

 

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3.

Remember that during the systole, the AV-valves have been closed. Blood that has meanwhile flown from the veins (VCS = vena cava superior, VCI = vena cava inferior and the pulmonary veins) into the atria have accumulated in that space.

4.

At the beginning of the diastole, when the AV-valves open, this accumulated blood will suddenly and rapidly flow into the ventricles; this the rapid filling phase.

 

5.

Once this accumulated blood has flown into the ventricles, blood keeps streaming in from the veins into the atria. This blood can immediately flow further into the ventricles: passive filling phase.

6.

At the end of the ventricular diastole, the sinus node will become excited again and the atrial systole will start (remember that the atrial systole starts before the ventricular systole). This atrial excitation will induce atrial contraction and this will squeeze the last remaining blood into the ventricles; active filling phase.

 

B. Blood pressure during the diastole:

1.

As I said before, the diastole begins when the AV-valves open and ends when the AV-valves close (at the next systole)

2.

The pressure in the ventricles during diastole is very low, close to zero mmHg. This is important as it must be lower than the low pressures in the atria, or else, blood would not flow into the ventricles.

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3.

Therefore, as shown in the diagram, there is not much change in the ventricular pressure during diastole, in contrast to the big change during systole.

 

 

C. Additional notes:

1.

Note that the blood flow from the veins is constant and continuous and is never interrupted during systole and diastole.

2.

But, because of the pumping of the heart, the blood that flows out of the heart, into the arteries is not continuous but pulsatile (in “spurts”).

3.

While the ventricles contract, the AV-valves are closed but there must be space where meanwhile the venous blood can be collected or “pooled”.

4.

This pooling of blood is done in the atria. This is the major function of the atria.

5.

Note also that the atrial contraction only plays a minor role in the blood flow from the atria into the ventricles; estimated at about 25% of the total blood flow.

6.

But, if the ventricles don’t contract, then of course there will be NO blood flow into the arteries at all!

 

D. Major difference between the Right heart and the Left heart:

1.

The behaviour of the heart and the flow of blood during systole and diastole is very similar in the right and the left heart.

2.

The valves work near simultaneously in closing and opening; both AV-valves close and open simultaneously and so do the SL-valves.

3.

The big difference between the right heart and the left heart is the blood pressure difference in the ventricles.

4.

As shown in the diagrams, during systole, the pressure in the left ventricles increases from 0 to 120 mmHg.

5.

But in the right ventricles, the increase is much less, and reaches a maximum of “only” 25 mmHg.

6.

This difference is caused by the fact that the wall of the left ventricle is much thicker than that of the right ventricle. Therefore, the left ventricular contraction is much stronger and the pressure achieved much higher; i.e. 120 mmHg instead of 25 mmHg.

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E. Cardiac Output:

1.

The function of the heart is to pump blood out of the ventricles. This amount of blood is called the cardiac output.

 

2.

Cardiac output is defined as the amount of blood pumped out of the left (or the right) ventricle in one minute.

3.

This cardiac output (=CO) can be easily calculated by multiplying two values; the frequency of the heart (=F) and its stroke volume (=SV): Frequency*StrokeVolume = Cardiac Output)

4.

The frequency of the heart is the amount of times the heart beats every minute. At rest, when the body is not exercising, it is about 60-80 beats/minute.

5.

The stroke volume is the amount of blood that is pumped out of the ventricle in every beat. This is, again at rest, about 70 ml.

6.

Suppose that the heart beats at 70 beats/min and that the stroke volume is 70 ml; then the cardiac output is:
       70 b/min * 70 ml = 4900 ml

     (or approximately 5 litres/min).

7.

Eh! Wait a minute! FIVE LITRES per MINUTE??
Yes, this is absolutely true; at rest, a normal heart in a normal size adult beats about 5 litres/min.

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8.

Just think about it; here is a 1-litre bottle of water. Multiply this with 5, then, this is the amount of blood that one ventricles pumps every minute. And then, there is a second ventricle, which also pumps the same amount of blood, so, the two hearts together pump 10 LITRES every minute!!!

9.

And this was at rest. When the body exercises, it will need more blood and the heart provides for this. Therefore, the heart has to beat faster (the frequency increases) and the force of contraction increases (to be discussed later). The increase in contraction force will increase the stroke volume.

10.

Therefore, as both the frequency and the stroke volume increases (in both ventricles!), this will easily increase the cardiac output (in each ventricle) to, for example, 15 litres/min.

11.

Just to give you an idea of how much all this really is; go to kitchen and open the tap wide open. Try to fill a bucket (typically 10 litre) in one minute!

 

HumanPhysiology.academy 2014