Download Heart As A Pump And Cardiac Cycle Mechanical events :

Heart As A Pump And Cardiac Cycle
Mechanical events :
The duration of one cardiac cycle is approximately 0.8 second,
making a heart rate of 75 beats/minute, (we use these values
for studying purposes, but they actually vary according to the
heart rate which ranges between 60 – 100 beats/minute in
normal cases). The mechanical events that happen during
one
cardiac
cycle
beginning
from
atrial
systole:
1) Atrial systole takes 0.1 second, and the atrial diastole takes the
rest of the cycle‟s time, 0.7 second.
2) The AV delay makes the ventricles contract after the atria finish
their contraction, so the ventricular systole begins at 0.1 second
on the diagram, its duration is 0.3 second and therefore it finishes
at 0.4 second. Ventricular diastole obviously takes 0.5 second,
and it lasts from 0.4 to 0.9 second on the diagram.
AS
AS
VS
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
*continuous line means systole, dotted line means diastole, the one above is for the Atria and the one
below is for the ventricles.
- The atria contract while the ventricles are in diastole, as seen in (00.1) and (0.8-0.9).
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The electrical response precedes the mechanical one, so the P wave is
recorded before the atrial systole, and QRS before the ventricular
systole, also T precedes ventricular diastole in a way similar to the
diagram below.
P
R
Q
0
S
0.1
T
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
We can add the action potential on the diagram, (you‟re supposed to
understand the relationship between the action potential and ECG from
previous lectures).
atrial
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ventricular action potential
Volume changes :
Now we‟ll talk about volume changes, we don‟t care about the atrial
volume, the ventricular volume is the important one. The blood volume
of the right ventricle equals that of the left ventricle, the ventricles differ
only in pressure (pressure in the left > right).
Before the atria contract, there is a certain amount of blood in the
ventricles, let‟s say there‟s 100 ml in the left ventricle before the atrial
contraction, now when the atria contract, the AV valves would be
already open (remember that they open passively due to pressure
gradient at the beginning of ventricular diastole). The atrial pressure is
almost 0, the ventricular pressure is almost -2 mmHg (negative,
because we‟re measuring relative to the atmospheric pressure which is
our reference, so a value of zero here equals the atmospheric pressure
= 760 mmHg). When the atria contract, they push certain amount of
blood to the ventricles, so the volume in the left ventricle (our example)
becomes 125 ml at the end of the ventricular diastole, we call it End
Diastolic Volume “EDV”. The atrial systole contributed to about 25%
of this volume.
if they didn‟t contract (as in atrial fibrillation), blood would also be
flowing to the ventricles because the AV valves are open, but they will
move with less amounts and speed in this case (the volume in the
ventricle would increase by 15% or 10% only.)
At the beginning of ventricular systole, pressure will gradually build up
in the ventricles, exceeding that of the atria, when blood tries to move
from the ventricles to the atria, the AV valves will close. At this stage,
the 4 valves of the heart (AV & semilunar) are closed, and the volume in
the ventricle doesn‟t change, therefore we call this phase Isovolumic
contraction, which is a short phase in which ventricular volume is
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constant and the 4 valves are closed, and with contraction, the pressure
sharply increases.
When the pressure in the left ventricle exceeds that of the aorta, the
aortic valve opens, and the blood is ejected from the ventricle, the first
ejection phase is called the rapid ejection, after it the blood is ejected
slowly.
At the end of systole, 55 ml stays in the ventricle and it‟s called the
End Systolic Volume “ESV”, the amount of blood ejected in each
beat (cardiac cycle) from either right or left ventricle is called stroke
volume, and here it equals (125 – 55 = 70 ml/beat). Stroke volume =
EDV – ESV.
During ventricular diastole, pressure in the left ventricle starts to
decrease, when it becomes less than the aortic pressure, blood tries to
move from the aorta to the left ventricle, closing the semilunar / aortic
valve, and the AV valves are still closed, and this diastolic phase where
the 4 valves are closed, is called Isovolumic relaxation.
The pressure in the Isovolumic relaxation phase keeps decreasing
until it becomes lower than the atrial pressure, then the AV valve opens
 rapid filling then slow filling. Before the atrial systole, the blood
volume in the ventricle rises to 100 ml (passively by pressure gradient),
when the atria contract, another 25 ml are added.
* Cardiac output (ml/minute) = stroke volume X heart rate
The stroke volume in the left ventricle equals that in the right
ventricle, they have to be so ! Let‟s suppose that the stroke volume in
the right ventricle is 70 ml/beat, and in the left ventricle 69ml/beat,
consequently, 1 ml accumulates in the left ventricle per beat, if the
heart rate was 70 beat/minute, 70 ml will accumulate every minute,
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4200 ml every hour! this is called „heart failure‟. The left ventricle
normally pumps what it receives from the right ventricle, even if a small
amount is accumulated in one beat, compensation will happen in the
next one.
Pressure changes :
Left ventricular pressure during ventricular diastole equals -2 or -1 or
0 (doesn‟t matter), while the aortic pressure is 80 mmHg. Because of
the atrial systole, the left ventricular pressure increases a little bit (rises
to be around +3 mmHg), when the isovolumic contraction starts, left
ventricular pressure increases sharply, until it exceeds 80, causing the
aortic valve to open, and it keeps rising to be higher than the aortic
pressure.
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At the last part of the ventricular systole, the aortic pressure will be
higher than the left ventricular pressure, although the blood is still
moving from the ventricle to the aorta, and that happens because the
blood has a momentum, when the force of the aortic pressure
overwhelms the force of the blood‟s momentum, it closes the aortic
semilunar valve, starting the isovolumic relaxation, the left ventricular
pressure decreases until we reach the end of this phase where
ventricular pressure falls to be lower than atrial pressure and the AV
valves open.
When the aorta pushes the blood closing the semilunar valve, the
blood is compressed; so aortic pressure slightly rises, making a small
peak, called dicrotic notch or incisura.
The pressure in the left ventricle varies between 0 – 120 mmHg, in
the aorta it varies between 80 – 120 mmHg. The right ventricular
pressure varies between 0 – 25 mmHg, in the pulmonary artery 8 – 25
mmHg. If we want to make a similar diagram for the right side, the
waves (below) are the same, but we just change the scale (pressure
values).
 Heart sounds :
The 1st heart sound “lup” (S1) appears when the AV valves close. The
2nd heart sound “dup” (S2) appears when the semilunar valves close.
The time between S1 and S2 represents ventricular systole (0.3
second), the time between S2 and the next S1 represents the
ventricular diastole. There are S3 & S4 but we don‟t hear them
normally, however; the Phonocardiograph will record them.
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