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Hind Leys Biology
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The cardiac cycle
Objectives
 Describe the cardiac cycle with reference to the action of the valves in the
heart.
Sequence of contraction
It is important that the chambers of the heart all contract in a coordinated fashion,
otherwise inefficient pumping of blood will occur.
The cardiac cycle is the sequence of events in one contraction.
Since a cycle has no beginning or end, it is best to describe the cycle as a series of
stages. The term for a stage in the cardiac cycle when the heart muscles are
contracting is systole. The relaxation stage is called diastole. The three stages shown in
Figure 1 and detailed in Table 1 are atrial systole, ventricular systole and diastole.
Figure 1 The three
stages in the cardiac
cycle. The atria
contract (atrial
systole), the ventricles
contract (ventricular
systole) and then the
ventricles relax
(diastole).
http://www.kscience.co.uk/animations/heart.swf
http://www.youtube.com/watch?v=D8LK34hoVpU&feature=related
http://www.youtube.com/watch?v=vGP08SEYdJc&feature=related
http://www.youtube.com/watch?v=nx-XRC_1n-Q&feature=related
http://www.youtube.com/watch?v=q0s-1MC1hcE&feature=related
http://www.youtube.com/watch?v=P_d0ykpzQgY&feature=related
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Phase
Heart
Atrial systole
Atria contract in
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Ventricular systole Diastole
Ventricles
Atria and ventricles
later stage. Volume
contract. Volume
relax. Internal
decreases so
decreases,
volume increases,
pressure increases.
pressure increases.
pressure decreases
Atria relaxed, so
lower pressure
Blood flow
Pressure
Blood forced from
Blood forced from
Blood flows into
atria into
ventricles into
heart from major
ventricles.
aorta and
veins, into atria,
pulmonary artery.
then ventricles.
Pressure in atria
Pressure in
Pressure in
rises gradually.
ventricles
ventricles
Pressure drops
increases. Atrial
decreases
when atria stop
pressure decreases
contracting.
Valves
AV valves closed at
AV valves close
SL valves close as
start. SL valves
when pressure in
pressure in
shut
venticles greater
ventricles drops
than in atria. SL
below that in
valves open when
aorta. AV open as
pressure in
pressure in atria
ventricles exceeds
greater than that
that in aorta.
of ventricles.
Table 1 Events occurring during the cardiac cycle
http://www.youtube.com/watch?v=D8LK34hoVpU&feature=related
http://www.youtube.com/watch?v=vGP08SEYdJc&feature=related
http://www.youtube.com/watch?v=nx-XRC_1n-Q&feature=related
http://www.youtube.com/watch?v=q0s-1MC1hcE&feature=related
http://www.youtube.com/watch?v=P_d0ykpzQgY&feature=related
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How heart valves work
The valves ensure that the blood flows in the correct direction. They are opened and
closed by changes in the blood pressure in the various chambers of the heart.
Atrioventricular valves
When the walls of the venticles relax and recoil following contraction, the pressure in
the ventricles falls below that of the atria. This causes the AV valves to open. Blood
entering the heart flows straight through the atria into the ventricles. The pressure in
both chambers slowly increases as they fill with blood. The valves remain open as the
atria contract, forcing more blood into the ventricles.
The pressure in the ventricles increases as the ventricles contract. This causes the
blood to push upwards into the valve pockets which closes the valves, preventing
backflow into the atria.
Figure 2 Action of the
atrioventriclular valves
Semilunar valves
When the ventricles start to contract, the pressure in the major arteries is higher than
the pressure in the ventricles. The semilunar valves are closed. The pressure rises as
the ventricles contract, and once the pressure is greater than in the arteries, the
semilunar valves open.
As the ventricles relax they return to their original size and the pressure drops rapidly.
When the pressure in the ventricles falls below that in the arteries, the blood flows
backwards towards the ventricles, closing the semilunar valves.
http://www.youtube.com/watch?v=D8LK34hoVpU&feature=related
http://www.youtube.com/watch?v=vGP08SEYdJc&feature=related
http://www.youtube.com/watch?v=nx-XRC_1n-Q&feature=related
http://www.youtube.com/watch?v=q0s-1MC1hcE&feature=related
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Hind Leys Biology
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Figure 3 The atrioventricular and semilunar valves during systole and diastole
The heart sounds
The lub-dub sound of the heart is made
by the heart valves closing.
 ‘lub’ AV valves closing
 ‘dub’ SL valves closing
The AV valves snap shut producing a
louder sound than when the SL valves
close due to the accumulation of blood
in their pockets.
1. Use the information and Figure 4
to complete Table 1.
2. Which chamber of the heart
produces the greatest pressure?
Figure 4 Pressure and volume changes in the
heart chambers and vessels, and ECG
activity during one heart beat
http://www.youtube.com/watch?v=D8LK34hoVpU&feature=related
http://www.youtube.com/watch?v=vGP08SEYdJc&feature=related
http://www.youtube.com/watch?v=nx-XRC_1n-Q&feature=related
http://www.youtube.com/watch?v=q0s-1MC1hcE&feature=related
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3. Use Figure 5 to answer the following questions.
c)
d)
e)
f)
a) Describe and explain what
happens to the pressure in the
atrium between the start of
the cycle and A. the pressure
rises as the muscles in the
atrium wall contract and force
blood through the valve into
ventricle. The pressure falls
again as the atrium empties.
b) The atrioventricular valve
closes at A. When does it open
again? Explain your answer.
The valve opens at D- curves
for pressure in ventricle and
atrium cross here, so pressure
on either side of valve is briefly the same. The pressure in the ventricle
then falls slightly below the atrial pressure, so blood flows again thru valve
into ventricle.
Soon after A, the pressure falls as the atrium expands after atrial systole.
Explain why the pressure then rises again until D. the pressure in the
pulmonary vein tho quite low, is pushing blood into the atrium and raising the
pressure in the atrium as it fills up.
Explain why the maximum pressure in the atrium is much lower than the
maximum pressure in the ventricle. There is less muscle in the walls of the
atria. They exert much less force when they contract compared with thick
ventricular walls.
Between which points on the curve is blood entering the aorta from the left
ventricle?. Blood enters the aorta between B and C. the valve opens at B
and closes again when the pressure in the ventricle falls below that in the
arota. Pressure in aorta stays high thru cycle. Has elastic walls which are
stretched during ventricular systole, exerting pressure on the blood.
Figure X shows the pressures on the left side of the heart. How would you
expect a similar graph showing the pressure on the right side to differ?
Explain your answer. The max pressure in the right ventricle is lower as
walls are thinner and exert less force. About ¼ of that of left side.
http://www.youtube.com/watch?v=D8LK34hoVpU&feature=related
http://www.youtube.com/watch?v=vGP08SEYdJc&feature=related
http://www.youtube.com/watch?v=nx-XRC_1n-Q&feature=related
http://www.youtube.com/watch?v=q0s-1MC1hcE&feature=related
http://www.youtube.com/watch?v=P_d0ykpzQgY&feature=related
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Pressure in pulmonary artery is much lower. However, overall pattern and
timing of events would be the same.
This work can be reinforced using pages 67-69 of your textbook.
http://anatimation.com/cardiac/intro.html
http://library.med.utah.edu/kw/pharm/hyper_heart1.html
http://library.med.utah.edu/kw/pharm/1Atrial_Systole.html
http://bcs.whfreeman.com/thelifewire/content/chp49/49020.html
http://www.cvphysiology.com/Heart%20Disease/HD002.htm
http://www.youtube.com/watch?v=D8LK34hoVpU&feature=related
http://www.youtube.com/watch?v=vGP08SEYdJc&feature=related
http://www.youtube.com/watch?v=nx-XRC_1n-Q&feature=related
http://www.youtube.com/watch?v=q0s-1MC1hcE&feature=related
http://www.youtube.com/watch?v=P_d0ykpzQgY&feature=related
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Hind Leys Biology
http://www.youtube.com/watch?v=D8LK34hoVpU&feature=related
http://www.youtube.com/watch?v=vGP08SEYdJc&feature=related
http://www.youtube.com/watch?v=nx-XRC_1n-Q&feature=related
http://www.youtube.com/watch?v=q0s-1MC1hcE&feature=related
http://www.youtube.com/watch?v=P_d0ykpzQgY&feature=related
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Transport in animals 5.3
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