Download The Cardiac Cycle

The Cardiac Cycle
Waves of excitation from the sino-atrial node (SAN) start a series of changes that make up the cardiac cycle.
Imagine that the individual drawings in Figure 1 are still pictures from a film that shows the heart beating.
The small arrows in the heart show where the blood is flowing at each stage.
Figure
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aorta
2
left atrium
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left
ventricle
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4
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Task 1
Work in pairs
Arrange the cards in Envelope 1 to show the pathway taken by blood as it travels through
the heart. Check your answer with a text book or with your tutor.
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Imagine pressure sensors are placed into the left ventricle, the left atrium and the aorta.
These sensors record the changes in blood pressure as the heart beats and relaxes.
Work in groups of 3 or 4 through the following tasks
Task 2
You are provided with three acetate sheets and a grid marked in pressure units (kPa).
Acetate sheet 1 shows the changes in pressure that occur in the left atrium during one
cardiac cycle.
Follow the pressure changes in the left atrium and answer these questions.
1. What is the lowest blood pressure and at what time does it occur in the cycle?
2. What is the highest blood pressure and at what time does it occur in the cycle?
3. State the time when the left atrium starts to contract and the time when it stops
contracting?
4. Where does the blood go after leaving the left atrium?
Task 3
Acetate sheet 2 shows the changes in pressure that occur in the left ventricle during one
cardiac cycle.
Follow the pressure changes in the left ventricle and answer these questions.
5. What is the lowest blood pressure and at what time does it occur in the cycle?
6. What is the highest blood pressure and at what time does it occur in the cycle?
7. State the time when the left ventricle starts to contract and the time when it stops
contracting?
8. Where does the blood go after leaving the left ventricle?
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Task 4
Acetate sheet 3 shows the changes in pressure that occur in the aorta during one cardiac
cycle.
Follow the pressure changes in the aorta and answer these questions.
9. What is the lowest blood pressure and at what time does it occur in the cycle?
10. What is the highest blood pressure and at what time does it occur in the cycle?
11. Where does the blood go after leaving the aorta?
Task 5
Now overlay all three acetate sheets.
12. State the time when the pressure in the left ventricle first becomes greater than the
pressure in the left atrium.
13. What prevents the blood travelling back into the left atrium at this time? If you want
help with this question look at a diagram or a model of the heart.
14. State the time when the pressure in the left ventricle first becomes greater than the
pressure in the aorta.
15. What happens in the heart to allow the blood to leave the left ventricle and enter the
aorta?
16. State the time when the pressure in the ventricle falls so that it becomes less than
the pressure in the aorta.
17. What prevents the blood flowing back into the left ventricle at this time?
18. State the time when the pressure in the left ventricle falls so that it becomes less
than the pressure in the left atrium.
19. What happens to the flow of blood when the pressure in the left ventricle is lower
than that in the left atrium?
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Figure 2 shows the changes in blood pressure in the left atrium, left ventricle and in the
aorta. This should be similar to the graph you have made by overlaying the three acetate
sheets.
Figure
2
blood
pressure /
kPa
time / s
How many beats of the heart are shown in Figure 2?
If the heart continues to beat at the same rate, what is the heart rate in beats per minute?
Using the information from tasks 2 to 5, identify the three lines drawn on the Figure.
The points numbered 1 to 4 represent times during the cardiac cycle when valves open
and close.
Complete the table below using the numbers 1 to 4.
valve
opens
closes
atrio-ventricular valve
semi-lunar valves
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Task 6
Work in groups of 3 or 4
Envelope 2 contains a set of cards giving the stages of the cardiac cycle. Arrange the
cards in the correct sequence so that they form a cycle.
Use the answer sheet to check the sequence you have made.
Task 7
Extension exercise
Read the following passage
Cardiac muscle cells, unlike other muscle cells, are described as myogenic as they have
the intrinsic ability to contract without nervous stimulation, provided they have a
supply of the raw materials for the production of ATP.
In order for the heart beat to be regular, the contractions of the cardiac muscle
cells must be controlled and co-ordinated. This is the function of a specialised region
of the heart known as the sino-atrial node (SAN) or pacemaker. It is situated in the
wall of the right atrium near to where the vena cava enters. It consists of specialised
muscle tissue, which not only has the ability to contract, but also is capable of
generating waves of excitation that spread throughout the heart. Cardiac cells are
branched and joined by intercalated discs that convey the wave of excitation to the
next cell.
The septum between the atria and the ventricles is non-conducting. Here the waves of
excitation are picked up by another group of specialised muscle cells called the atrioventricular node (AVN). This leads to a small delay so that the atria empty fully
before the ventricles contract. The waves of excitation continue down to the tips of
the ventricles and then up through their side walls via the Purkinje fibres.
You are provided with three blank cards.
Use the passage above, to write a statement describing the activities of each of these three
structures and incorporate the cards into the cycle you have made in Task 6.
Sino-atrial node
Atrio-ventricular node
Purkinje fibres
Weblink: you can find an animation of the cardiac cycle at
http://medlib.med.utah.edu/kw/pharm/hyper_heart1.html
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