The document describes the stages of the cardiac cycle and how blood flows through the heart due to pressure changes. It discusses: - There are four stages: atrial systole, ventricular systole, diastole. - Valves between chambers open and close based on pressure differences, ensuring blood flows in one direction through the heart. - As chambers contract and relax, pressure within them rises and falls, pushing blood into the next chamber or vessel and causing valves to open and close.

1. The cardiac cycle
WAL: about the sequence of events in one
heart beat
All
Most
Some
• What are the stages in the cardiac cycle?
• How are the pressures in the
heart related to the different stages
in the cycle?
• How do valves control the flow of blood
through the heart?

2. Today we are covering from the
specification:

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Preventing backflow
Blood always flows in the same direction as it moves
through the heart during each circulation of the body.
Why is it important that blood does not flow backwards?

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The chambers of the heart are separated by valves
which prevent blood from flowing in the wrong direction.
Heart valves
There are valves between the atria and the ventricles…
…and there are valves leading out of the ventricles.
valve between
right atrium and
right ventricle
valve between
left atrium and
left ventricle
valve leading
out of
right ventricle
valve leading
out of
left ventricle

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Naming the heart valves

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How are valves held in place?
The valves between the atria and ventricles are connected
to the inner walls of the heart by tough tendons.
valve open

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How are valves held in place?
The tendons allow the valves to close and hold the valve
flaps in place. They prevent the valves from flipping up
and turning inside out. Why is this important?
valve open valve closed

8. Definitions
Systole = period of ventricular contraction.
Diastole = period of ventricular relaxation.
NOTE: Normally diastole is longer than
systole.

9. Cardiac cycle
 General Principles.
 Contraction of the myocardium generates
pressure changes which result in the orderly
movement of blood.
 Blood flows from an area of high pressure to an
area of low pressure, unless flow is blocked by
a valve.
 Events on the right and left sides of the heart are
the same, but pressures are lower on the right.

10. As a chamber fills with blood, the pressure
is going to rise. When a chamber
contracts, the pressure is going to rise.
Changes in pressure affect whether a
valve is open or closed.
Fluids always move from areas of high
pressure to areas of low pressure.

11. Let us think through the cardiac cycle in
terms of pressure:
As the blood passes into the atria, the
valves are open so most will fall
immediately into the ventricle.
There is a gradual rise in pressure in the
atria until the end of atrial systole when the
blood has moved into the ventricles.

12. The intraventricular pressure rises as the
ventricles fill with blood. This closes the
AV valves.
Contraction of the ventricles means that
the intraventricular pressure is higher than
the pressure in the artery which forces the
blood out of the ventricle and into the aorta
or pulmonary artery (depending on which
side of the heart you’re looking at).

13. The increase in pressure of the artery
causes the closing of the semilunar valves
preventing the back flow of blood into the
ventricle.

14. Heartbeat animation

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Cardiac output
The amount of blood pumped around the body is called the
cardiac output, and depends on two factors:
 the heart rate – the number of times the heart beats per
minute. A typical value for an adult at rest is 70bpm.
 the stroke volume – the volume of blood pumped by
the left ventricle in each heart beat. A typical value for an
adult at rest is 75ml.
cardiac output = stroke volume × heart rate
A typical resting cardiac output is 4–6 litres per minute.
This can rise to as much as 40 litres per minute in highly
trained endurance athletes.

16. Atrial systole
 The heart is full of blood
and the ventricles are
relaxed
 Both the atria contract
and blood passes down
to the ventricles
 The atrio-ventricular
valves open due to blood
pressure
 70% of the blood flows
passively down to the
ventricles so the atria do
not have to contract a
great amount.

17. Ventricular systole
 The atria relax.
 The ventricle walls
contract, forcing the
blood out
 The pressure of the
blood forces the atrio-
ventricular valves to
shut (producing the
heart sound ‘lub’)

18. Ventricular systole
The pressure of
blood opens the
semi-lunar valves.
Blood passes into
the aorta and
pulmonary arteries.

19. Diastole
 The ventricles relax
 Pressure in the ventricles
falls below that in the
arteries
 Blood under high
pressure in the arteries
causes the semi lunar
valves to shut. This
produces the second
heart sound, ‘dub’.
 During diastole, all the
muscle in the heart
relaxes.

20.  Blood from the vena
cava and pulmonary
veins enter the atria.
 The whole cycle
starts again.

22. 5 mins – try it yourself!
Match the letter on the graph to the following
events
Semi-lunar valves open
Atrio-ventricular valves close,
Semi-lunar valves close
Atrio-ventricular valves open
Pressure in
ventricle higher
than in atria – shuts
atrioventricular
valves to prevent
backflow
A

23. Pressure in
ventricle higher
than in atria – shuts
atrioventricular
valves to prevent
backflow
Pressure in
ventricle higher
than in aorta–
opens semi lunar
valves
Pressure in aorta
higher than
ventricles – shuts
semi lunar valves
Pressure in atria
higher than in
ventricles –
atrioventricular
valves open

24. Activities
1. Card sort – put the statements into the
correct order to show the events that
occur during the cardiac cycle.
2. Try the questions on your worksheet.
You may wish to use the textbook to help
you interpret the information on the
graph.