The Cardiac Cycle
• describe the cardiac cycle, with reference
to the action of the valves in the heart;
• explain how heart action is coordinated
with reference to the sinoatrial node
(SAN), the atrioventricular node (AVN) and
the Purkyne tissue;
Exam tip
• When referring to the bicuspid (on the left)
and tricuspid (on the right) valves, use the
word atrioventricular valves- that way, you
don’t have to remember which is which!
Sequence of Contraction
• The sequence of events involved in one
heartbeat is called the cardiac cycle
• It is important that the chambers of the
heart contract in a co-ordinated fashion, if
not, this leads to inefficient pumping
Task
• Using this diagram,
outline the main
stages of diastole,
atrial systole and
ventricular systole
during the cardiac
cycle
Phase 1: Diastole
• Both the atria and ventricles
are relaxing
• the internal volume
increases and blood flows
into the heart from the major
veins
•The blood flows first into the
atria, then through the
atrioventricular valves into the
ventricles
Phase 2: Atrial Systole
• The atria contract- right and
left together
• The small increase in
pressure pushes blood into
the ventricles
• This stretches the walls of
the ventricles and ensures
they are full of blood
Phase 3: Ventricular Systole
• Once the ventricles are full they begin to
contract
• Blood fills the atrioventricular valve flaps
causing them to snap shut preventing
blood returning to the atria
• There is now a short period where all 4
valves are closed
• The walls of the ventricles continue to
contract raising the pressure
• The contraction starts at the base of the
ventricles pushing blood upwards towards
the arteries
• The semi-lunar valves open and blood is
pushed out of the heart
•The ventricles walls then relax and the
cycle begins again
Valves
• The valves ensure blood flows in the
correct direction
• They are opened and closed by changes
in the blood pressure in the various
changes in the heart
Task: You will be given a series of statements about the blood pressure and
valves, arrange them in a sequence and stick them in your book
Atrioventricular Valves
• When the ventricular walls relax and recoil after contracting, the
pressure in the ventricles drops below the pressure of the atria- this
causes the atrioventricular valves to open
• Blood entering the heart flows straight through the atria and into the
ventricles
• The pressure in the atria and the ventricles slowly rises as they fill
with blood
• The valves remain open while the atria contract
• As the ventricles begin to contract, the pressure of the blood in the
ventricles rises. When the pressure rises above that in the atria, the
blood starts to move upward.
• This movement fills the valve pockets and keeps them closed
• This prevents blood flowing back into the atria
Semilunar Valves
•
When the ventricles start to contract, the pressure in the major arteries is higher than the
pressure in the ventricles- this means the semilunar valves are closed
•
As the ventricles contract, the pressure rises very quickly because the blood cannot
escape
•
Once the pressure in the ventricles rises above the pressure in the aorta and pulmonary
arteries, the semilunar valves are pushed open
•
The blood is under very high pressure, so it is forced out of the ventricles in a powerful
spurt
•
Once the ventricle walls have finished contracting, the heart muscles start to relax
•
Elastic tissue in the walls of the ventricles recoils to stretch the muscle out again and
return the ventricle to its original size
•
This causes the pressure in the ventricle to drop quickly
•
As it drops below the pressure in the major arteries, the semilunar valves are pushed
closed by blood starting to flow back towards the ventricles and collecting in the pockets
of the valves
•
This prevents blood returning to the ventricles
Control of Heart Beat
1. What is the patch of muscle called that
controls the heart beat? (2)
2. Name the 2 nerves that carry impulses
from the brain to the heart (2)
3. How do acetylcholine and noradrenaline
affect the heart? (5)
4. What does digoxin do to the heart? (3)
5. What does propanolol do to the heart? (3)
Control of Heart Beat
1.
2.
3.
4.
5.
What is the patch of muscle called that controls the heart beat? (2)
pacemaker (1) or sino-atrial node (1)
Name the 2 nerves that carry impulses from the brain to the heart
(2) vagus nerve (1) sympathetic nerve (1)
How do acetylcholine and noradrenaline affect the heart? (5)
acetylcholine released from the vagus nerve (1) slots into
receptors on the plasma membrane of the heart cells (1) this
makes the heart beat more slowly (1) noradrenaline released from
the sympathetic nerve (1) makes the heart cells beat more rapidly
(1)
What does digoxin do to the heart? (3) inhibits and slows the Na-K
pump in the plasma membrane (1) sodium ions accumulate in the
cells (1) this increases calcium ions which increases the force of
muscle contraction (1)
What does propanolol do to the heart? (3) a beta blocker (1) which
decreases the effect of noradrenaline on the SAN (1) reducing
heart rate (1)
Pressure Practice Questions
• Try to answer SAQ Q5 on p67
• Leave any you cannot do blank
• Remember to answer in full sentences to
help with revision