The document describes the cardiac cycle, which is the sequence of events in one heartbeat. It defines systole as the period of ventricular contraction and diastole as the period of ventricular relaxation. The cycle begins with atrial systole, where the atria contract and blood passes to the relaxed ventricles. This is followed by ventricular systole, where the ventricles contract and force blood out through the arteries. Finally, during diastole the ventricles relax and refill with blood from the veins, completing the cycle.

1. The cardiac cycle
Describing the sequence of
events in one heart beat

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

3. 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.

4. 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.

5. Ventricular systole
 The atria relax.
 The ventricle walls
contract, forcing the
blood out
 The pressure of the
blood forces the atrioventricular valves to
shut (producing the
heart sound ‘lub’)

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

7. 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.

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

10. Cardiac cycle
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

11. atrio-ventricular valves open

12. atrio-ventricular valves close
atrio-ventricular valves open

13. semi-lunar valves open
atrio-ventricular valves close
atrio-ventricular valves open

14. semi-lunar valves open
semi-lunar valves close
atrio-ventricular valves close
atrio-ventricular valves open