5.2 The cardiac cycle
Learning outcomes
Students should understand the following:
 Myogenic stimulation of the heart and transmission of a
subsequent wave of electrical activity.
 Roles of the sinoatrial node (SAN), atrioventricular node
(AVN) and bundle of His.
 Cardiac output as the product of heart rate and stroke
volume.
 Pressure and volume changes and associated valve
movements during the cardiac cycle.
Candidates should be able to analyse and interpret data
relating to pressure and volume changes during the cardiac
cycle.
The heart
 Heart muscle does not need to be stimulated by a
nerve before it will contract. The heart beat
originates in the muscle itself and, for this reason,
it is described as being myogenic.
 There are 2 phases to the beating of the heart:
 Contraction (systole)
 Relaxation (diastole)
The Cardiac Cycle
 Cardiac cycle = sequence of events that makes up one
heart beat
 Continuous but can be divided into 3 stages:
 Atrial systole
 Ventricular systole
 Ventricular diastole
1. Atrial Systole
• Heart is full of
blood
• Contraction of
both atria
• AV valves open
• blood flows 
ventricles
• No backflow as
veins have valves
2. Ventricular Systole
• Atria relax
• Ventricles contract (0.1s
after atria)
• Increase in pressure in
ventricle
– AV valves close
– Semi-lunar valves
open
– blood flows from
ventricles  arteries
• Lasts ~0.3 s
3. Ventricular Diastole
• Ventricles relax
• Blood flows from veins
(at low pressure)  atria
• Some blood flows
through atria 
ventricles
Atrial Systole
The walls of the atria
contract. This reduces the
volume of the atria,
increasing the pressure.
More blood is forced through
the atrio-ventricular valves
into the ventricles.
Ventricular Diastole
Ventricular Systole
The ventricle walls relax and
the pressure in the ventricles
falls. Blood starts to flow
from the atria into the
ventricles again.
The walls of the ventricles
now contract, reducing the
volume in the ventricles. The
pressure increases and blood
is forced into the arteries.
Cardiac cycle
 Cardiac cycle - narrated.swf
Cardiac Cycle: Summary
Control of Heartbeat
 Cardiac muscle is
myogenic – naturally
contracts & doesn’t
need to receive
impulses
 Need to coordinate
contractions with
pacemaker = sinoatrial node (SAN)
Control of heartbeat
 Cardiac cycle - SAN & AVN.swf
The Sino-atrial Node
• Patch of specialised
muscle in wall of
right atrium
• Sets rhythm for rest
of heart
• The SAN has its own
rhythm, but this may
be modified by nerve
impulses from the
brain.
The Sino-atrial Node
• Contraction of SAN
sends excitation
wave (wave of
depolarisation)
through atrial walls
• Cardiac muscle in
atrial walls contracts
at same rhythm
• Atria contact
simultaneously
The Atrio-ventricular Node
• Ventricles don’t contract
until atria have finished
contracting.
• Delay due to band of fibres
between atria & ventricles
that does not conduct
excitation wave
• Wave can only pass
through patch of
conducting fibres at top of
septum = atrio-ventricular
node (AVN).
Purkyne Fibres
• AVN picks up excitation wave and
the signal now passes rapidly
down the specialised conducting
fibres (Purkyne fibres) which form
the bundle of His in the wall or
septum separating the two
ventricles.
• wave spreads from base outwards
& upwards
• ventricles contract from bottom up
• squeezes blood upwards into
arteries
Cardiac cycle overview
Ventricular
diastole
Atrial systole
Ventricular
systole
Control of heartbeat
 Cardiac cycle - SAN & AVN.swf