© SSER Ltd.
Lesson Aims
• To examine the cardiac cycle
• To p12 “the pressure gets less and less…”
At the end of atrial
systole, the volume of
blood in the ventricles
is such that the AV
valves are forced closed
There is now a situation
where the AV and the
semilunar valves are
both closed, the atria
are relaxed and the
ventricles enter a
phase of contraction
or SYSTOLE
The ventricles are now
contracting but the sheer
load of blood on the
ventricle muscles prevents
them from shortening
Ventricular systole is taking
place but the muscle cells are
unable to change in length
When muscles contract without
changing in length, this is known
as isometric contraction
Muscle contraction involving the
shortening of muscle fibres is
known as isotonic contraction
The ventricles are
undergoing isometric
contraction with the
volume of blood in the
ventricles remaining
constant and the pressure
building up
When the rising pressure
exceeds that in the aorta
and pulmonary arteries,
the semilunar valves are
forced open and blood is
ejected from the heart as the
ventricle muscles shorten
This phase of the heartbeat
is known as
This phase of the heartbeat is known as
VENTRICULAR
ISOMETRIC VENTRICULAR CONTRACTION
EJECTION
To summarise:
Ventricular Systole involves two phases
Isometric ventricular contraction when;
• the ventricle muscles contract without
changing in length
• the volume of blood in the ventricles
remains constant
• pressure builds up in the ventricles
Ventricular ejection when;
• the semilunar valves are forced open
• the ventricle muscles contract in an
isotonic fashion (they shorten)
• blood is ejected into the aorta and
pulmonary arteries
At the end of ventricular systole, the ventricles begin to relax and the
pressures drop below those in the aorta and pulmonary arteries
A brief backflow of blood in the arteries closes the semilunar valves
As the ventricles relax, closure of the semilunar valves occurs due to a
brief backflow of blood from the aorta and pulmonary arteries
The pressures in the
ventricles continue to fall
and reach very low
values
When the pressures
in the ventricles fall
below those of the
atria, the AV valves
open
This phase of the heartbeat
is known as
VENTRICULAR
RELAXATION
(Early Diastole)
As the ventricles relax, closure of the semilunar valves occurs due to a
brief backflow of blood from the aorta and pulmonary arteries
The pressures in the
ventricles continue to fall
and reach very low
values
This phase of the heartbeat
is known as
VENTRICULAR
RELAXATION
(Early Diastole)
When the pressures
in the ventricles fall
below those of the
atria, the AV valves
open
Passive filling of the ventricles (late diastole) occurs
as the cycle begins again
Late Diastole &
Atrial Systole
Ventricular Systole
(Isometric Phase)
AV valves close
at the end of
atrial systole
All valves closed
Passive filling
as the ventricle
of the ventricles
muscles contract
followed by
without shortening
atrial systole
(Isometric
Contraction)
Pressure builds
up in the
ventricles
Ventricular Systole
(Ejection)
Semi-lunar valves
Semi-lunar valves
close as the ventricles
open and blood
begin to relax
is ejected into
the aorta and
Pressure in the
pulmonary artery
ventricles falls
to a very low
Muscles shorten
value and the
as they contract
AV valves open
AV valves open,
semi-lunar valves
closed
Ventricular
Relaxation
Throughout the cardiac cycle, pressure changes take place
in the atria, ventricles and arteries
Pressures in the right and left atrium, right and left ventricle, aorta
and pulmonary arteries can be recorded and illustrated in
graphical form
The graph on the next slide shows pressure changes in the left side of
the heart and the aorta
A similar graph can be drawn for the right side of the heart and the
pulmonary arteries
Such a graph is similar in shape to that obtained for the left side
of the heart but all the pressures readings are of a lower value
Pressure Changes in the
Left Side of the Heart
During One Cardiac
Cycle
120
aortic
pressure
pressure (mm Hg)
100
The pressure changes
in the left ventricle,
left atrium and aorta
can be related to the
phases of the cardiac
cycle
80
left ventricular
pressure
60
40
20
left atrial
pressure
0
0
0.1
0.2
0.3
0.4
time (s)
0.5
0.6
0.7
0.8
Pressure Changes in the
Left Side of the Heart
A
WX
Y
A
Z
120
aortic
pressure
pressure (mm Hg)
Period Z to A represents
the phase of Passive Filling
of the ventricles when the
AV valves are open and
100
the semi-lunar valves are closed
Period A to W represents
the phase of Atrial Systole
when the atria contract
80
and the ventricles are
filled to full capacity
Period W to X represents
the first phase of
60
Ventricular Systole when
the ventricles contract in an
isometric fashion; the
greatest rise in ventricular 40
pressure occurs during this
phase and the ventricular
volume remains constant
Period X to Y represents the 20
second phase of Ventricular
Systole when ejection of blood
takes place and pressure in the
aorta rises
0
Period Y to Z represents relaxation 0
of the ventricles (diastole) when the
ventricular pressure drops sharply
left ventricular
pressure
left atrial
pressure
0.1
0.2
0.3
0.4
time (s)
0.5
0.6
0.7
0.8
The facts you need to know
• From p11 “systole is the term used…”
• To p12 “the pressure gets less and less…”