1. © SSER Ltd.

2. heart receives oxygenated blood from the lungs via the
Mammalian Heart Structure
The left side of the
heart receives oxygenated
blood from the lungs via the
PULMONARY VEINS
The heart is the major
organ of the circulatory system
Superior
VENA
CAVA
Inferior
AORTA
It is a fist-sized muscular
pump consisting of
four chambers
The left side of the
heart pumps oxygenated
blood out into
the body’s arteries via
the AORTA
The human heart
recirculates the entire
blood volume (5 dm3)
every minute when
the body is at rest
PULMONARY
ARTERY
Deoxygenated blood
returns to the right side
of the heart via the
VENA CAVA
The ability of the
heart to perform such
work is due to the
presence of specialised
cardiac muscle in
its walls
CORONARY
ARTERIES
Deoxygenated blood
is pumped to the
lungs via the
PULMONARY
ARTERY
The job of the heart is to
pump blood around two
separate circuits
Heart muscle receives
its own supply of blood
from the CORONARY
ARTERIES

3. Mammalian Heart Structure
Aorta
Pulmonary artery
Vena cavae
Pulmonary veins
Semilunar valves
Left atrium
Right atrium
Bicuspid valve
Tricuspid valve
Right ventricle
Left ventricle
Septum
(dividing wall)
Cardiac muscle

4. Mammalian Heart Structure
The mammalian heart is
a muscular pump that consists
of four chambers
Two upper chambers, called the
atria, are thin walled cavities that
receive blood from veins
Two lower chambers, called the
ventricles, are thick walled cavities
that receive blood from the atria and
pump blood away from the heart
The cavity of the heart is
divided completely by a
partition called the
SEPTUM
Right
atrium
Left
The muscular walls of the
heart are referred to as the
myometrium and consist of
specialised cardiac muscle
cells
Right
ventricle
Left
Septum
The thicker walled structure of the left
ventricle is a consequence of the distance
over which it is required to pump blood

5. The direction of blood flow through the heart is maintained be valves
Between the right atrium and
the right ventricle is the
TRICUSPID VALVE
Right
atrium
Left
ventricle
Aorta
This valve prevents the backflow
of blood from the right ventricle
to the right atrium
Pulmonary
Artery
Between the left atrium and
the left ventricle is the
BICSUPID VALVE OR
MITRAL VALVE
This valve prevents the
backflow of blood from
the left ventricle to
the left atrium
Semilunar
valves
Bicuspid
or Mitral
valve
The bicuspid and tricuspid
valves are collectively
known as the
ATRIO-VENTRICULAR
VALVES or AV valves
Tricuspid
valve
Pocket-shapes valves known
as SEMILUNAR VALVES are
located at the base of the arteries
responsible for transporting
blood away from the heart

6. The average human heart rate at rest is 72 beats a minute
The Mammalian Heart
The average human heart rate at rest is 72 beats a minute
Each heart beat lasts for approximately
0.8 of a second at rest
Each heart beat involves a series of events
referred to as THE CARDIAC CYCLE

7. THE CARDIAC CYCLE is the sequence of events taking place during
ONE COMPLETE HEARTBEAT
A single heartbeat may be divided into two major phases known as
SYSTOLE AND DIASTOLE
The
Cardiac Cycle
Systole describes periods when the heart is contracting
Diastole describes periods when the heart is relaxing

8. Pressure Changes during the Cardiac Cycle
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

9. aortic pressure left ventricular left atrial
Pressure Changes in the
Left Side of the Heart W A X Y A Z
Period Z to A represents
the phase of Passive Filling
of the ventricles when the
AV valves are open and
the semi-lunar valves are closed
aortic
pressure
left ventricular
left atrial
Period A to W represents
the phase of Atrial Systole
when the atria contract
and the ventricles are
filled to full capacity
Period W to X represents
the first phase of
Ventricular Systole when
the ventricles contract in an
isometric fashion; the
greatest rise in ventricular
pressure occurs during this
phase and the ventricular
volume remains constant
Period X to Y represents the
second phase of Ventricular
Systole when ejection of blood
takes place and pressure in the
aorta rises
Period Y to Z represents relaxation
of the ventricles (diastole) when the
ventricular pressure drops sharply

10. Period Z to A represents the phase
of Late Diastole when all chambers of
the heart are relaxed, atrial and
ventricular pressures are low and
the aortic pressure is falling
The AV valves open at the
beginning of this phase and
the semi-lunar valves are already
closed
Passive filling of the
ventricles takes place
during this phase

11. Period A to W (Atrial Systole) begins
as the atria contract filling the
ventricles to their full capacity
Both the atrial and ventricular
pressure curves rise slightly
at this time, as additional blood
is forced into the left ventricle
At the end of atrial systole,
the increased blood pressure
in the ventricles forces
the AV valves to close

12. Period W to X (the first phase of Ventricular Systole) begins
as the ventricles contract in an
isometric manner
Both the AV and semi-lunar valves
are closed and the steeply rising
pressure in the left ventricle
reflects the increasing muscle
tension created by the
ventricular muscles
No blood enters or leaves the ventricles during this phase and the aortic semi-lunar valve is forced open at the end of this phase

13. Period X to Y represents the second phase of Ventricular Systole when
blood is ejected from the left ventricle
As the semi-lunar valves open, blood
is ejected into the aorta and
pulmonary arteries (right side)
The ventricular and aortic pressures
are the same throughout this
period and both pressures
reach their highest value
(around 120 mm Hg)

14. Period Y to Z represents the phase
of Ventricular Relaxation (Diastole)
Backflow of blood from the aorta
closes the semi-lunar valve at the
beginning of this phase
The ventricular pressure drops
sharply as the ventricle relaxes
and the ventricular blood volume
remains constant as the AV
and semi-lunar valves are
both shut
The AV valve opens at the END of this phase as the atrial pressure is slightly greater than that in the ventricle

15. As the AV valve opens, passive filling of the ventricle starts
during phase Z to A and the
cycle begins again

16. Summary of Valve Movements during the Cardiac Cycle W A X Y A Z aortic
pressure
left ventricular
left atrial
The AV bicuspid valve opens
at the beginning of
Phase Z to A
(Passive filling of the ventricles)
Aortic semi-
lunar valve
closes
The AV valve opens as
the pressure in the atrium
is slightly greater than
that in the ventricle
Aortic semi-
lunar valve
opens
The AV valve closes at the
end of atrial systole
(Period A to W)
The aortic semi-lunar valve
opens at the end of
Isometric Ventricular Systole
following a steep rise in
pressure in the left ventricle
AV bicuspid
valve opens
The aortic semi-lunar
valve closes at the end of
Ventricular Ejection due
to a slight backflow of
blood from the aorta
AV bicuspid
valve closes
As the ventricle relaxes during
diastole, the pressure falls
slightly below that of the
atrium and the AV bicuspid
valve opens again

17. Ventricular Volume during the Cardiac Cycle
relaxation
(diastole)
Ventricular Systole
(isometric phase)
Atrial
systole
Ventricular
ejection
(systole)
Passive
filling
(late diastole)
100%
70% 0%
% Capacity
Time (seconds)
Ventricular volume
decreases sharply
as blood is
ejected into
the arteries
Ventricular volume rises
sharply and then levels
off as ventricles fill to
70% of their capacity
Ventricular
volume
rises
sharply as
ventricles
fill to
capacity
Ventricular volume
remains constant as
all valves are closed
Ventricular volume
remains constant as
all valves are closed

18. The Origin of the Heartbeat
The mammalian heart is special in that the electrical stimulation
necessary for contraction of its muscles originates from within the heart itself
Within the heart there is a network of specialised cardiac muscle cells
designed for initiating each heart beat and for the rapid and
co-ordinated spread of excitation
This network of specialised cardiac muscle cells is known as
THE CONDUCTION SYSTEM
This conduction consists of:
the Sino-atrial node (known as the SA node or pacemaker)
the Atrio-ventricular node or AV node
the Bundle of His
conduction fibres called Purkinje fibres
As the stimulus for contraction of the heart originates from within
cardiac muscle, the heartbeat is described as being
MYOGENIC

19. The Conduction System of the Heart
The origin of the heartbeat is
from within a specialised patch
of cardiac muscle tissue, located
in the wall of the right atrium,
and known as the sino-atrial
node or SA node
Another node of
specialised tissue known
as the AV node is
located in the right
portion of the septum
between the atria
and close to the
AV valves
AV node
SA node
in wall of
right atrium
Bundle of His
with left and right
bundle branches
The left and
right bundle
branches divide
into smaller
branches
called Purkinje
fibres that
spread
throughout the
ventricular muscle
The AV node connects
with a bundle of large
fibres called the bundle
of His, which divides into
left and right bundle
branches

20. The Conduction System of the Heart
When the SA node emits spontaneous
electrical impulses, they spread rapidly
across both atria due to the inter-connecting
nature of the cardiac muscle cells
As the impulses spread
across the atria, they
stimulate a wave of
contraction within the
atrial walls and
atrial systole is
triggered
When the electrical
impulses reach the
border between the
atria and ventricles
they are blocked by
a band of non-
conducting
fibrous tissue
Fibrous Tissue
Impulses are
conducted from
AV node along
the bundle of His AV
Node
In order to reach
the ventricles,
electrical impulses
must pass through
the AV node, which
slows down the
speed of electrical
transmission
The bundle fibres
divide into
numerous
Purkinje fibres
that permeate
throughout the
ventricular
muscles
This delay, called the AV delay,
is extremely important as it
allows the atria to complete their
contraction before the ventricles
begin to contract
The spread of
electrical impulses
throughout the ventricles triggers ventricular systole

21. The Electrocardiogram (ECG)
The contraction of muscles is associated with electrical changes called
‘depolarisation’, and these changes can be detected by electrodes attached
to the surface of the body
When the electrical changes associated with cardiac muscle contraction
are inscribed on a ruled strip of paper, they provide an electrocardiogram
that is a permanent record of cardiac activity
In order to understand the ECG trace, it is necessary to consider the electrical
properties of cardiac muscle

22. Electrical Properties of Cardiac Muscle
During diastole, when cardiac muscle cells are at rest, they display an
unequal distribution of charge across the membrane
At rest, cardiac muscle cells are internally negative and externally positive
Cardiac Muscle Cell
Intercalated
Disc
This charge difference creates a small voltage across the membrane, which
can be detected by electrodes across the chest wall
When cardiac muscle cells are at rest and therefore internally negative and
externally positive, they are described as being POLARISED

23. Electrical Properties of Cardiac Muscle
When a stimulus from the SA node spreads along the cardiac muscle cell
membranes, there is a reversal of the charge distribution
Stimulus from
SA node
Cardiac Muscle Cell
Intercalated
Disc
The muscle cell is now internally positive and externally negative and the
cell is described as being DEPOLARISED
Depolarisation stimulates the cardiac muscle to contract
Depolarisation results in a voltage change across the membrane and this is
detected by electrodes applied to the chest

24. Electrical Properties of Cardiac Muscle
Cardiac muscle cells form an interconnected network
The stimulus originating from the SA node spreads from cell to cell
creating a wave of depolarisation throughout the muscle network
Depolarised Cardiac Muscle Cell
Depolarisation results in muscle contraction and thus
a wave of contraction spreads throughout the network

25. Electrical Properties of Cardiac Muscle
As the stimulus dies away, the muscle cells return to their
POLARISED STATE and relax
This event is termed REPOLARISATION
The ECG electrodes detect the waves of depolarisation and
repolarisation occurring during the cardiac cycle and the ECG trace is a record of these events

26. The ECG Trace R T wave P wave Q S
The ECG trace for each heartbeat displays a P wave, a QRS wave or complex and a T wave

27. The ECG Trace R T wave P wave Q S
The P wave is the result of depolarisation spreading across the atria from the
SA node; it coincides with atrial contraction or systole
The QRS wave or complex is the result of depolarisation of the ventricles and
coincides with ventricular systole
The T wave is the result of repolarisation of the ventricles as the ventricles
begin to relax; repolarisation of the atria is not detected as the small voltage
changes involved are masked by the QRS wave

28. The ECG Trace P – R interval R T wave P wave T – P interval Q S
The P – R interval is the time, which elapses between the events of atrial
systole and ventricular systole
This period represents the time taken for the impulse to spread from the SA node
through the atria, plus the delay in transmission to the AV node, together
with the conduction time through the bundle of His and Purkinje fibres
The T – P interval is the time spent by the heart in diastole before the next
atrial systole begins

29. Control of the Heart Rate
Although the origin and transmission of the heartbeat are
properties of the heart itself, it is necessary for the heart rate
to be modified to meet the different demands of the body
The heart rate is regulated by both the nervous
and hormonal systems of the body
The autonomic nervous system is responsible
for the regulation of the heart rate
The autonomic nervous system has two divisions, i.e. the
sympathetic and parasympathetic nervous systems

30. Control of the Heart Rate
AUTONOMIC NERVOUS SYSTEM
SYMPATHETIC NERVOUS
SYSTEM
PARASYMPATHETIC NERVOUS
Sympathetic nerves release
the neurotransmitter noradrenaline
at their terminals
Parasympathetic nerves release
the neurotransmitter acetylcholine
at their terminals
The heart is supplied with both sympathetic and parasympathetic nerves
and the chemicals that they secrete modify the heart rate

31. Control of the Heart Rate
Two autonomic nerves link the
cardiovascular centre in the brain
with the SA node of the heart
A sympathetic nerve, when
stimulated, releases noradrenaline
at its terminus with the SA node
and this chemical speeds the
heart rate
This parasympathetic
nerve is a branch of
the vagus nerve
A parasympathetic nerve, when
stimulated, releases acetylcholine
at its terminus with the SA node
and this chemical slows the
heart rate
The heart rate is therefore
determined by the balance
between sympathetic and
parasympathetic nerve
activity
Numerous sympathetic
nerves also innervate
(link to) the walls of the
two ventricles where they
increase the force of
contraction of these
chambers
Sympathetic activity
dominates during
periods of exercise, stress
and excitement
Parasympathetic activity
dominates during
periods of rest and sleep
Increased sympathetic activity also stimulates the release of the hormone adrenaline from
the adrenal glands; adrenaline increases both the heart rate and its force of contraction

32. Acknowledgements
Copyright © 2003 SSER Ltd. and its licensors. All rights reserved. All graphics are for viewing purposes only.