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Mammalian Heart Structure
The heart is the major
Superior
organ of the
VENA
circulatory system
CAVA
It is a fist-sized muscular
pump consisting of
four chambers
The human heart
recirculates the entire
blood volume (5 dm3)
every minute when
the body is at rest
The left side of the
heart pumps oxygenated
blood out into
the body’s arteries via
the AORTA
PULMONARY
ARTERY
The ability of the
heart to perform such
work is due to the
presence of specialised
cardiac muscle in
its walls
The job of the heart is to
pump blood around two
separate circuits
AORTA
CORONARY
ARTERIES
Inferior
VENA
CAVA
The left side of the
heart receives oxygenated
blood from the lungs via the
PULMONARY VEINS
Deoxygenated blood
returns to the right side
of the heart via the
VENA CAVA
Deoxygenated blood
is pumped to the
lungs via the
PULMONARY
ARTERY
Heart muscle receives
its own supply of blood
from the CORONARY
ARTERIES
Mammalian Heart Structure
Aorta
Vena cavae
Semilunar valves
Pulmonary artery
Pulmonary veins
Left atrium
Right atrium
Bicuspid valve
Tricuspid valve
Right ventricle
Septum
(dividing wall)
Left ventricle
Cardiac muscle
Cardiac Muscle
Cardiac muscle tissue consists of branched, faintly striated fibres (cells), each of
which contains a centrally placed nucleus
Thickenings of
the plasma
membrane
between
individual
cardiac muscle
cells form
intercalated
discs
Cardiac muscle tissue forms networks due to its branching structure
When one muscle fibre is electrically excited, an impulse is quickly
transmitted to neighbouring fibres such that impulses spread
in all directions when cardiac muscle is stimulated
This photomicrograph of cardiac muscle tissue shows the branching nature
of the fibres, the intercalated discs and the nuclei
Nuclei
Intercalated
disc
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 Right
atrium
partition called the
SEPTUM
The muscular walls of the
heart are referred to as the
myometrium and consist of
specialised cardiac muscle Right
cells
ventricle
The thicker walled structure of the left
ventricle is a consequence of the distance
over which it is required to pump blood
Left
atrium
Left
ventricle
Septum
The direction of blood flow through the heart is maintained be valves
Between the right atrium and
the right ventricle is the
TRICUSPID VALVE
This valve prevents the backflow
of blood from the right ventricle
to the right atrium
Between the left atrium and
the left ventricle is the
BICSUPID VALVE OR
MITRAL VALVE
This valve prevents the
backflow of blood from
Right
the left ventricle to
atrium
the left atrium
The bicuspid and tricuspid Tricuspid
valve
valves are collectively
known as the
ATRIO-VENTRICULAR
Right
VALVES or AV valves
Pocket-shapes valves known ventricle
as SEMILUNAR VALVES are
located at the base of the arteries
responsible for transporting
blood away from the heart
Aorta
Pulmonary
Artery
Left
atrium
Semilunar
valves
Bicuspid
or Mitral
valve
Left
ventricle
Heart Valves
Pulmonary
semi-lunar
valve
Aortic
semi-lunar
valve
Bicuspid or
mitral
valve
Thickerwalled left
ventricle
Tricuspid
valve
Thinnerwalled right
ventricle
Valves viewed from above
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
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
Systole describes periods when the heart is contracting
Diastole describes periods when the heart is relaxing
The Cardiac Cycle
Since the heartbeat is a cycle of events, there is no absolute first or last phase
This account begins
Passive filling of the ventricles
with the phase of
continues until the ventricles
late diastole when
have filled to about 70% of
both the atria and
their capacity to hold blood
the ventricles of the
heart are relaxed
The increasing volume of
During late diastole,
blood in the ventricles
all chambers of the
presses against the AV
heart are relaxed
valves and they begin to
with the atriodrift towards a
ventricular valves
closed position
(AV valves) open
and the semi-lunar
At this point both of the
valves closed
atria contract, rapidly
Blood flows passively
propelling blood into the
from the atria, through
ventricles which stretch
the open AV valves, into
to accommodate their
the ventricles that stretch
full capacity for blood
to accommodate the
extra volume of blood
This phase of the heartbeat
This phase of the heartbeat is known as
is known as
PASSIVE FILLING of the ventricles
ATRIAL SYSTOLE
The Cardiac Cycle
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
AV valves
and pulmonary arteries,
shut
the semilunar valves are
forced open and blood is
ejected from the heart
This phase of the heartbeat is known as
ISOMETRIC VENTRICULAR CONTRACTION
The Cardiac Cycle
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
This phase of the
heartbeat is known as
This phase of the heartbeat is known as
VENTRICULAR
ISOMETRIC VENTRICULAR CONTRACTION
EJECTION
The Cardiac Cycle
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
The Cardiac Cycle
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)
The Cardiac Cycle
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
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
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
120
SL valve closes
pressure (mm Hg)
100
SL
valve
opens
aortic
pressure
80
left ventricular
pressure
60
40
Period Z to A represents the phase
of Late Diastole when all chambers of 20
the heart are relaxed, atrial and
ventricular pressures are low and
the aortic pressure is falling
0
The AV valves open at the
0
beginning of this phase and
the semi-lunar valves are already ATRIA
closed
VENTRICLES
Passive filling of the
ventricles takes place
during this phase
AV
valve
opens
AV
valve
closes
left atrial
pressure
0.1
0.2
0.3
0.4
0.5
0.6
time (s)
= systole
= diastole
0.7
0.8
120
SL valve closes
SL
valve
opens
pressure (mm Hg)
100
aortic
pressure
80
left ventricular
pressure
60
40
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
AV
valve
opens
AV
valve
closes
20
left atrial
pressure
0
0
0.1
0.2
0.3
0.4
0.5
0.6
time (s)
ATRIA
At the end of atrial systole,
the increased blood pressure
in the ventricles forces
the AV valves to close
VENTRICLES
= systole
= diastole
0.7
0.8
120
SL valve closes
pressure (mm Hg)
100
SL
valve
opens
80
aortic
pressure
left ventricular
pressure
60
40
AV
valve
closes
AV
valve
opens
Period W to X (the first phase of
Ventricular Systole) begins
left atrial
as the ventricles contract in an 20
pressure
isometric manner
Both the AV and semi-lunar valves
are closed and the steeply rising 0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
pressure in the left ventricle
time (s)
reflects the increasing muscle ATRIA
tension created by the
ventricular muscles VENTRICLES
= systole
diastole
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
0.8
120
SL valve closes
pressure (mm Hg)
100
SL
valve
opens
aortic
pressure
80
left ventricular
pressure
60
40
Period X to Y represents the second
phase of Ventricular Systole when
blood is ejected from the left ventricle 20
As the semi-lunar valves open, blood
is ejected into the aorta and
0
pulmonary arteries (right side)
0
The ventricular and aortic pressures
are the same throughout this
ATRIA
period and both pressures
reach their highest value VENTRICLES
(around 120 mm Hg)
AV
valve
opens
AV
valve
closes
left atrial
pressure
0.1
0.2
0.3
0.4
0.5
0.6
time (s)
= systole
= diastole
0.7
0.8
120
SL valve closes
pressure (mm Hg)
100
80
SL
valve
opens
aortic
pressure
left ventricular
pressure
60
40
AV
AV
Period Y to Z represents the phase
valve
valve
of Ventricular Relaxation (Diastole)
opens
closes
Backflow of blood from the aorta
20
left atrial
closes the semi-lunar valve at the
pressure
beginning of this phase
The ventricular pressure drops
0
sharply as the ventricle relaxes
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
and the ventricular blood volume
time (s)
remains constant as the AV
ATRIA
and semi-lunar valves are
VENTRICLES
both shut
= systole
= diastole
The AV valve opens at the END of this phase as
the atrial pressure
is slightly
greater than that in the ventricle
120
SL valve closes
SL
valve
opens
pressure (mm Hg)
100
aortic
pressure
80
left ventricular
pressure
60
40
As the AV valve opens, passive
filling of the ventricle starts
during phase Z to A and the
cycle begins again
AV
valve
opens
AV
valve
closes
20
left atrial
pressure
0
0
0.1
0.2
0.3
0.4
0.5
0.6
time (s)
ATRIA
VENTRICLES
= systole
= diastole
0.7
0.8
Summary of Valve
Movements during the
Cardiac Cycle
A
WX
Y
A
Z
120
pressure (mm Hg)
The AV bicuspid valve opens
at the beginning of
Phase Z to A
100
(Passive filling of the ventricles)
The AV valve opens as
the pressure in the atrium
is slightly greater than
80
that in the ventricle
The AV valve closes at the
end of atrial systole
(Period A to W)
60
The aortic semi-lunar valve
opens at the end of
Isometric Ventricular Systole
following a steep rise in
40
pressure in the left ventricle
The aortic semi-lunar
valve closes at the end of AV bicuspid
Ventricular Ejection due valve
20 closes
to a slight backflow of
blood from the aorta
As the ventricle relaxes during
diastole, the pressure falls
0
slightly below that of the
0
0.1
atrium and the AV bicuspid
valve opens again
aortic
pressure
Aortic semilunar valve
closes
left ventricular
pressure
Aortic semilunar valve
opens
AV bicuspid
valve opens
left atrial
pressure
0.2
0.3
0.4
time (s)
0.5
0.6
0.7
0.8
Ventricular Volume during the Cardiac Cycle
Ventricular
relaxation
(diastole)
Ventricular Systole
(isometric phase)
Atrial
systole
% Capacity
100%
70%
Ventricular
ejection
(systole)
Passive
filling
(late diastole)
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
0%
Ventricular volume
remains constant as
all valves are closed
Time (seconds)
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
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
SA node
in wall of
right atrium
The AV node connects
with a bundle of large
fibres called the bundle
of His, which divides into
left and right bundle
branches
AV 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
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 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
When the electrical
impulses reach the
border between the
atria and ventricles
they are blocked by
a band of nonconducting
fibrous tissue
In order to reach
the ventricles,
electrical impulses
must pass through
the AV node, which
slows down the
speed of electrical
transmission
This delay, called the AV delay,
is extremely important as it
allows the atria to complete their
contraction before the ventricles
begin to contract
As the impulses spread
across the atria, they
stimulate a wave of
contraction within the
atrial walls and
atrial systole is
triggered
Fibrous Tissue
AV
Node
Impulses are
conducted from
AV node along
the bundle of His
The bundle fibres
divide into
numerous
Purkinje fibres
that permeate
throughout the
ventricular
muscles
The spread of
electrical impulses
throughout the
ventricles triggers
ventricular systole
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
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
+ + + + + + + + + + + + + + + +
- - - - - - - - - - - - - - - Intercalated
Disc
Cardiac Muscle Cell
-+ +- +- +- +- +- +- +- +- +- +- +- +- +- +- +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
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
-++ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+
- - - - - - - - - - - - - - - Intercalated
Disc
Cardiac Muscle Cell
-++- +-+- +-+- +-+- +-+- +-+- +-+- +-+- +-+- +-+- +-+- +-+- +-+- +-+- +-+- +-+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
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
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
The ECG Trace
R
P wave
T wave
Q
S
The ECG trace for each heartbeat displays a P wave, a QRS wave
or complex and a T wave
The ECG Trace
R
P wave
T 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
P–R
interval
R
P wave
The ECG Trace
T 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
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
Control of the Heart Rate
AUTONOMIC NERVOUS SYSTEM
SYMPATHETIC NERVOUS
SYSTEM
PARASYMPATHETIC NERVOUS
SYSTEM
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
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
The heart rate is therefore
determined by the balance
between sympathetic and
parasympathetic nerve
activity
Sympathetic activity
dominates during
periods of exercise, stress
and excitement
Parasympathetic activity
dominates during
periods of rest and sleep
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
Numerous sympathetic
nerves also innervate
(link to) the walls of the
two ventricles where they
increase the force of
contraction of these
chambers
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
Acknowledgements
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