Cardiac Function
Learning objectives
To be able to describe the structure of the heart and the
function.
To understand how the heart works and the conduction
system.
To be able to explain the cardiac cycle and changes with
exercise.
To define Stroke Volume, Cardiac Output & Heart Rate.
To understand the neural control of heart rate & the long
term training adaptations to the cardiac system.
Cardiac Function
What does my heart do?
Although your heart is only about the size of your closed
fist, this small muscle is able to pump approximately 5 to 6
litres (about 1.5 gallons) of blood per minute, even when
you are at rest!
Structure of the Heart
The heart is a muscular pump that is divided into 4 chambers.
The two top chambers are called atria, and the two lower
chambers are called ventricles.
The two types of chambers
perform different functions:
The atria collect the blood
that enters the heart and
push it to the ventricles.
While the ventricles push
blood out of the heart and
into the arteries to go to
the rest of the body
Vessels of the heart
The right atrium of the heart receives deoxygenated blood
from two major veins:
The superior vena cava and the inferior vena cava.
The pulmonary artery carries
deoxygenated blood from the
right ventricle to the lungs.
The pulmonary vein delivers
oxygenated blood from the lungs to
the left atrium.
Vessels of the heart
The aorta carries oxygenated blood from the left ventricle
to the body
For the heart to work effectively,
it requires a good blood supply.
This is provided by the coronary
artery, which carries oxygenated
blood.
Deoxygenated blood is removed
by the veins of the heart into the
right atrium through the
coronary sinus.
Heart Valves
Your heart valves act as gates that allow blood to pass
between heart chambers or from heart chambers to their
associated blood vessels.
They include the tricuspid, pulmonary, bicuspid (or mitral),
and aortic valves.
How the Heart Works – Conduction System
The heart works by producing impulses which spread and
innervate the specialised muscle fibres.
The heart produces its
own impulses (myogenic),
and it is the conduction
system of the heart which
spreads the impulses and
enables the heart to
contract.
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VIDEO CLIP
Conduction System
The electrical impulse begins at the pacemaker: a mass of
cardiac cells known as the sino-atrial node (S.A. node).
As the impulse is
emitted, it spreads to
adjacent
interconnecting fibres
of the atrium, which
causes the atria to
contract.
Conduction System
It then passes to another specialised mass of cells
called the atrioventricular node (A.V. node).
The A.V. node acts as
a distributor and
passes the action
potential to the
Bundle of His,
Conduction System
This action potential filters to the branching Purkinje fibres,
spread the excitation throughout the ventricles.
There is a delay of
about 0.1 second from
the time when the A.V.
node receives
stimulation, to when it
distributes action
potential through the
ventricles.
Conduction System
The relationship between the electrical activity of the heart
and the cardiac cycle can be shown in an electrocardiogram
(ECG)
Conduction System - Summary
1)
2)
3)
4)
5)
6)
Heart generates own electrical impulses,
At the sino-atrial node (pacemaker),
Impulse spreads through cardiac tissue in the atria,
This causes contraction of the atria,
The impulse carries on to Atrio-ventricular node,
The action potential moves into Bundle of His and
spreads throughout the Purkinje fibres causing the
ventricle to contract.
The Cardiac Cycle
This refers to the process of cardiac contraction and
blood transportation through the heart.
The heart has dual purpose,
and the cardiac cycle explains
the sequence of events that
takes place during one
complete heartbeat.
Each cycle takes
approximately 0.8 seconds
and occurs on average 72
times per minute.
The Cardiac Cycle
The filling of the heart and subsequent emptying follows a
particular sequence.
There are four stages to each heart beat:
1)
2)
3)
4)
Atrial diastole
Ventricular diastole
Atrial systole
Ventricular systole
Important points to note
Diastole
The heart
filling with
blood.
----------------The heart is
relaxing
Systole
The heart
emptying.
------------------The heart
contracts
1. Atrial diastole
This is the 1st stage of the cardiac
cycle.
The upper chamber of the heart are filled with blood
returning from:
• The body via the vena cava to the right atrium.
• The lungs via the pulmonary vein to the left atrium.
At this time the atrio-ventricular
valves are shut but as the atria fill
with blood, atrial pressure
overcomes ventricular pressure.
2. Ventricular diastole
High pressure forces the atrio-ventricular values open
and ventricular diastole now takes place.
During this stage the
ventricles fill with blood and
the semi-lunar valves
remain closed.
The atria now contract,
causing atrial systole.
3. Atrial Systole
The atrial contraction ensures that all the blood is ejected into
the ventricles.
As the ventricles continue going through diastole, the
pressure increases, which causes the atrioventricular valves
to close.
4. Ventricular Systole
Ultimately the ventricular pressure overcomes that in
the aorta and the pulmonary artery.
The semi-lunar values open and
the ventricles contract, forcing
all the blood from the right
ventricle into the pulmonary
artery and the blood in the left
ventricle into the aorta.
Once completed, the semilunar values snap shut.
The cycle is now complete
and ready to be repeated.
http://www.youtube.com/watch?
Cardiac Output
The amount of blood the heart pumps per minute is known
cardiac output (Q) (measure in litres).
The cardiac output is a product of stroke volume and heart
rate.
Q = SV x HR.
HR = numbers of BPM
Lance Armstrong’s rest heart rate = 34 bpm
Stroke Volume
SV Definition - The amount of blood ejected from
LV each contraction (measured mm per beat).
Factors Affecting Stroke Volume
• How much blood is being returned to the heart
(venous return).
• How far the ventricles will stretch (elastic
property of muscle tissue).
• The contractility of the ventricles.
• The pressure in the arteries leading from the
heart.
Factors Affecting Stroke Volume
The first 2 factors relate to how much blood can enter
the ventricles and ...
...the last 2 relate to how much blood can be ejected
from the heart during systole
Starling’s Law of the Heart
During exercise, venous return increases and therefore
cardiac output increases. This is caused by the myocardium
being stretched, resulting in the myocardium contracting
with greater force.
Therefore, the stimulus that
causes the greater force of
contraction is the stretching of
the muscle fibres themselves.
This relationship is known as starling’s law of the heart.
Heart Rate During Exercise
Heart rate will change in response to exercise and is
determined by the intensity of such activities.
Heart Rate During Submaximal Exercise
200
C
180
160
D
140
E
E
120
B
F
F
100
Max
A
80
Min
60
40
0
2
4
6
8
10
12
14
16
18
A = Anticipatory rise due to hormonal actions of
adrenaline
20
Interpretation of Graph
Heart Rate During Submaximal Exercise
200
C
180
160
D
140
E
E
120
B
F
F
100
Max
A
80
Min
60
40
0
2
4
6
8
10
12
14
16
18
20
B = Sharp rise – mainly anaerobic work – due to
proprioceptors / sensory stimulation, continued release of
hormones and action of muscle pump.
Interpretation of Graph
Heart Rate During Submaximal Exercise
200
C
180
160
D
140
E
E
120
B
F
F
100
Max
A
80
Min
60
40
0
2
4
6
8
10
12
14
16
18
20
C = continued high HR due to maximal work loads which
continue to stress anaerobic systems.
Interpretation of Graph
Heart Rate During Submaximal Exercise
200
C
180
160
D
140
E
E
120
B
F
F
100
Max
A
80
Min
60
40
0
2
4
6
8
10
12
14
16
18
20
D = steady state (plateau) and some recovery of O2 debt
Interpretation of Graph
Heart Rate During Submaximal Exercise
200
C
180
160
D
140
E
E
120
B
F
F
100
Max
A
80
Min
60
40
0
2
4
6
8
10
12
14
16
18
20
E = rapid recovery due to cessation of Proprioceptive
stimuli / muscle pump / withdrawal of hormones.
Interpretation of Graph
Heart Rate During Submaximal Exercise
200
C
180
160
D
140
E
E
120
B
F
F
100
Max
A
80
Min
60
40
0
2
4
6
8
10
12
14
16
18
20
F = Slow recovery. Clearance of metabolites (lactic acid).
Heat loss hence muscle cooling.
Cardiac Output and Exercise
The resting HR of an individual can vary greatly,
although we all need to produce roughly the same
cardiac output at rest.
If a person does a lot of aerobic activity their heart rate
often drops to 60 BPM or lower.
In order to produce the same cardiac output, the stoke
volume must increase to compensate for this drop.
Cardiac Output and Exercise
This is an obvious benefit because more O2 can be
delivered to the working tissue, enabling them to work
harder or for a longer period.
This is known as Bradycardia
Neural control mechanism
The autonomic nervous system comprises the sympathetic
system and the para sympathetic system. The sympathetic
system stimulates the heart to beat faster; the
parasympathetic system returns the heart to its resting
level.
The cardiac control centre in
the medulla oblongata of the
brain coordinates these two
systems.
Neural control mechanism
The cardiac control centre is stimulated by
chemoreceptors, baroreceptors and proprioceptors.
Chemoreceptors detect increases in carbon dioxide and
lactic acid and decreases in oxygen.
Baroreceptors detect increases in blood pressure.
Proprioceptors detect increases in muscle movement
Neural control mechanism
When exercise stops, carbon dioxide levels, blood pressure
and muscle movement all decrease. This is detected by the
receptors, which send impulses to the cardiac control centre.
An impulse is then
sent through the
parasympathetic
system, which
stimulates the SA
node and heart rate
decreases.
Cardiovascular Drift
Research has shown that heart rate does not remain the
same but instead increases slowly. This is known as
cardiovascular drift.
It occurs during
prolonged exercise
in a warm
environment
despite the
intensity of the
exercise remaining
the same.
Effects of training on the heart
If you perform continuous, fartlek or aerobic interval
training over a period of time, physiological adaptations
take place.
Athlete's heart is enlarged heart caused by repeated
strenuous exercise.
Hypertrophy of the myocardium means that the heart
gets bigger and stronger. This results in bradycardia.
Effects of training on the heart
Maximum cardiac output increases
Increased capillarisation of the heart muscle
increasing the efficiency of oxygen diffusion into the
myocardium.
Increased contractility - resistance or strength training
causes an increase in the force of heart contractions
Plenary
Exam Style Question.
1. Describe the fluctuations in heart rate as exercise
increases.
(4 marks)
2. Define the terms Cardiac Output, Heart Rate & Stroke
Volume and the relationship with exercise
(4 marks)