Download Heart structure and function

Heart structure and function
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The Human Heart
The Human Heart
The heart is located slightly to the left of the body
Right side of body
Left side of body
Right side of heart
Left side of heart
Heart Chambers
 Left and right sides of the heart are separated by the septum.
RA, RV Right atrium, Right ventricle
Thinner atrium muscle
LA, LV Left atrium, Left ventricle
LA
Septum
RA
LV
RV
Thicker ventricle muscle
 The heart has four chambers: two atria and two ventricles.
 The heart is composed of cardiac muscle.
 Heart muscle contracts approximately 70 times per minute generating
blood pressure and flow.
Atrio-Ventricular Node
• The septum consists of non-conducting tissue, thus the impulse from the
SAN can only pass to the ventricles via the AVN. The AVN has greater
electrical resistance and this delays the impulse by 0.1s, ensuring that
atria contract before ventricles.
AVN
Bundle of His
Walls containing
Purkine fibres
• From the AVN the excitation passes to the base of the heart via the
Bundle of His and Purkine fibres, causing the ventricles to contract
(ventricular systole).
• Excitation and contraction (ventricular systole) spread upwards
across the ventricle walls.
Summary
sino-atrial node
(SAN/pacemaker) in wall of
right atrium
aorta
1. wave of excitation that starts
each heart beat is sent from
the SAN
2. spreads over atrial walls
causing contraction
artri-ventricular node (AVN)
in the atrioventricular
septum (under the aorta in
this diagram)
3. delayed at non-conducting
atrioventricular septum
2
Left Atrium
contraction
1
3
Bundle of His
4
Left ventricle
Right Atrium
Purkine fibres
5
contraction
Right ventricle
intraventricular septum
4. wave of excitation is sent from
AVN
5. passes along bundles of
His/Purkine fibres to the base
of the ventricles which then
contract
Pressure changes
120
Pressure
(mm Hg)
60
AP
VP
0
0
0.2
0.4
0.6
0.8
Time (seconds)
Atrial pressures (AP) are much lower than those generated in the
ventricle (VP).
0.1- 0.2 seconds
Systole
Diastole
120
Aortic Pressure
Pressure
(mm Hg)
60
Atrial Pressure (AP)
Ventricular pressure (VP)
0
0
0.2
0.4
0.6
Time (seconds)
•
At about 0.1 seconds atrial systole ends and
ventricular systole begins.
•
VP rises rapidly and becomes higher than
in the diastolic (non-contracting) atrium.
•
Shortly before 0.2 seconds ventricular
pressure overtakes atrial pressure and the
atrio-ventricular valve closes.
0.8
AP>VP
VP>AP
0.5 – 0.6 seconds
Systole
Diastole
120
Aortic Pressure
Pressure
(mm Hg)
60
Atrial Pressure (AP)
Ventricular Pressure (VP)
0
0
0.2
0.4
0.6
Time (seconds)
0.8
• VP falls rapidly as the ventricle continues to relax.
– Between 0.5 and 0.6 seconds VP becomes less than AP.
Atrio-ventricular valve
opens
Heart rate
120
Aortic Pressure
Pressure
(mm Hg)
60
Atrial Pressure
Ventricular Pressure
0
0
0.2
0.4
0.6
Time (seconds)
0.8
• The cardiac cycle is about 0.8 seconds in duration.
• Heart rate is variable - related especially to age and fitness.
Heart rate
• Heart rate can be estimated from graphical representations of the
cardiac cycle
One cycle
• The start and finish of a single cycle are determined.
• For convenience the spike of ventricular pressure has been used in the
diagram.
Electrocardiograms (ECGs)
ECGs are a record of the electrical currents that cause the heart muscle to contract.
• Electrodes placed on the skin detect the electrical activity.
• Electrode jelly ensures good electrical contact and the patient is required to keep still
because movement causes interference to the trace due to theelectrical impulses from
the muscles.
P: atrial systole (contraction) - the wave of
depolarisation in atrial walls causes contraction& blood
flows into the ventricles. Heart rate can be calculated
from the interval between successive P waves.
QRS-complex
R
Ventricular
depolarisation
QRS: ventricular systole (contraction) - the wave
of depolarisation in the ventricle walls causes
contraction and the valve between atrium and
ventricle closes.
Ventricular
repolarisation
Electrical
potential/mv
T
P
Q
S
0
0.2
0.4
Time (s)
0.6
0.8
T : ventricular diastole - the ventricles relax.
• PR interval = the time taken for an electrical
impulse to travel from the atria to the ventricles.
• QT interval = the contraction time because the
ventricles are contracting.
• The interval between T of one cardiac cycle and
Q of the following cycle = the filling time
because blood is firstly filling the atria, then the
ventricles.
Summary
Time (m sec)
100 200 300 400 500 600 700 800
One cardiac cycle
0
1. Atria contracting so blood flowing into
ventricle
2. Ventricles start to contract so ventricular
pressure > atrial pressure and the
atrioventricular valve closes
ECG
3. Ventricular pressure > aortic pressure
forcing open aortic valve so blood flows
from the ventricle into the aorta
Atrial
Ventricular
120
Systole
Diastole
Systole
Diastole
Systole
Diastole
4. Ventricular pressure falls below aortic
pressure, so aortic valve closes
5. Ventricular pressure falls below atrial
pressure so that blood flows from the
atria to the ventricles and ventricular
volume rises rapidly
6
4
90
Atrium filling with blood from pulmonary
vein until atrial pressure> ventricular
pressure and blood flows from atrium to
ventricle
3
Aorta pressure
Pressure 60
(mm Hg)
30
5
1
6
0
Left AV valve
closes
2
Aortic valve
opens
Aortic valve
closes
Left Atrial pressure
Left Ventricular pressure
Left AV valve
opens
Cardioinhibition
• Receptors in the carotid artery and
aorta directly stimulate the
cardioinhibitory centre of the
medulla.
CARDIOACCELERATORY
centre
• The medulla responds by sending
inhibiting signals via the
parasymphathetic (vagus) nerve to the
heart.
CARDIOINHIBITORY
centre
Vagus
Medulla
oblongata
Parasympathetic
Sympathetic
• Parasympathetic nerve ends release the
neurotransmitter acetylcholine.
Spinal
cord
Cardiac
nerve
• Acetylcholine reduces the rate and strength of the
heartbeat and transmission of stimulation to the
ventricles.