CARDIOVASCULAR SYSTEM
PREPARED
BY MR. ABHIJIT DAS
HUMAN HEART
Heart is situated in the thoracic cavity, in between
the two lungs, slightly tilted to the left.
It is covered by a double layered membranous bag called pericardium.
Human heart has four chambers, two relatively small
upper chambers called atria and two larger lower chambers called ventricles.
Inter-atrial septum,
which is a thin muscular wall, separates the right and left atria. Similarly,
the inter-ventricular septum, which is a
thick muscular wall, separates the left and right ventricles.
The atrium and ventricle of the same side are also
separated by a thick fibrous tissue called the atrio-ventricular
septum.
The opening between the right atrium and right
ventricle is guarded by a muscular valve known as tricuspid
valve.
Similarly, the opening between the left atrium and
the left ventricle is guarded by bicuspid valve (or
mitral valve).
The openings of right and left ventricles into the
pulmonary artery and aorta respectively are provided with the semilunar valves.
The valves in the heart allow the blood flow only in
one direction and prevent any backward flow.
CIRCULATORY PATHWAY IN HUMANS (DOUBLE CIRCULATION)
Humans possess a four chambered heart with two atria
and two ventricles. Oxygenated blood is received by the left atrium and
deoxygenated blood is received by the right atrium. Then the blood passes on to
the ventricles of the same sides.
The right ventricle pumps blood to the lungs (which
is known as the pulmonary circulation),
where the blood collects oxygen from the lungs and excess carbon dioxide
diffuses into the lungs.
Similarly, the left ventricle pumps blood to the
rest of the body (which is known as the systemic
circulation).
So two separate circulatory pathways are present in
humans, hence, we have double circulation.
THE HEART WALL
The heart wall is composed of three layers of tissue
pericardium, myocardium and endocardium.
The pericardium is the outermost layer and is made
up of two membranes. The outer membrane is fibrous pericardium which consists
of fibres. The inner membrane is serous pericardium formed by single layer of
squamous epithelium. The pericardial fluid lie between the two layers, so that
the heart can beat without rubbing against the fibrous pericardium.
The myocardium is composed of specialised cardiac
muscle only found in the heart.
Endocardium lines the chambers and valves of the heart.
It consists of single layer of squamous epithelium.
CONDUCTING SYSTEM OF THE HEART
The entire heart is made of cardiac muscles. The
heart has auto-excitable cells which means it beats independently.
A small mass of specialised cells called the sino-atrial
node (SA node) present in the wall of right
atrium near the opening of superior venacava.
Another mass of these cells are seen in the lower
left corner of the right atrium called the atrio-ventricular node (AV node).
A bundle of nodal fibres called atrio-ventricular
bundle/bundle of His continues from the AV
node divides into a right and left bundle branches.
These branches give rise to minute fibres throughout
the ventricular musculature of both ventricles and are called purkinje fibres.
The cells of SA node are electrically unstable. This
instability leads them to depolarise regularly within 60-90 times a minute.
This depolarization is followed by repolarization.
Because the SA node depolarises faster than any
other part of the heart, it normally sets the heart rate and is called the
pacemaker of the heart. Depolarisation of SA node triggers atrial contraction.
Then the electrical signal moves to the ventricular
side by AV node.
Then the bundle of His, bundle branches and purkinje
fibres transmit electrical signals from the AV node to the ventricular
myocardium, where the ventricular contraction begins by pumping blood into the
pulmonary artery and the aorta.
CARDIAC CYCLE
The healthy adult heart beats at a rate of 60-90
beats per minute.
During each heartbeat or cardiac cycle the heart
contacts (systole) and then relaxes (diastole).
Each cycle lasts about 0.8
second.
Each cardiac cycle consists of:
·
Atrial contraction (atrial systole)
·
Atrial relaxation (atria diastole)
·
Ventricular contraction (ventricular
systole)
·
Ventricular relaxation (ventricular
diastole)
To begin with, all the four chambers of the heart
are in a relaxed stage that means they are in joint diastole.
Now the atria are being filled with blood from the
superior and inferior vena cava (into the right atrium) and the pulmonary veins
(into the left atrium).
The semilunar valves are closed at this stage.
The SA node now triggers a wave of contraction that
spreads over the myocardium of both atria to undergo a simultaneous contraction-
the atrial systole (atrial systole lasts 0.1 second). This increases the flow
of blood into the ventricles.
Now the signal is moved to the ventricular side by
AV node. Then AV node triggers its own electrical signal, which quickly spreads
to the ventricular muscle via the bundle of His, bundle branches and purkinje
fibres.
This causes the ventricular muscle to contract,
pumping the blood into the pulmonary artery and the aorta (ventricular systole
lasts 0.3 second).
Contraction of ventricles is followed by complete
cardiac relaxation (joint diastole lasts 0.4 second) when atria and ventricles
are relaxed. During this time the myocardium recovers.
HEART SOUNDS
If the ear or the diaphragm of the stethoscope is
placed on the chest wall a little below the left nipple the heart sounds (lub
and dub) can be heard.
When the ventricles contract, there is a rapid
increase in the ventricular pressure, and when the ventricular pressure is more
than the atrial pressure the atrioventricular valves (tricuspid and bicuspid
valves) close, making a sound ‘lub’.
The first heart sound ‘lub’ is loud and is due to
the closure of the atrioventricular valves.
When the ventricular pressure rises above the
pressure in the pulmonary artery and in the aorta, the semilunar valves open
and blood flows in these vessels.
When the ventricles relax, the semilunar valves
close, making a sound ‘dub’.
CARDIAC OUTPUT
The cardiac output is the amount of blood pushed out
by each ventricle in one minute.
Formula for cardiac output (CO) =Stroke Volume X
Heart Rate
STROKE VOLUME=volume of blood pushed out by each
ventricle during one cardiac cycle (which is about 70ml)
HEART RATE=72 beats/min
So, Cardiac Output (CO) = 70 X 72= 5040ml (approx. 5litres)
ELECTROCARDIOGRAM (ECG)
Electrocardiogram is
a graphical representation of the electrical activity of the heart during
cardiac cycle.
To obtain a standard ECG, a patient is connected to
the machine (electrocardiograph) by using
electrical leads.
Generally the test involves, attaching 9 small,
sticky electrical leads to the arms (one on
left arm, one on right arm), ankle (one on the left ankle) and chest (six on
the chest).
These leads are connected by wires to the ECG
recording machine.
WAVES OF ECG
P wave represents
atrial contraction.
QRS complex represents
ventricular contraction.
T wave represents
ventricular relaxation.
There is no visible wave representing atrial
relaxation in the ECG because it occurs during ventricular contraction.
CLINICAL SIGNIFICANCE
ECG is a fundamental clinical tool, when read
carefully, can identify possible cardiac abnormalities or disorders.
REGULATION OF CARDIAC ACTIVITY
Normal activities of the heart are auto regulated by
specialised muscles hence heart is myogenic.
A special neural centre in the medulla oblangata can
also regulate the function of heart through autonomic nervous system.
Neural signals from sympathetic
nerves can increase the heart rate, the strength of ventricular
contraction and thereby the cardiac output.
On the other hand, parasympathetic
neural signals decrease the heart rate, ventricular contraction and
thereby the cardiac output.
Adrenaline,
which is a hormone, released from the adrenal gland can also increase the
cardiac output and heart rate will be increased.
BLOOD PRESSURE
Blood pressure is the pressure exerted by the
flowing blood on the walls of the blood vessels.
It is usually measured in ‘mm
Hg’.
SYSTOLIC BLOOD PRESSURE
Systolic pressure is the maximum arterial pressure
during during ventricular contraction.
Normal systolic blood pressure in a young adult is 120 mm Hg.
Systolic blood pressure sometimes shows fluctuations
that means it is increased during exercise and meals, and is decreased during
sleep and rest.
DIASTOLIC BLOOD PRESSURE
Diastolic blood pressure is the arterial pressure
during ventricular relaxation.
Normal diastolic blood pressure in a young adult is 80 mm Hg.
NORMAL BLOOD PRESSURE
Blood pressure of a normal person is written as 120/80 mmHg.
Blood pressure is measured with a device called as sphygmomanometer and is usually expressed with the
systolic pressure written above the diastolic pressure.
FACTORS AFFECTING BLOOD PRESSURE
The arterial blood pressure is the product of
cardiac output (CO) and total peripheral resistance (PR).
Blood Pressure = Cardiac Output (CO) X Peripheral
Resistance (PR)
So blood pressure is affected by conditions that
affect either cardiac output or peripheral resistance.
Changes in cardiac output affect the systolic pressure
while changes in peripheral resistance affect diastolic pressure.
The cardiac output depends
upon:
·
Stroke volume and
·
Heart rate
So these are important factors of blood pressure.
The peripheral resistance
depends upon:
·
The viscosity of blood
·
Elasticity of the vessel walls and
·
Velocity of the blood flow
So these are factors affecting the blood pressure as
well.
FUNCTIONS OF THE HEART
1. The
heart pumps nutrients and oxygen throughout the body through blood circulation.
2. The
heart collects carbon dioxide and other waste products from different organs
and pumps them to excretory organs for excretion.
3. The
heart also pumps hormones to the target tissues for blood circulation.
4. The
heart ensures that adequate blood pressure is maintained in the body.
5. The
heart also delivers antibodies, platelets, WBCs etc. to help body’s defense
system.