STRUCTURE AND FUNCTIONS OF HEART - ENGLSH
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STRUCTURE AND FUNCTIONS
OF HEART-
• CIRCULATORY
SYSTEM- The circulatory system includes the organs which are involved in
circulation . It is also known as cardio-vascular system . In this system we will study about -
• Blood
and blood vessels.
• Heart.
• The
heart weighs 250–390 g in men and 200–275 g in women and is a little larger
than the owner’s closed fist, being approximately 12 cm long and 9 cm wide . It
is located in the thoracic cavity (chest) in the mediastinum (between the
lungs), behind and to the slight left of the sternum
• The
heart is a muscular organ containing four chambers. Its main function is to
pump blood around the circulatory system of the lungs and the systemic
circulation of the rest of the body. In the average day the heart beats about
100,000 times and never rests. It must continue its cycle of contraction and
relaxation in order to provide a continuous blood supply to the tissues and
ensure the delivery of nutrients and oxygen and the removal of waste products.
Anatomy of heart is easily understood under following
headings-
Ø Wall
of the heart
Ø Chambers
of heart
Ø Valves
of heart
Ø Great
vessels of heart
Ø Conduction
system of heart
WALL OF THE HEART- The heart wall is composed of three layers of
tissue
Ø Pericardium,
Ø Myocardium
and
Ø Endocardium.
• PERICARDIUM-
Pericardium is the outermost layer of
heart wall . It consists of two layers, one outer is the fibrous pericardium
and inner is the serous pericardium. The fibrous pericardium, a tough,
inelastic layer made up of dense, irregular, connective tissue. The role of
this layer is to prevent the overstretching of the heart. It also provides
protection to the heart and anchors it in place
• The
serous pericardium, a thinner, more delicate, layer that forms a double layer
around the heart:
• The
parietal pericardium, the outer layer attached to the fibrous pericardium; and
• The
visceral pericardium (also known as the epicardium) attached tightly to the
surface of the heart.
Between the parietal and
visceral pericardium is a thin film of fluid (pericardial fluid) that reduces
the friction between the membranes as the heart moves during its cycle of
contraction and relaxation. The space containing the pericardial fluid is known
as the pericardial cavity
• MYOCARDIUM-The
myocardium is composed of specialised cardiac muscle found only in the heart .
It is not under voluntary control but is striated, like skeletal muscle. Each
fibre (cell) has a nucleus and one or more branches. The ends of the cells and
their branches are in very close contact with the ends and branches of adjacent
cells.
There are ‘joints’, or intercalated
discs, in between the muschles. This arrangement gives cardiac muscle the
appearance of being a sheet of muscle rather than a very large number of
individual cells. Because of the end-to-end continuity of the fibres, each one
does not need to have a separate nerve supply.
• ENDOCARDIUM-
This lines the chambers and valves of the heart. It is a thin, smooth,
glistening membrane that permits smooth flow of blood inside the heart. It
consists of squamous epithelial cells, and it is continuous with the
endothelium lining the blood vessels.
CHAMBERS OF HEART- The heart has four chambers. The two
superior receiving chambers are the atria , and the two inferior pumping
chambers are the ventricles
The right
atrium forms the right border of the heart and receives blood from three
veins: the superior vena cava, inferior vena cava, and coronary
sinus. The left atrium forms most of the base of the heart and
receives blood from the lungs through four pulmonary veins. Between the
atria is a thin dividing wall, the inter-atrial septum
The right ventricle receives blood from the right
atrium and pumps this blood out into the pulmonary circulation (the lungs). As
the pressure in the pulmonary circulation is quite low because the right
ventricle has a thinner wall than the left ventricle.
The left ventricle receives blood from the left
atrium and pumps this blood out into the systemic circulation (the rest of the
body) via the aorta. As the left ventricle has to pump against a higher
pressure and over a greater distance it has a much thicker (more muscular)
wall.
VALVES OF HEART
• The
valves allow one way flow of the blood,
human heart have 4 valves.
1. The
tricuspid valve is made up of three cusps (leaflets) and lies between
the right atrium and the right ventricle;
2. The
bicuspid (mitral) valve is made up of two cusps and lies between the
left atrium and the left ventricle.
3. The
pulmonary valve is made up of three semilunar cusps (leaflets) and lies between
the pulmonary artery and the right ventricle;
4. The
aurtic valve valve is also made
up of three semilunar cusps (leaflets) and lies between the aorta and the left
ventricle.
GREAT VESSELS OF HEART-The two largest veins of the body,
the superior venae cavae and inferior venae cavae, empty their contents
into the right atrium.
• The pulmonary
artery is a blood vessel which delivers deoxygenated blood to the
lungs. Like all arteries, the pulmonary artery pumps
blood away from the heart.
• The aorta is
the main artery that carries blood away from your heart to the rest of your
body.
CONDUCTION SYSTEM OF HEART
The heart possesses the property of autorhythmicity,
which means it generates its own electrical impulses and beats independently of
nervous or hormonal control. The source of this electrical activity is a
network of specialized cardiac muscle fibers called autorhythmic fibers or
conduction system of heart. This system includes-
~SA Node
~AV Node
~Bundle of His
~Bundle branch ~Purkinje fibers
Normal electrical impulse begins in the sino-atrial (SA)
node, which is located in the right atrium, and is rapidly transmitted across
the atria . This ensures that the right
and left atria are excited together and beat as one unit. The impulse is transmitted
to the AV node, where further transmission is delayed for approximately 0.1s.
This ensures that the atria have completely contracted before ventricular
contraction is initiated.
Once the impulse has been ‘held’ in the AV node it is
then transmitted down the bundle of His (AV bundle) to the fast pathways of the
two bundle branches (one bundle branch per ventricle). The bundle branches then
divide into the smaller and smaller branches of the Purkinje system, which
transmits the impulses to the muscles of the ventricles.
CARDIAC CYCLE-
At rest, the healthy adult heart is likely to beat at a
rate of 60–80 bpm. During each heartbeat, or cardiac cycle , the heart
contracts and then relaxes. The period of contraction is called systole and
that of relaxation, diastole. Taking 74 bpm as an example, each cycle
lasts about 0.8 of a second and consists of:
Atrial systole – contraction of the atria
Ventricular systole – contraction of the
ventricles
Complete cardiac diastole – relaxation of the
atria and ventricles.
HEART SOUND-
There are four heart sounds, each corresponding to a
particular event in the cardiac cycle. The first two are most easily
distinguished, and sound through the stethoscope is like “lub dup”. The first
sound S1, ‘lub’, is fairly loud and is due to the closure of the
atrioventricular valves. The second
sound S2, ‘dup’, is softer and is due to the closure of the aortic and
pulmonary valves. S3 is due to blood turbulence during rapid ventricular
filling, and S4 is due to blood turbulence during atrial systole.
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