The most important feature of the blood
is its incessant movement. The main effector of the
blood flow is the heart. The heart is a hollow muscular
organ about the size of
the owner’s fist and shaped like a blunt cone.
It is located centrally between the lungs with nearly
two-thirds of its mass towards the left of the midline.
The heart is enclosed and held in position by the
pericardium which is a connective tissue membrane
consisting of two layers. Between the pericardial
layers is a fluid filled cavity that prevents friction
between the two surfaces.
The wall of the heart is divided into three layers
– epicardium (outer layer), myocardium (middle
layer) and endocardium (inner layer). The muscle of
the heart called the cardiac muscle is characterised
by the capacity to contract rhythmically in response
to self-generated impulses.
Hence two types of muscle fibers are recognised in
the heart – the myocardium that insures the
contractibility and the second type to which the pacemaker
belongs. This type of fiber generates and transmits
impulses. The pacemaker is a small region of muscle
in the upper, posterior wall in the right atrium.
The SAN triggers an impulse that causes both atria
to contract. Very quickly the impulse reaches the
AV node at the bottom of the right atrium. Immediately,
the AV node triggers an impulse that causes both ventricles
Chambers of the heart.
The interior of the heart is divided into four chambers
that receive the circulating blood.
Atria (auricles) – the two superior chambers are
called right and left atria. They are separated by a partition
called the inter-atrial septum. Located in the upper wall
of the right atrium is the sinoatrial node (SAN) or the
pacemaker. The pacemaker is a specialised tissue that regulates
the contraction and relaxation of the heart muscles.
Ventricles – the two inferior chambers of the heart
are the right and left ventricles. They are separated from
each other by the interventricular septum. The atria and
ventricles are separated by connective tissue that also
forms the valves. The muscular layer of the walls of the
ventricle especially of the left ventricle is more developed
than the walls of the atria. The different muscular development
is related to their different functions. The left ventricle
pumps blood into the aorta under high pressure and also
against the force of gravity. As such they are adapted to
withstand greater pressure.
Valves of the heart.
Valves are muscular flaps that prevent the blood that has
once passed through it, to flow back through it. The opening
and closing of the valves is due to the pressure difference
across the valves. The two types of heart valves are distinguished.
The atrio ventricular valve – these valves separate
the atria from the ventricles. They consist of fibrous tissues
that grow out of the heart. The pointed ends of the flaps
project into the ventricles. Tendon like fibrous chords
called chordae tendinea connect the pointed ends to small
conical projections called papillary muscles, located on
the inner surface of the ventricles. The right side of the
heart posses a tricuspid valve that prevents the back flow
of blood into the right auricle during the contraction of
the right ventricle. Located on the left side of the heart
is a bicuspid valve consisting of two flaps.
Semilunar valves – these are three half-moon shaped
folds. They are located on the arteries leaving the heart.
They prevent the blood from flowing back into the heart.
The pulmonary semilunar valve lies at the opening of the
pulmonary trunk while the aortic semilunar valve lies at
the opening of the aorta.
Blood flow through the heart.
The right atrium receives deoxygenated blood from all the
parts of the body. When the right atrium is full of blood
it contracts, the tricuspid valves open under pressure and
the blood is delivered into the right ventricle. When the
right ventricle is full of blood the ventricle contracts
and the blood is pumped into the pulmonary trunk. This blood
then gets oxygen from the lungs. The oxygenated blood returns
to the heart via the pulmonary veins that empty into the
left auricle. When the left auricle contracts the blood
is passed into the left ventricle through the opening of
the bicuspid valve. On the contraction of the left ventricle
the blood is pumped into the aorta which then supplies it
to all the parts of the body.
The Lymphatic System
The lymphatic system consists of a fluid called lymph,
vessels that transport lymph called lymphatic vessels and
a number of structures and organs that contain lymphatic
Working of Lymphatic System
Under pressure from the heart fluid seeps through the porous
walls of the capillaries carrying nutrient to tissues and
absorbing waste matter. These fluids called tissue fluids
contain proteins, salts and water. The salts and water pass
back into the veins but the protein cannot directly re-enter
the venous system. If these proteins accumulate, their excess
concentration may damage the surrounding tissue. The proteins
have to be distributed to other parts of the body where
they may be required for growth and repair.
The lymphatic vessels originate as microscopic, blind-ended
vessels in spaces between the cells. With blotter like action
the lymphatics soak up the protein-laden filtrate. Tiny
flap like valves on the interior of lymphatic vessels prevent
any backward movement of the fluid. Unlike the blood stream
that depends on the pumping action of the heart, the lymph
has no driving force behind it. Its movement depends on
the body’s breathing and muscular movements. The lymph
vessels also wind up around the aorta and utilise its powerful
pulsations to propel the lymph.
Usually the lymph moves at a sluggish pace but exercise,
deep breathing and massage can speed up its movement. Though
it has its own circulatory system the lymph proceed towards
the heart and pour into the vena cava just before it enters
Lymph Nodes And Glands.
Scattered throughout the body usually in groups, these
oval shaped structures act as filtering stations. The lymph
passing through the nodes is filtered of foreign substances,
trapped by the reticular fibers within the nodes. The nodes
also contain lymphocytes that destroy microbes either by
phagocytosis or by releasing antibodies. A large number
of lymph nodes are located in the armpit and the groin.
These nodes localize infection, for example, there is an
infection in the hand that may cause painful swelling in
the lymph glands in the armpit.
Sometimes the trapped bacteria infect the lymph nodes causing
acute inflammation. This is commonly observed in the case
of tonsils, which are multiple aggregation of large lymphatic
nodules embedded in a mucous membrane. It then becomes necessary
to remove these glands. The spleen is the largest mass of
lymphatic tissue in the body. It is located in the region
between the fundus of the stomach and the diaphragm. It
produces antibodies, phagocytizes bacteria and worn out
or damaged red blood cells and platelets.