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The Heart

Heart

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 to contract.

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 tissue.

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 the heart.

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.

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