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The Heart :-
The heart is a muscular organ responsible for pumping blood through the blood vessels by repeated, rhythmic contractions. The heart is composed of cardiac muscle, an involuntary muscle tissue which is found only within this organ.
In the human body, the heart is usually situated in the middle of the thorax with the largest part of the heart slightly offset to the left. The heart is enclosed by a sac known as the pericardium. The pericardium comprises two parts: the fibrous pericardium made of dense fibrous connective tissue; and a double membrane structure containing a serous fluid to reduce friction during heart contractions.
The right side of the heart contains de-oxygenated blood collected by the superior and inferior vena cava (which are large veins), drained into the right atrium, from the body and pumps it, via the right ventricle, into the lungs through the pulmonary artery - an exceptional artery since arteries usually carry oxygenated blood so that carbon dioxide is diffused into the alveoli and oxygen is diffused into the blood. This happens through the passive process of diffusion.
The left side contains oxygenated blood from brought to the lungs into the left atriumvia four pulmonary veins (exceptional veins since veins usually carry deoxygenated blood). From the left atrium the blood moves to the left ventricle which pumps it out to the body through the largest artery - the aorta. On both sides, the lower ventricles are thicker and stronger than the upper atria. The muscle wall surrounding the left ventricle is thicker than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the systemic circulation.


External structure of heart


Pumping of blood

Starting in the right atrium, the blood flows through the right atrio- ventricular aperture gaurded by the tricuspid valve to the right ventricle. Here it is pumped out through the pulmonary artery to the lungs for oxygenation. From there, oxygenated blood flows back through pulmonary veins to the left atrium. It then travels through the left atrio- ventricular aperture guarded by the mitral/bicuspid valve to the left ventricle, from where it is pumped to the aorta which is gaurded by the semilunar valve (valves allow the unidirectional flow of blood and prevent its backflow) The aorta forks and the blood enters the major arteries which supply the upper and lower body. The blood travels in the arteries to the smaller arterioles, then finally to the tiny capillaries which feed each cell. The deoxygenated blood then travels to the venules, which coalesce into veins, then to the inferior and superior venae cavae and finally back to the right atrium where the process began.


Transportation of blood in the heart


Transportation in Human Beings
On an average, our body has about 5 liters of blood continuously flowing through our body by way of the circulatory system. The important cellular components of the blood are RBC [erythrocytes] WBC [leucocytes] and Platelets [thrombocytes]. The heart, the lungs and the blood vessels work together to form the circulatory system. The pumping of the heart forces the blood to undertake its journey.
In a general sense, a vessel is defined as a hollow utensil for carrying something e.g. a cup or a bucket or a tube. Therefore, blood vessels are hollow utensils for carrying blood. Located throughout our body, our blood vessels are hollow tubes that circulate our blood.
There are three varieties of blood vessels: arteries, veins, and capillaries. During blood circulation, the arteries carry blood away from the heart. The capillaries connect the arteries to veins. Finally, the veins carry the blood back to the heart.
The body's circulatory system really has three distinct parts: pulmonary circulation (the lungs), coronary circulation (the heart), and systemic circulation (the rest of the system).

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