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Circulation in Animals Biology Help

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Circulation in Animals
Every structural unit of the biological machinery requires continuous supply of both food and oxygen to sustain life.To ensure constant supply of food and oxygen in different cells and to remove metabolic wastes,a system known as circulatory or vascular system has been devised.
Types of Blood Vascular Systems
The blood vascular system is mainly of two types:
(1)Open Circulatory System-It is found in invertebrates such as annelids(e.g. Leech),arthropods(e.g.Prawn),and molluscs(e.g.Snail).He re,the haemolymph does not remain confined to blood vessels but flows through open spaces and channels called sinuses or haemocoel in the tissues.In sinuses,the blood is in direct with the tissue cells.The flow of blood through open sinuses is very slow.
(2)Closed circulatory system-It is found in all vertebrates.Here,blood runs in closed vessels and never comes in direct contact with the tissue cells.Blood vascular system is a network of arteries and veins centrally connected with heart.
Blood Vascular System of Humans
HEART
It is situated almost in the middle of thoracic cavity between the lungs.Its lower conical portion is tilted to the left.It is hollow,muscular cone shaped organ.It weighs about 300 gm enclosed in a membranous sac called pericardium.Pericardium protects heart from injury and against friction.Percardial fluid keeps the heart moist.
Structure of the heart
The  human heart is four chambered having two auricles and two ventricles.Heart id distinctly divided into anterior smaller part i.e. auricles and the posterior larger part i.e. ventricles with the help of transverse groove called auriculoventricular groove.The right auricle is separted from right ventricle by tricuspid valve and left auricle is seprated from left ventricle by biscuspid valve.
Working of the heart
The deoxygenated blood from different parts of the body is poured into right auricle by three vena cavae.The left auricle on the other hand,receives oxygenated blood from the lungs by pulmonary veins.The auricles when filled,contract and force the blood into the respective ventricles through auriculo-ventricular aperatures.The tricuspid and biscuspid valve guide the blood into the ventricles.The blood does not return to the venae cavae and pulmonary veins as the wave of contraction,auricles being anterior passes on to them and then towards the ventricles thus pushing the blood into the latter.The contraction of ventricles exerts pressure on the enclosed blood.When the pressure in the ventricles exceeds the semilunar valves present at the base of aorta open up resulting in the flow of deoxygenated from right ventricle into aortic arch.The  pressure in the ventricles falls below the level prevailing in aorta resulting in the closing down of aperatures with the help of semilunar valves.The pulmonary  aorta carries the deoxygenated blood to the lungs,whereas aortic arch distributes the oxygenated blood to all parts of the body.

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