External structure of human heart:
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1. Shape and Size: A human heart is muscular hollow pumping organ which can maintain continuous flow of blood circulation inside the body. It is roughly triangular in shape. It is about the size of a person’s fist and weighs about 300 grams. It is reddish brown in colour. It is situated ventrally in the middle of thoracic cavity in between two lungs. The narrow end of the heart is slightly displaced to the left side.
2. External structure: The heart is enclosed in a double walled membranous sac called #pericardium. The inner membrane is attached to the heart. In between two membranes, there is presence of a #pericardial fluid. The fluid is shock absorbing so protect heart from any shock and minor injuries. It also allows free movement to the heart.
a) Auricles (Atria): Anterior broad part is called #auricular part and posterior narrow part is called #ventricular part. Towards right side of auricular part there is superior and inferior venacavae carrying impure blood from different parts of body except lungs. Auricles are thin walled chambers. The left auricle is smaller than right auricle. The left and right auricles are externally separated by a vertical inter-auricular groove.
Towards the left side of auricular part, the #pulmonary veins carry pure blood from lungs. The auricular part receives blood.
b) Ventricles: Ventricles are thick walled. The left ventricle is somewhat longer and about three times thicker than right ventricle. Right ventricle receives the impure blood from right auricle whereas the left ventricle receives pure blood from the auricle. There is no mixing of pure and impure blood.
c) Aorta and pulmonary trunk: An aorta arises from the left auricle. Aorta supplies pure blood throughout the body. The pulmonary trunk arises from the right ventricle that carry deoxygenated blood to the lungs.
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