Explain External & internal Heart
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The upper broader part of the heart is called atrium and the lower conical part ventricle. A coronary sulcus present on the surface of heart separates atrium from ventricle. A vertical inter atrial sulcus divides the atrium into a large right atrium and small left atrium . Similarly an oblique vertical inter ventricular sulcus divides the ventricle into right and left ventricles.
The cavity of the heart is divided into 4 chamber internally also. Right and left atria are present on the upper side while right and left ventricles form the lower side of the heart. Right and left atria are separated from each other by an interatrial septum. During foetal stage there is an aperture in the interatrial septum called foramen ovalis.
It close automatically after the birth and is represented as a depression called fossa ovalis, on the right side of inter atrial septum. Right atrium has an opening on the upper side for superior vena cava and lower side for inferior vena cava. Through these apertures deoxygenated blood from the upper and lower side of the body reaches right atrium. A Part form the openings of superior and inferior vena cavae right atrium has another opening on its left side through which deoxygenated blood from the wall of heart it self reaches right atrium through coronary sinus.
Four pulmonary veins bringing oxygenated blood from the lungs open into left atrium separately through four openings. Left and right atria collect the body and pump it to left and right ventricles respectively . As atria simply collect the blood they are thin walled. Ventricle is divide into left and right ventricles by an interventricular septum. Right ventricle contracts to pump the deoxygenated blood into lungs while left ventricle contracts to pump oxygenated blood to different part of the body.