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| Heart - Shape and Position |
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| It is a thick muscular, reddish brown, conical organ present in the mediastinal space of thoracic cavity between 2 pleura enclosing the lungs. Its broader side is called base and it is forward and upward while the pointed side called apex is backward and downward. It is 9 cm broad and 12 cm long and about 300 gms in weight. |
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| Protective covering |
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| The heart is enclosed in a tough sac called pericardium. It is made up of an inner pericardium and an outer fibrous pericardium. The 2 layers have between them a very narrow space, the pericardial cavity filled with a watery coelmic fluid called pericardial fluid. |
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| The pericardial fluid keeps the heart moist and prevents friction between the heart wall and the surrounding tissues during the heart beat. The fibrous pericardium is formed of tough, white fibrous tissue which protects the heart from mechanical injury and checks its over stretching or overfilling with blood. |
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| External view of a Mammalian Heart |
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| The heart is a 4 chambered muscular pump located inside the chest or the thoracic cavity. It is a mesodermal derivative and is a myogenic heart. The heart is surrounded externally by a thin, transparent 2 layered serous sac called pericardium. The narrow cavity between the 2 layers is called the pericardial cavity which is filled with a watery fluid called the pericardial fluid. The fluid performs 2 functions |
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| a) it allows frictionless movements of the heart and |
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| b) it protects the heart from mechanical shocks. The wall of the heart is primarily made up of cardiac muscles called myocardium. |
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| The heart is formed of 4 chambers, namely 2 auricles and 2 ventricles. The auricles are named as right and left auricles and the ventricles are right and left ventricles. |
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| A groove is present externally between the auricles and the ventricles called the coronary sulcus. |
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| The ventricles also home two grooves present on them called anterior interventricular sulcus and posterior interventricular sulcus. These sulci receive coronary arteries through which the heart receives the blood. |
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| The right and the left auricles are separated by a fibrous partition called interauricular septum. Both the auricles have very thin walls because they have to push the blood only into the ventricles. The inner surface of the auricles is very smooth except for a network of low ridges called the musculi pectinati |
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| Diagram of Heart cut open |
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| The right auricle receives deoxygenated blood through the 2 major veins, namely the inferior vena cava, superior vena cava and the coronary vein. The left auricle receives oxygenated blood from the lungs through the pulmonary veins. |
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| An oval depression is present on the interatrial septum called the fossa ovalis. It is actually the remnant of foramen ovale, which is an opening in the interatrial septum of the foetal heart, which closes at the time of birth. |
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| The auricles are separated from the ventricles by an auriculo ventricular septum. The right auricle opens into the right ventricle by a right auriculo ventricular aperture. The left auricle opens into the left ventricle by a left auriculo ventricular aperture. The left auriculo ventricular aperture is guarded by a valve called bicuspid valve. It has two cusps (flaps) and hence the name bicuspid. The right auriculo ventricular aperture is guarded by a tricuspid valve containing 3 flaps. Both the tricuspid and the bicuspid valves are fastened to small conical muscles called the papillary muscles on the ventricular wall through several tendinous strands called the chordae tendinae. |
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| Section through Mammalian Heart |
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| The ventricles have thicker walls than the auricles. The walls of the left ventricle is about 3 times as thick as that of the right ventricle. This is because the left ventricle has to pump the blood to the farthest end of the body, while the right ventricle has to send the blood only to the nearby lungs. |
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| Two main blood vessels carry blood from the ventricles. One large aortic arch (aorta) carries blood from the left ventricle to the various parts of the body except the lungs. The pulmonary artery takes blood from the right ventricle to the lungs. |
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| The base of the aortic arch and the pulmonary artery are guarded by semi lunar valves. Each semi lunar valve is made up of 3 half moon cusps (flaps), attached to the wall of the aorta by one border with the curved edge free inside the lumen of the aorta. They open during ventricular systole and close during ventricular diastole. They allow the blood to flow into the aorta from the ventricles and the reverse flow is prevented. |
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| On the right wall of the right auricle is the sino-auricular node or SA node. It represents the sinus venosus which has completely merged into the wall of the right auricle. It is called as the pacemaker as the cardiac impulse originates from here and it determines the rate of heart beat. |
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