The heart is a four chambered organ situated in the mediastinum within the thoracic cavity. Approximately two thirds of the heart lie on the left side of the midline. The apex of the heart (tip of left ventricle) lies at the level of the 5th intercostal space and points diagonally to the left hip. The base of the heart (atria / mainly left atrium) lies in the 2nd intercostal space towards the midline. The heart is about the same size as an adult’s closed fist and weighs approximately 300g in an adult. Tortora G. J. and Grabowski S. (1993)
The heart is surrounded by the pericardium, this is a three layered membrane consisting of : -
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Fibrous Pericardium - This is made of fibrous tissue and it serves to protect the heart, prevents overstretching of the myocardium and anchors the heart in the mediastinum. |
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Serous Pericardium - This is made up of a further two thin serous membranes. The outer (Parietal) layer is fused to the fibrous pericardium whilst the inner (epicardium) is fused to the myocardium. Between each of these membranes is serous fluid that prevents friction between the membranes as the heart moves. The space containing this fluid is known as the pericardial cavity. |
The heart also is made up of three layers: -
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Epicardium – Inner layer of the serous pericardium that is fused to the myocardium |
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Myocardium – Middle layer composed of cardiac muscle. |
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Endocardium – Within the heart and is made up of endothelium, it serves to provide a smooth lining within the heart and covers the valves of the heart. |
The endocardium is continuous with the endothelial lining of the large blood vessels and the rest of the cardiovascular system. Its purpose is to reduce friction thereby increasing flow.
There are four chambers of the heart: -
Right and Left Atrium – each of these contain an auricle that increases the atrial capacity. Blood enters the heart by the atria as will be described later.
Right and Left Ventricles – blood is fed to these by the atria, the ventricles are responsible for providing the pressure to pump the blood out of the heart. They are larger in capacity than the atria.
The right and left sides of the heart are separated by a septum (the interatrial separates the atria and the interventricular separates the ventricles).
The muscle mass of the atria is less than the ventricles because they only have to deliver blood to the ventricles. The muscle mass of the right ventricle is less than the left. The right ventricle is responsible for generating enough pressure to circulate blood around the pulmonary circulation. The left ventricle is responsible for generating enough pressure to pump blood around the peripheral and neurological circulation.
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Tricuspid Valve – This is found between the right atrium and right ventricle, it is composed of three leaflets and serves to prevent back flow to the right atrium during ventricular contraction. |
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Pulmonary Valve – This is found in the pulmonary artery where it exits the heart. It is consists of three half moon shaped leaflets and serves to prevent back flow into the right ventricle during ventricular relaxation. |
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Mitral Valve (Bicuspid Valve) – This is found between the left atrium and left ventricle. It is composed of two leaflets and serves to prevent backflow into the left atrium during ventricular contraction. |
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Aortic Valve – This is found in the aorta where it exits the heart. It consists of three half moon shaped leaflets and serves to prevent backflow into the left ventricle during ventricular relaxation. |