It is a hollow muscular organ of approximately the size of one’s own first. It consist of four chambers called the right atrium, left atrium, right ventricle and left ventricle. It lies in the middle mediastinum, within a pericardial sac and rest on the superior surface of the diaphragm. Great vessels look upwards, diaphragmatic surface downwards, and apex will be directed downwards, forwards and to the left. Size and weight of the heart of human circulatory system are as follows — Base to apex – 12 cm, transverse diameter – 9 cm, anterioposterior diameter – 6 cm. The weight of heart varies from 280 to 340 gm. It is heavier in males than that of females.
APEX – The apex is formed only by left ventricle and it’s direction is downwards, forwards and to the left. Apex is situated at the left 5th intercostal space about 9 cm away from the middle line. It lies 4 cm deep from the skin.
BASE OF THE HEART – It is the posterior surface of the heart, formed mainly by the left atrium and partly by the right atrium. It directed backward to the right. On standing position the heart lies against T6 – T9 vertebrae and on supine position it lies against T5 – T8.
SURFACE OF THE HEART – The sternocostal surface is formed by the left and right ventricles, of which left contributes 1/3rd and the right 2/3rd, Infundibulum, right atrium with auricle and the left auricle. It is covered by two layers of the serous pericardium, and fibrous pericardium which forms the posterior boundary of the anterior mediastinum. The surface is overlapped by both of the lung with pleura. An area on the right ventricle is not covered by the lung, due to cardiac notch at the anterior border of the left lung. The diaphragmatic surface Is formed by both the ventricles of which right contribute 1/3rd and the left 2/3rd. It lies with pericardium on the central tendon and a portion of muscular part of the diaphragm, peritoneum, liver fundus of the stomach. The Left surface Is formed by the left auricle and left ventricle. And the right surface is formed by the wall of the right atrium. It is covered by pericardium and overlapped by mediastinal surface of right lung with pleura.
RIGHT ATRIUM – It is the upper right chamber of the heart with a quadrilateral shape. It forms the sternocostal surface of the heart, right border of the heart and the right portion of the base of the heart. Anteriorly, laterally and posteriorly it is covered by the two layers of serous pericardium and fibrous pericardium. A smooth muscular ridge namely the crista terminals Is on the lateral wall of the interior of right atrium of heart of the human respiratory system separating the rough part of the right atrium from its smooth part which is developed from the right horn of the sinus venosus. It starts from the upper part of the atrial septum, passes anterior to the opening of the superior vena cava, which descends on the lateral wall of the right atrium up to the orifice of the inferior vena cava.
Opening of the superior vena cava is the devoid of valve and opens into the upper and posterior part of the right atrium opposite the right sternocostal junction. The opening of the inferior vena cava opens into lower and posterior part of the right atrium at the level of the 6th sternocostal junctiont. This opening is guarded by a semilunar valve called the valve of inferior vena cava , whose right end is continuous with the crustal terminals.The right atrioventricular orifice is guarded by a fibrous ring, oval shaped from which muscles of the heart arise and it is called the skeletal element of the heart.
RIGHT VENTRICLE – It contributes right portion of the sternocostal and diaphragmatic surface and inferior border of the heart. The upper part of this ventricle forms a smooth conical pouch called the Infundibulum which is continued as the pulmonary trunk. Interior of the right ventricle consist of an upper smooth part called the Infundibulum and lower rough part, separated by the muscular ridge called supraventricular crest.
LEFT ATRIUM auricle projecting from its upper part. The smooth part is the incorporated common portions of the pulmonary veins of the two sides. Inside the left atrium there have the opening of the four pulmonary veins, the left atrioventricular orifice with bicuspid valve and a semilunar depression called fossa lunata situated on the surface of the atrial septum looking towards the left atrium.
LEFT VENTRICLE – It contributes the left portion of the sternocostal and diaphragmatic surfaces, left surface and border, and apex of the heart. In the interior of the left ventricle there have the left atrioventricular orifice with bicuspid or mitral valve, the aortic orifice with valve, the aortic vestibule, and the rough part.
ORIFICE OF HEART WITH VALVES – 1) Opening of the superior vena cava. It has no valve. (2) Opening of the inferior vena cava with valve. (3) Opening of the coronary sinus with valve. (4) Right atrioventricular orifice with tricuspid valve. (5) Pulmonary orifice with valve. (6) Orifice of four pulmonary veins. (7) Left atrioventricular orifice with bicuspid valve. (8) Aortic orifice with valve.
RIGHT ATRIOVENTRICULAR ORIFICE WITH VALVE – It is an aperture communicating the right atrium with the right ventricle. It is oval in shape and surrounded by a fibrous ring from which muscles of the heart arise, and to which tricuspid valve is attached. Cusps are triangular and guard the right atrioventricular orifice.
PULMONARY ORIFICE WITH VALVE – this orifice lies at the junction of the pulmonary trunk and infundibulum of the right ventricle. Its swhape is circular. Pulmonary cusps are three in number, two anterior and one posterior. Each cusp is semilunar and formed by duplication of the endocardium containing only fibrous tissue.
ORIFICE OF FOUR PULMONARY VEINS – There are two right and two left pulmonary veins carrying oxygenated blood from the corresponding lung open into the left atrium.
LEFT ATRIOVENTRICULAR ORIFICE WITH MITRAL VALVE – This orifice make the communication with the left atrium to left ventricle. Cusps of the mitral valve are two in number called the anterior and posterior.
MITRAL VALVE – It comprises the mitral orifice with fibrous ring, The mitral valve, chordae tendineae and the papillary muscles.
AORTIC ORIFICE WITH VALVE – this orifice is situated at the junction of the ascending aorta and aortic vestibule. It is circular in shape.