The human circulatory system consist of the heart and blood vessels. Heart is situated inside the thoracic cage and consists of four chambers. The four chambers are two atria and two ventricles. The right atrium receives impure blood through large veins from the upper and lower regions of the body. Through an opening namely atrioventricular opening the right atrial blood passes into the right ventricl From the right ventricle arises the pulmonary trunk. The pulmonary trunk divides and breaks up into capillaries in the lung. Due to contraction of the right ventricle, the blood is carried to the lungs. In the alveoli of the lungs the impure blood gives of carbon dioxide and takes up oxygen and thus the gas purification is done. The pure blood coming from the lungs is carried through atrioventricular opening to the left ventricle. A large artery known as the aorta arises from the left ventricle. The left ventricle contracts and the pure blood is pumped out to the aorta. The aorta and its atrial branches which break up into capillaries carry the pure blood to the head and neck regions, superior and inferior extremities, thorax, abdomen and trunk. The veins carries the impure blood from different regions of the body provided valves into the right atrium. This course of human circulatory system is called major circulation. The right atrium sends blood to the right ventricle and the cycle is repeated. The amount of blood ejected per ventricle per minute is about 5 litres and which is known as cardiac output. Heart beats from 70 to 80 times per minute.
BLOOD AND BLOOD VESSELS: – It is well known to us that the blood is the most important fluid in our body. So in human circulatory system the blood plays most important role. In a human body there has a quite bit of blood that is about 5 litres, weighing about 1/13 of the total body weight. The blood carries nutrition and oxygen to all the cells and tissue of the body. It is not possible without this vital fluid to feed all of the organs of the body. Blood is a salty taste red fluid. Apparently it is a red fluid, but when viewed in the microscope, it is seen to contain different types of cells floating in a liquid called plasma. The cells are known as red blood cells, and white blood cells which are divided further into different type of cells. The quantity of red blood cells which are also known as erythrocytes in a healthy person is about 4 to 5 million per cubic millimetre of blood. The red colour of the blood is due to a pigment which contains iron named the haemoglobin . Without the red blood cells it will be impossible for the blood to carry oxygen which is mainly due to to the haemoglobin which have a property to absorb oxygen in the lungs.
The white blood cells in human circulatory system which are medically known as leucocytes, and number of which varies from 5000 to 9000 per cubic millimetre. The lymphocyte perform the protective function of the body, that means, the destroy bacteria and other harmful particles which may damage or interrupt the function of the body or the organs of the body. Blood also contains varying amounts of salts like potassium, sodium etc. and also some carbohydrates proteins etc.
The blood vessels which carry the pure oxygenated blood to different organs are called the arteries , where as the blood vessels which bring impure blood from different organs called the veins. One of the important phenomena associated with this important fluid is that of clotting of the blood. The body which take the responsibility of clotting the blood called the platelet . The substance called fibrinogen in the blood plasma for which due to a cut or injury would leads to continuous oozing of blood from the blood vessels leading ultimately to death. Platelet is the substance which is marvel in nature changes into fibrin after coming in contact with the air and entangles the blood cell into a clot which is very important for the human circulatory system.
HEART : – 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/b>.
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. There are novalves.
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.
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