The volume of blood in our body is limited, but it has to perform unlimited work continuously. This leads to one conclusion that, the same quantity of blood must be used over and over again. In other words, blood must circulate.

Blood does not come into free contact with the tissue cells. It flows through a system of closed channels, known as the Circulatory System. Heart acts as the central pump. Blood gets reduced in the tissues and oxygenated in the lungs.

Consequently, it has to pass alternately through lungs and tissues, doing opposite function at these two places. Hence circulatory system has been divided into two parts. The Systemic Circulation passing through the tissues and the Pulmonary Circulation passing through the lungs. The two systems meet in the heart.

Function of Circulation:


It helps blood to do its work properly in two ways.

(i) By maintaining an efficient circulation, blood supply to every part of the body can be assured in rest as well as in activity;

(ii) By maintaining an optimum blood pressure it helps in capillary exchange;

(Front View of the Heart, Great Vessels, a, d Lungs)


The Lungs have been drawn aside in order to show l the other structures fully. I he outer layers of the pericar­dium and the pleurae have been removed.

R. V.-Right Ventricle; L. V. Left Ventricle; R A – Right Auricle; A.A -Left Auricle; A-Aorta; PA -Pulmonary Artery; EP. V.-Pulmonary Veins; R. L.-Right Lung ;L.L.Lef Lung; Lung; V. S.-Vega cava Superior; S. C -Subclavian vessels; C-Carotids; R. J. V and L. J. V. Right and Left Bronchi’s.B. P. V-Branch of Pulmonary Vein.

Structure of the Heart:

The heart is a muscular organ situated within the chest cavity and is inclined more to the left than eye right side The pointed end of the heart, called the apex, may be felt beating a little below the left nipples between the fifth and sixth ribs, The heart has got four chambers which are separated from each other by partitions.


The two upper cavities are called the right and left auricles, and the two lower the right and left ventricles respectively. The right auricle opens into the right Ventricle, and the left auricle into the left ventricle.

Each of those opening is provided with little valves or doors, which open and shut alternately and helps blood to pass in one way from upper chamber to lower chamber smoothly, From the left ventricle the Aorta arises which carries oxygenated blood to the tissues.

From the right ventricle the pulmon­ary artery arises, which carries reduced blood to the lungs. The right auricle receives all the venous blood from the body through three veins. The superior venae cave carry blood from the upper part of the body the inferior venae cava from the lower Part of the body and the coronary sinus from the heart, the left auricle receives all the oxygenated blood from the lungs through the pulmonary veins.

Mechanism of Circulation:


The four chambers of the heart perform four different Functions. The left ventricles propel oxygenated blood to the tissues. Here, it gives up oxygen and becomes reduced. The reduced blood comes back to the heart through the vein and is received by the right auricle.

From the right auricle it passes into the right ventricle, which then propels into the lungs. Here, it becomes re-oxygenated, and is returned to the left auricle through the pulmonary veins; from here it enters the left ventricle and is pumped on into the greater circulation again. In this way circulation goes on the systematic circulation, therefore, begins in the left ventricle and ends in the right auricle.

The pulmonary circulation starts in the right ventricle and ends in the left auricle. The right half of the heart is concerned with venous blood; while the left half with arterial blood. When auricles contract, the right auricular, ventricular valve or tricuspid valve as well as the left auricle ventricular valve or bi-cusped valve open and blood passes into the ventricles, when ventricles contract, auricle-ventricular valves close.

But semi-lunar valves present in the openings of the aorta and pulmonary artery open. This prevents regurgitation; of blood in to the auricles but allows it to flow out of the ventricles. In this way circulation becomes one way.


Heart is a miracle of constants. The two ventricles contract simultaneously, and also the two auricles. The same amount of blood passes out of the ventricles at the same time during contraction or systole. The same amount of blood enters the heart at the same time during relaxation or diastole. Any discrepancy in the time or in the quantitative relations may ultimately cause heart failure.