Our circulatory system consists of the heart, blood vessels and blood. Together, these are responsible for the transport of materials to and from different parts of the body. The materials are carried by blood, an extra cellular fluid which flows throughout the body within blood vessels. The flow of extraneous fluid in an orderly manner is called circulation. The heart regulates circulation by pumping blood, and thus causing it to move through the vessels.

Function of the circulatory system

The circulatory system performs the following function in human beings.

a) It transports digested food from the site of absorption o the sites where it is stored and utilized.

b) It carries oxygen from the lungs to the tissues, and the carbon dioxide produced in the tissues to the lungs.

c) It carries the waste products formed during the body’s metabolic process to the excretory organs for removal from the body.

d) It also transports hormones, enzymes, and so on, to the where they are required.


As you know, blood consists of solid elements, like corpuscles, and plasma, the fluid matrix in which the corpuscles float.


This is a straw –colored solution of organic and inorganic substances. The main constituent is water, which accounts for 92% of plasma .the organic constituent include plasma proteins, blood sugar, cholesterol and urea, while the organic substances include gases and various salts.


These are also called red blood cells, or red blood corpuscles (RBCs). They are disc-shaped and look as if they have been pressed in form the sides. The diameter of an RBC measures 7.5 um and its thickness is 2um. Each mm cubed of human blood contain 5-6 million RBCs. They are produced continuously by our bone marrow, and have a lifespan of 120 days.

A mature RBC has no nucleus. Most of the cell is filled with hemoglobin, which consists of an ion-containing pigment, haem, and a protein called goblin. The pigment gives blood its red color.

Haemoglobin is vital for respiration. It combines with the oxygen entering the blood from the lungs, forming compound called oxyhaemoglobin. Through the blood the circulation, oxyhaemoglobin reaches the tissues, where the oxygen part of it is utilized for internal respiration. Haemoglobin also carries the carbon dioxide formed in the tissues during internal respiration to the lungs. It combines with the carbon dioxide to form carbaminohaemoglobin. When this reaches the lungs, the carbon dioxide is realized and the haemoglobin becomes free to combine with oxygen again.


These cells are commonly called white blood cells (W B Cs). They are larger than RBCs and are colorless. They have no definite shape and can change their contours like an amoeba. W B Cs has a nucleus and lives for 12 hours to 12 days. The number of W B Cs in our blood is much less than that of R B Cs (4000-1300o per mm cubed). Different types of W B Cs are produced in different sites.

Leucocytes defend the body against infection. Since they can change heir shape, they are able to reach any part f the body. Thus, they reach the site of infection, where they surround the germs and destroy them by digesting them. They product us from infections also by releasing antibodies, which inactivate foreign bodies entering the blood.


Thrombocytes, also called blood platelets, are minute, disc –shaped structures without a nucleus. They are formed in the bone marrow and live for 3-7 days. There are 200,000-400,000 platelets perm cubed of blood. Their main function is coagulation, or clotting of blood. Clotting, this is the thickening of blood, ensures that we do not lose much blood when we are injured.




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