Circulatory System : Composition and Functions of Blood

Circulatory System

Filling the petrol tank of a motor car is of no use, unless there is a pipeline to carry the petrol from the tank to the engine. Similarly, digesting food in alimentary canal and absorbing O2 from external environment in respiratory organs shall be of no use to an animal unless the digested nutrients and O2 are transported from these organs to all the body cells where these are actually needed and utilized. We are going to discuss about Circulatory System.

Circulatory System

That is why all the higher multicellular animals where there is complete division of labour, possess an extensive pipeline system called circulatory system, for transporting materials between various parts of the body via Circulatory System.

Circulatory System also helps in exchanging materials from the external environment as well as in the elimination of metabolic wastes, All humans have blood, an amazing fluid, circulating in their bodies.

Circulatory System constitutes distributing system (to supply substances) as well as a collecting system (to pick up substances) to and from various parts of the body including the remotest cells. All the parts of the body that help in the transportation of various materials in the body, collectively constitute the circulatory system.

In animals it consists of three major parts :

  1. A fluid transport medium (blood and lymph). 
  2. Controlling centre - the heart. 
  3. Path through which transport medium circulates - The blood vessels and the lymph vessels.

Fluids in Circulatory System

In all, 40 litre fluid is present in the body of a fully grown up human, 25 litre called intracellular fluid is present within the cells. The remaining 15 litre called extracellular fluid is present in various spaces in the body.
The extracellular fluid that forms the transport medium is of three principal types :
(i) Blood : The remarkable red coloured fluid that keeps on circulating through the heart and various blood vessels (arteries, veins and capillaries).

(ii) Tissue fluid : Colourless fluid occupying spaces between various cells in the organs. As the blood flows in the capillaries of tissues, the plasma and WBCS move out through their walls. This fluid bathes the cells and is called as tissue fluid. The cells absorb oxygen from this fluid and gives out CO2 and wastes back into it.

(iii) Lymph: It is a faint yellow coloured fluid that is contained in the lymph vessels and lymphatic organs (like spleen and tonsils). Most of the tissue fluid enters these minute channels (lymph vessels) where it is called lymph.

The lymph vessels on the way bear lymph nodes. Various lymph vessels unite to form two lymphatic trenks. They pour lymph into the nearby veins which open into the right auricle. Lymph is again in general circulation. Lymph consists of only WBCS (mostly lymphocytes). No blood platelets are present in them. RBCS are also absent hence not red in colour.

In Circulatory System, Lymph supplies oxygen (dissolved in plasma) and nutrition to those parts where blood cannot. reach. Lymphocytes and monocytes of the lymph defend our body by removing bacteria from the tissues. These three principal fluids are intercon- vertible. There is a relationship between the blood, lymph and tissue fluids as they circulate through their respective vessels and interstitial spaces.


Blood is bright red (when oxygenated) and dark red (when deoxygenated) coloured fluid. It forms about 8 – 10 % of the body weight of a vertebrate. It is saltish in taste, slightly alkaline (pH 7.4) in nature, heavier than water and viscous sticky fluid. It is the most important part of Circulatory System.

It always keeps on circulating through blood vessels. The heart pumps blood into arteries that carry it to various organs. After exchange of various materials with the organs it is returned back to the heart through veins. Blood is required by every body organs and systems.

Digestive system digests and absorbs nutrients which get transported to every body cell through blood. Oxygen absorbed from the lungs is carried to all parts of body by blood, and on the other hand, carbon dioxide collected from entire body has to be carried to lungs for giving it out.

Excretory products are also transported by blood to kidneys for filtration and urine formation. Hormones are circulated to whole body through blood.

Composition of blood 

Blood consists of two major parts:

  •  Plasma - the fluid part of the blood  
  •  Formed (cellular) elements : RBCS, WBCS and platelets found floating in the blood 

(A) Plasma : 

It is the liquid portion of the blood. It is slightly yellow coloured and alkaline. It forms 50 60% of the total blood, It is sticky and slightly heavier than water. 90 – 92% of plasma is made-up of water, 7-8% proteins, 1% inorganic salts and other substances in traces.

Inorganic salts include sodium chloride and sodium bicarbonate. Other and substances contained in plasma are blood proteins (albumins, globulins, fibrinogen and prothrombin), glucose, amino acids, fatty acids, urea, uric acid, ammonium salts, hormones and gases like O2 and CO2.

(B) Formed (cellular) elements

Formed (cellular) elements are shaped structures visible under the microscope as cells and cell-like structures present in the blood. They form 42 – 45% of blood. These are of three types i.e., (1) Red Blood Corpuscles or Erythrocytes. (2) White Blood Corpuscles or Leucocytes. (3) Blood platelets or Thrombocytes.

Functions of Blood: 

  1. Transport of digested food : Glucose, amono acids, vitamins, mineral salts, etc. are supplied from alimentary canal to all body cells by way of blood. 
  2. Transport of oxygen : About 3% of oxygen supplied to body cells is transported in dissolved state in the plasma of blood. Rest is transported by means of red blood cells in combination with haemoglobin in the form of an unstable compound oxyhaemoglobin, which on reaching the tissues break up to deliver oxygen. 
  3. Transport of carbon dioxide : It occurs partly in combination with haemoglobin and partly as solution in blood plasma. 
  4. Transport of metabolic wastes: Various nitrogenous waste products are transported to kidneys by way of blood for elimination. 
  5. Transportation of hormones : These are secreted by endocrine glands, directly into blood.
  6.  Transportation of heat : Blood distributes heat, keeping the temperature of the body constant. 
  7. Homeostasis : Maintenance of the internal environment constant is called homeostasis. Blood helps to maintain the internal environment steady by maintaining different salts and organic substances in constant amount.
  8. Protection : Blood helps in forming a clot wherever there is a cut in blood vessel, in order to prevent further loss of blood and entry of disease causing germs in the body. 
It was all about Circulatory System.

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