Grade 7, Science Olympiad (CBSE) - Circulatory system in humans 

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Grade 7  |   Science  |   Circulatory system in humans, Transportation in animals and plants, Olympiad, CBSE, ICSE, SOF, ITO

Circulatory system in humans

The blood circulatory system (cardiovascular system) delivers nutrients and oxygen to all cells in the body. It consists of the heart and the blood vessels running through the entire body.


Blood is a type of connective tissue which transports all the substances in our body. It is the Carrier of various substances. Some of the important functions performed by the blood are-

1. Transportation of oxygen and nutrients to the body cells.

2. Carries waste products away from the cells.

3. Carries hormones and chemicals wherever they are needed in the body.

4. Maintains the body's temperature to prevent it from getting too hot or too cold.

5. It is a vital part of the immune system.

Composition of the blood

There are four main substances in the blood.

1. Plasma

2. Red blood cells

3. White blood cells

4. Platelets


It is a fluid in which RBCs, WBCs, and platelets are transported. It forms the largest portion of the blood an is pale in colour. 

Blood cells

  • Red blood cells

RBC's contains a pigment called hemoglobin which binds oxygen and thus is responsible for the transportation of oxygen in the body. It also transports some amount of carbon dioxide. Hemoglobin consists of haem complex which contains iron. The red colour of  the blood is due to this haemoglobin only. When the oxygen enters our lungs the haemoglobin combines with the oxygen to form oxyhaemoglobin. The shape of a red blood cell is biconcave disc. Due to their small size and flexibility they easily move through capillaries and get closer to the cells. RBCs don't have a nucleus. 

  • White blood cells

They are responsible for killing the foreign particles and harmful microbes which enters the body. They protect us from various diseases and make the immune system of the body.


Platelets are responsible for clotting or coagulation of blood. You might have noticed that in a case of injury, the blood stops flowing by itself after sometimes. This happens because of the clotting of blood at the site of damage to prevent the excessive loss of blood. 

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