Theory:

What is blood?
Have you seen the red coloured fluid that flows out when you have a cut? That is known as blood.
The red colour fluid which flows in the blood vessels of our body is known as blood.
Functions of blood
Blood serves as the medium of transportation for various substances like digested food from the small intestine to the other parts, oxygen from the lungs to the cells of the body. It also transports the wastes to the excretory system for its removal from the body. It also plays a significant role in the regulation of body temperature.
Components of blood
The blood is capable of performing several functions due to its composition. The various components of blood are blood plasma, red blood cells, white blood cells and blood platelets.
 
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Plasma:
The liquid component of the blood in which the various cells are suspended is known as plasma.
Plasma is made up of  water, salt, fats, proteins and sugar. It is responsible for the transportation of various substances throughout the body.
 
Red blood cells:
  
They are also known as erythrocytes. They are known as red blood cells as they contain a red pigment known as hemoglobin. The oxygen inhaled by us binds to this pigment and is transported to all the cells of the body. Hence, it is known as the oxygen carrier. The blood appears red in colour due to the presence of this pigment.
 
White blood cells:
  
They are also known as leucocytes. These cells kill and fight against the germs which enter our body.
 
Blood platelets:
 
The blood platelets are the cells which prevent the blood from flowing out of the body during injury by forming clots.