Class 11 Biology Body Fluids Circulation Blood


  • Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements.


  • Plasma is a straw coloured, viscous fluid constituting nearly 55 per cent of the blood.
  • Fibrinogen, globulins and albumins are the major proteins of plasma.
  • Fibrinogens are needed for clotting or coagulation of blood, globulins are involved in defense mechanisms of the body and the albumins help in osmotic balance.
  • Plasma contains small amounts of minerals like Na+, Ca++, glucose, amino acids etc.
  • Factors for coagulation of blood are present in the plasma in an inactive form.
  • Plasma without the clotting factors is called serum.


Fig. Plasma in blood 

Formed elements

  • Erythrocytes, leucocytes and platelets are collectively called formed elements.
  • Erythrocytes
  • Erythrocytes or red blood cells (RBC) are formed in the red bone marrow in the adults, are devoid of nucleus with a biconcave shape.
  • Erythrocytes are known as red blood cells because these have a red coloured, iron containing complex protein called
  • Haemoglobin molecules play a significant role in transport of respiratory gases.
  • RBCs have an average life span of 120 days after which they are destroyed in the spleen, known as the graveyard of RBC.
  • Leucocytes
  • Leucocytes are also known as white blood cells (WBC) as they are colourless due to the lack of haemoglobin.
  • They are nucleated and short lived.
  • There are two main categories of WBCs
  1. granulocytes
  • Neutrophils, eosinophils and basophils are different types of granulocytes, while lymphocytes and monocytes are the agranulocytes.
  • Neutrophils are the most abundant cells of the total WBCs and basophils are the least among them.
  • Neutrophils and monocytes are phagocytic cells, basophils secrete histamine, serotonin etc,.and are involved in inflammatory reactions.
  • Eosinophils resist infections and are also associated with allergic reactions.
  • Lymphocytes are of two major types: ‘B’ and ‘T’ forms.
  • Both B and T lymphocytes are responsible for immune responses of the body.


Fig. White blood cells 

  • Platelets
  • Platelets also called thrombocytes, are cell fragments produced from megakaryocytes.
  • Megakaryocytes are the special cells in the bone marrow.
  • Platelets can release a variety of substances most of which are involved in the coagulation or clotting of blood
  • A reduction in their number can lead to clotting disorders which will lead to excessive loss of blood from the body.


Fig. Platelets

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