Blood is a mobile connective tissue composed of a fluid, the plasma and formed elements. Formed elements includes erythrocytes, leucocytes and platelets.


Blood corpuscles.
(a) Erythrocytes are most prevalent corpuscles (approx. 5 to $5.5$ million $/ \mathrm{mm}^3$ of blood). They contain carbonic anhydrase enzyme which help in transportation of $\mathrm{CO}_2$ and haemoglobin pigment which helps in transportation of $\mathrm{O}_2$.
(b) Leucocytes lacks of any pigment and most active motile constituents of blood (approx. 6000-8000/ $\mathrm{mm}^3$ of blood). They may be of two types.
1. Granulocytes are the cells containing granules and a polymorphic nucleus.
(i) Neutrophils : resposible for protection against infection.
(ii) Eosinophils : play important role in allergic reaction.
(iii) Basophils : significant in inflammatory reaction.
2. Agranulocytes are cells which lack granules
(i) Lymphocytes play a key role in immunological reactions.
(iii) Monocytes are phagocytic in nature.
(c) Platelets
There are involved in the coagulation or clotting of blood