Blood is a fluid connective tissue.It is composed of plasma(55%) and formed elements/blood corpuscles(45%).
There are three types of formed elements(blood corpuscles):
1) Erythrocytes (red cells)
2) Leukocytes (white cells)
3) Platelets (thrombocytes)
Erythrocytes (RBC) - Most abundant type of blood cells. Their main function is transport of respiratory gases,mainly oxygen, but they also carry some carbon dioxide.
They also maintain the viscosity and pH of blood.
Leukocytes (WBC) - These cells have an important function in defence and immunity. They detect foreign or abnormal (antigenic) material and destroy it, through a range of defence mechanisms.
There are two main types:
Granulocytes (polymorphonuclear leukocytes)- neutrophils, eosinophils and basophils
Agranulocytes - monocytes and lymphocytes
Platelets (Thrombocytes) - These are very small discs, 2-4 micrometer in diameter. They are responsible for coagulation or clotting of blood.