Blood is a fluid connective tissue. There are two parts in blood, viz. 

Plasma and formed elements:

Plasma: Plasma composes about 55% of total blood. About 92% of plasma is composed of water and rest is composed of organic and inorganic substances.
Formed elements are composed of cells which are as follows:

Red Blood Cells or Red Blood Corpuscles (RBCs): RBCs comprise 99% of blood cells. RBCs contain a protein called haemoglobin which imparts red colour to them. They lack nucleus. The life of RBCs is 120 days.
White Blood Cells or White Blood Corpuscles (WBCs): These are colourless as haemoglobin is absent in them. They provide immunity to the body and fight against diseases. WBCs are produced in bone marrow. There are two types of WBCs, viz. granulocytes and agranulocytes.

• Granulocytes: Neutrophil, eosinophil and basophil are the granulocytes. Neutrophils are largest in number.
• Agranulocytes: Lymphocytes and monocytes are the agranulocytes. Lymphocytes are of three types, viz. B-lymphocytes, T-lymphocytes and natural killer cells. Lymphocytes provide immunity against diseases. Monocyte becomes macrophage after attaining maturity. Neutrophil, lymphocyte and macrophage are the prime defenders and eat away antigens.
• Platelets: They are also known as thrombocytes. There are about 3 lakh platelets per cubic mm of blood. The lifespan of platelets is of 10 days. Platelets facilitate clotting of blood.

Importance of Blood:

• Blood transports different substances in the body.
• It controls the temperature of body.
• It maintains pH of body.
• It plays an important role in immunity