Blood cells form about by volume of the blood. These include red blood cells, white blood cells and platelets.
a. Red Blood Cells
These are called erythrocytes. A single R.B.C is a circular flattened, biconcave disc and has no nucleus. RBCs have iron-containing pigment haemoglobin in its cytoplasm, which gives red colour to the blood. RBCs are formed in bone marrow after birth. Their life span is 120 days. When they are worn-out, they are destroyed in spleen and liver. RBCs carry oxygen and also transport a small amount of carbon dioxide.
b. White Blood Cells
White blood cells are called leukocytes. Their life span is 3-4 days. Each white blood cell is irregular in shape and contains a nucleus. WBC's can be divided into two main types: 1. Granular leukocytes 2. Aganular leukocytes
Granular Leukocytes
Their nucleus is variable in shape. Cytoplasm contains fine granules. These cells originate in bone marrow. Granular leukocytes are of three types, neutrophils, eosinophils and basophils.
Neutrophils: Neutrophils engulf pathogens during phagocytosis.