(i) To carry oxygen in the form of oxyhemoglobin from the respiratory organs to all cells of the body.

(ii) To carry carbon dioxide in the form of potassium bicarbonate from the tissue cells to the respiratory organs.

(2) Leucocytes or white blood corpuscles

Fig. 18.10 Types of Leucocytes

White blood corpuscles are less numerous, about seven thousand per cubic millimeter of blood, i.e. the ratio of RBCs to WBC (White Blood Cells) is about 600:1. They are colorless, nucleated and migrate anywhere in the body. Leucocytes are produced in spleen or yellow bone marrow and when worn out, they are degraded in the blood stream and engulfed by other phagocytes. Some leucocytes are also destroyed by microorganisms at the site of invasion (infection). These destroyed leucocytes, along with damaged tissue cells with some plasma, form a yellowish liquid called pus, causing inflammation or abscess. During infection the number of leucocytes in blood increases.

A chronic abnormal increase in leucocytes can lead to leukemia or blood cancer inhibiting the normal production of RBCs.

Functions of leucocytes: (i) They are mainly phagocytic in function ingesting bacteria, cell debris and other injurious materials occurring in blood.

(ii) They produce antibodies and antitoxins which are specific in action.