- Erythrocytes: Erythrocytes (red blood corpuscles or RBC) are the most numerous of the formed elements of blood. Their most characteristic feature is the presence of hemoglobin, the red oxygen carrying pigment.
The total number of erythrocytes per microlitre(1 = 1mm3 = 10-6) of blood is known as the Total Countof RBC. It averages 5 millions and 4.5 millions in adult man and adult woman respectively. The total countwould be low in anaemia and after profuse bleeding. On the contrary, the abnormal rise in the total count – of RBC is called Polycythemia.
The size and shape of erythrocytes vary in different classes of animals. In fishes, amphibians, reptiles and birds erythrocytes are usually nucleated, oval and biconvex. But in mammals they are non-nucleated, biconcave and circular. Only camel and llama possess oval red blood corpuscles. Human erythrocytes measure 7-8 mm (1 mm = 10-6 m) in diameter and 2 mm thickness near the rim.
Old and damaged erythrocytes are phagocytosed and destroyed by macrophages. The pigment part porphyrin of hemoglobin is then catabolised to the yellow pigment Bilirubin which is excreted the bile. The pale yellow colour of plasma is largely due to bilirubin.
If a sample of blood is rendered non-coagulable by adding potassium or sodium oxalate and then centrifuged at a high speed in a graduated centrifuge tube (hematocrit tube), the centrifugal force rapidly sediments the erythrocytes to the bottom of the tube They become packed into a solid, red, bottom layer while plasma forms a clear, fluid layer above. On the upper surface of the erythrocyte layer, leukocytes form a thin, buff-coloured layer. From the graduations on the tube, the relative volume of erythrocytes maybe read as a percentage of the total blood volume. This is called the Hematocrit Value or Packed Cell Volume. It normally forms 45 percent of the blood volume.
- Leukocytes: Leukocytes (white blood corpuscles or WBC) are devoid of hemoglobin and are consequently colourless. Leukocytes are nucleated blood cells. They are of two major classes: granulocytes (with cytoplasmic granules) and agranulocytes (without granules). Granulocytes are of three types, viz. neutrophils, eosinophils and basophils, each with lobed nucleus. Agranulocytes are of two types, viz, lymphocytes and monocytes. Neutrophils and monocytes protect the body against microbes by phagocytosing them. Lymphocytes secrete antibodies in the blood to destroy microbes and their toxins. The number of leukocytes per microlitre (1mL =1 mm3 = 10-6) of blood is called the Total Count of WBC. This is normally 5000 in humans. It may rise abnormally in acute infections (e.g., pneumonia), inflammations (e.g. appendicitis) and malignancies (e.g., leukemia). In some conditions such as folic acid deficiency, the total count falls abnormally (leukemia). The total count of WBC is also of diagnostic value in many diseases. Monocytes have kidney shaped nucleus.