Erythrocytes or Red Blood Corpuscles (RBC):
Erythrocytes are highly specialized cells. The mature human erythrocytes is a circular, biconcave, non nucleated disc. It contains homogeneous protoplasm. It is 7-8 μm in diameter, 2-2.5 μm thick at periphery and 1-2 μm thick at the centre. Under the microscope a single red blood cell seems to have a ligtht brown or yellowish colour. But in groups the cells appear to be red. The red colour of the blood is due to the presence of respiratory pigment haemoglobin.
The haemoglobin is an iron containing protein. It is composed of 600 amino acids and has a molecular weight of about 68,000. Haemoglobin consists of a protein-globin and four molecules of haem. The haem molecule contains an atom of iron which can combine with one oxygen molecule. The haemoglobin constitutes about 90 per cent of dry matter of RBC. The haemoglobin combines with oxygen to form oxyhaemoglobin and thereby carry the oxygen to the different parts of the body.
The number of erythrocytes differs in the different stages of life as well as with the physiological state of the individual. The number of RBC is biggest at birth which gradually decreases. There are about 5.4 million of erythrocytes in one cu. mm of blood in adult male and about 4.8 million in adult female. In infants the RBC count is about 6 to 7 million per cu. mm whereas in the foetus it is about 9.8 million. Moreover, the number of erythrocytes may vary during the different hours of a day. The number is less during sleep and after meals. This is known as diurnal variation.
The life span of RBC is very short. An erythrocyte of mammal is non-nucleated and so they are short lived. As they are non nucleated, they do not have any mechanism for their repair and are unable to divide or multiply. As the cells grow, they change their shape and size and become more brittle. They lose their structural integrity and become flask shaped having some pseudopodia like processes. Ultimately these processes are broken off and RBC cells are disintegrated. The distintegrated RBC are either disposed by the liver or spleen. However, spleen is the major site of disposal of the disintegrated RBC and so the spleen is known as graveyard of RBC. The haemoglobin of the RBC is decomposed into haem and globin. The iron part of the haem is again utilised in the formation of new RBC.
White Blood Corpuscles or Leucocytes (WBC):
White blood corpuscles are colourless as they do not contain any respiratory pigment. They are also known as leucocytes. Leucocytes are less in number as compared to erythrocytes. The average total number of WBC is 6,000 to 8,000 per cu. mm. The rate of WBC number with that of RBC is about 700. Leucocytes are nucleated and larger in size in comparison to erythrocytes. They are actively mobile and they Perform amoeboid movement. The leucocytes are rich in nucleoprotein. They also contain lipids, glycogen, cholesterol, ascorbic acid and number of proteolytic enzymes.
Leucocytes have a number of important physiological properties which are (i) Leucocytes can perform the amoeboid movement by forming the pseudopodia. (ii) Leucocytes have the ability to pass through capillary walls. The process is known as diapedesis. (iii) Leucocytes can ingest and digest microorganisms by the process of phagocytosis.
Classification of White Blood Corpuscles:
There are many varieties of leucocytes and each type of leucocyte possesses different morphological characteristics and staining property. WBC can be divided broadly into two groups. These are : (i) Granulocytes or granular leucocytes. (ii) Agranulocytes or agranular leucocytes.
Difference Between Erythrocytes and Leucocytes:
|1||They are smaller, more numerous and longer lived cells than the leucocytes.||They are larger, fewer, shorter-lived cells than erytnrocytes.|
|2||They have a fixed form. RBCs of man are circular, biconcave and enucleated.||They are rounded but can change their shape.|
|3||They occur only in blood vessels.||They can escape from capillaries into the tissues (diapedesis).|
|4||They lose cell organelles (ER, mitochondria,ribosomes, centrioles)
|They retain cell organelles (ER, mitochondria, ribosomes,centrioles).|
|5||They have haemoglobin.||They lack haemoglobin.|