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- Formed elements or blood corpuscles are of the following three types : Erythrocytes,Leucocytes and Thrombocytes.
- They constitute about 45 % of the blood.
Erythrocytes ( Red Blood Corpuscles or RBCs )
- They are the most abundant cells in the human body.
- Number. The total number of RBCs per microlitre of blood is termed as the total count of RBCs.
- A normal adult man and woman have 5 and 4.5 million RBCs per cubic millimetre of blood respectively.
- Thus the total count of RBCs is more in man than in a woman. It is due to the fact that women undergo menstruation.
- Less amount of haemoglobin leads to anaemia. Anaemia may be caused by loss of blood (haemorrhage), destruction of RBCS (haemolysis or faulty formation of blood ).
- The increase in number of RBCs may be during muscular exercise to meet the increased demand of oxygen and at high altitudes to cope with the low oxygen content of the air.
- An abnormal rise in RBC count in called polycythemia.
- Descrease in the number of RBCs is called erythrocytopenia which causes oxygen shortage in the blood and tissues.
- It is important to note that the oxygen shortage stimulates the kidney cells to secrete a hormone named erythropoietin, into the blood .
- Erythropoietin stimulates the bone marrow to increase the production of RBCs.
- Shape. Red blood corpuscles of all adult mammals are enucleated (non - nucleated).
- They are mostly biconcave and circular, however in camal and Llama (American animal with thick wooly coat used as a beast of burden) RBCs are oval.
- Biconcave form of human RBCs is advantageous in gas exchange because bíconcave disc has more surface area than a sphere.
- Size . Human erythrocytes are 7-8 um in diameter and 1-2 um thick near the rim.
- Colour. The RBCs look yellowish when seen singly and red when viewed in bulk . They impart red colour to the blood.
- Red colour is due to the presence of haemoglobin.
- Structure. Matured mammalian RBCs do not have cell organelles including nucleus, Golgi bodies, mitochondria, ribosomes, centrioles and endoplasmic reticulum.
- It increases the surface area of RBCs and enables these to contain more haemoglobin. Thus almost entire cytoplasm is filled with haemoglobin.
- In the absence of cell organelles, the consumption of oxygen is very low.
- Anaerobic respiration occurs in RBCs. Haemoglobin is a conjugated protein which is made up of a protein catted globin anda non protein group heme ( = háeme ). hence the name haemoglobin. Heme is an iron ( Fett ) - porphyrin complex.
- A mammalian haemoglobin molecule is complex of 4 heme molecules joined with 4 globin molecules.
- A red blood corpuscle has some 280 million haemoglobin molecules. Haemoglobin is the oxygen carrying pigment in most vertebrates except some ice fishes (Antarctic ice fish, Chaenocephalus) and eel larvae.
- Haemoglobin also occurs in the blood of some annelids and molluscs but in these invertebrates it remains in the plasma.
- Quantity of Haemoglobin in RBCs. 100 ml of blood of a normal healthy individual contains about 12-16 gms of haemoglobin.
- The quantity of haemoglobin is less in women as they undergo menstruation.
- Formation. Formation of erythrocytes is called erythropoiesis.
- In the early few weeks of embryonic life, primitive nucleated RBCs are produced in the yolk sac. (one of the embryonic membranes).
- In later embryonic stage, RBCs are mainly produced by the liver and spleen.
- But from birth onwards, RBCs are produced by bone marrow.
- Hemocytoblasts in red bone marrow give rise to mature RBCs.
- Iron and proteins are necessary raw materials for the synthesis of haemoglobin.
- However vitamin B12 and folic acid stimulate the matu ration of RBCs. Thus iron, protein, vitamin B12 and folic acid are essential for the formation of haemoglobin and RBCs.
- Deficiency of any of these nutrients can cause anaemia . Excess RBCs are stored in the spleen (graveyard of RBCs).
- Life Span. The life of a RBC is about 120 days. Functions of RBCs.
- Transport of O2. Haemoglobin of RBCs readily combines with oxygen to form oxyhaemoglobin.
- In the tissues oxyhaemoglobin readily gives up its oxygen. This oxygen is used for oxidation of food.
- Transport of CO2 . RBCs also participate in transporting carbon dioxide from tissues to lungs.
- Carbon dioxide combines with potassium carbonate of the red blood corpuscles to form potassium bicarbonate in the presence of an enzyme carbonic anhydrase.
- Carbon dioxide also combines with the amino group (-NH ) of the haemoglobin of red blood corpuscles to form carbamino - haemoglo bin.
- Maintenance of pH of blood. Moreover, the haemoglobin is an excellent acid base buffer which is largely responsible for maintaining the pH of blood Acidity of blood results haemoglobin to carry less oxygen.
- Erythrocyte Sedimentation Rate ( ESR ). If blood contain ing an anticoagulant (oxalate) is allowed to stand in a narrow vertical tube, the erythrocytes settle to the bottom half of the tube.
- The rate at whch this occurs is called the erythrocyte sedimenta tion rate (ESR).
- The erythrocytes settle to the bottom because their density is greater than that of plasma.
- ESR is very useful in diagnosing various diseases including tuberculosis . ESR in men is 0-5 mm in Ist hour and in women it is 0–7 mm in Ist hour (Westergren Method ).
- Rouleaux. In resting and slow flowing blood , the RBCs aggregate to form rouleaux (the RBCs are piled on top of each other). Fibrinogen favours rouleaux formation.
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