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The production of red blood corpuscles is called haemopoiesis or erythropoiesis.
In the first few weeks of embryonic life RBC are produced in the yolk sac. During the middle part of gestation, liver, spleen and lymph nodes are the main organs of haemopoiesis. During the later part of gestation and after birth, bone marrow is the main organ producing RBC.
Haemopoiesis is the process by which blood cells form, develop and mature in animals and man.
All blood cells originate from hemopoetic stem cell(HSC). Stem cell are differentiate into other types which are self renewing, maintain their population level by cell division. Stem cell differentiate into common lymphoid progenitor cell or a common myeloid progenitor cell. Progenitor cell control its differentiation. Progenitor cells lost capacity for self renewal and are committed to a particular cell lineage. Lymphoid progenitor cells gives B, T and NK(natural killer) cell. Myeloid progenitor cell produces red blood cells, white blood cells and platelets.
Haemopoiesis is the process by which the cellular elements of the blood are formed. The three types of cells are found in the blood i.e. red blood corpuscles (erythrocytes), which serve to carry oxygen, the white cells (leukocytes), which function in the prevention of and recovery from disease, and the thrombocytes, or platelets which function in blood clotting. The formation of these cells is one of the most active and important processes in the body. Most of the circulating cells live only for a short time and must be replaced in order to maintain life. For instance, in the human adult a red blood cell has a life of 120 days; 250 billion new red cells have to be produced daily to replace those that are destroyed.
Blood is the red corpuscles fluid connective tissue that circulates throughtout the body. It is five times viscous than water. It has pH of 7.4 and is alkaline. It is made to flow by the pumping
action of the heart. Adult human contains about 5 litres of blood. The blood is formed of two components, namely a fluid component called plasma and a cellular componentcalled formed elements.
Functions of Blood :-
1)Blood carries oxygen from the lung to the tissue and carbondioxide from the tissues back to the lungs.
2)It transports digested and absorbed food substances (nutrients) from the intestine to the various tissues.
3)Waste products of metabolism such as ammonia, urea,
uric acid are collected by the blood and are passed to the excretory organs like kidneys, skin etc.
4)It transports the hormones from the endocrine glands to the target organs.
II)Haemostatic function :-
It maintains the body temperature by transporting the interior of the body to the surface. It helps in maintaining the water balance of the body.
With the help of plasma proteins, it maintains a constant pH.
IV)Protective Function :- protect the organism from the infection of foreign substances.
V)Clotting function:- It protects it own loss by means of the clotting mechanism.
Composition of the Blood ;_
Plasma is the liquid portion of blood. It is structureless, straw coloured, slightly alkaline fluid and is 2.2 times more viscous than water. Out of five litres of blood 3.5 litres are plasma. It contains 90% water and the remaining 10% is formed of organic and inorganic substances. These materials include proteins, glucose, nitrogenous wastes enzymes, hormones, minerals etc.
Organic constituents :-
Plasma contains the following organic substances.
1)Plasma proteins (7-9%), Albumin, globulin and fibrinogen
2)Carbohydrates (1%); Glucose, fructose and polysaccharides
3)Lipids- Neutral fats, Phospholipids, cholesterol, fatty acids.
4)Non-protein nitrogenous substances :- Amino acid, urea, uric acid, creatine and ammonia
5)Others :-Enzymes hormones and vitamins
Inorganic Constituents ;- (0.9%)
6)Inorganic salts :- chlorides, bicarbonates, sulphate, phosphate, iodides of sodium, potassium, calcium, magnesium and iron
7)Gases – oxygen, CO2, and N2
In addition plasma also contains small amounts of bilurubin, carotene imparting a yellow teenage to it.
Plasma Proteins :-
Plasma proteins are of three types as albumin, globulin and fibrinogen. These types can be separated from one another by precipitation at temperatures below 00 with various concentration of alcohol and also by the process of electrophoresis.
The serum albumins are the spherical particles and are the most abundant of the plasma proteins synthesized in the liver. Albumin show the highest electrophoretic mobility. It is mainly the albumins that impart on blood an oncotic pressure of about
30mmHg. Albumins thus maintains an osmotic eqllibrium between the blood and tissue fluids by preserving their fluid balance.
Globulins can be further separated into alpha, alpha2, beta, and gamma globulins. Their molecules are or less oblong. Human blood plasma contains about 2.5gm%globulin. These with the exception of gamma globulin are all produce in the liver. Gamma globulin originate in lymphocytes, macrophages and reticulo-endothelial tissues. Alpha and beta globulinsare concerned with transport of various harmones and chemical substance. Gamma globulin forms defensive line of the body. All known antibodies are the gamma globulin fraction.
Fibrinogen is globulin which is easily precipitated. It is the precursor of fibre. It is form exclusively in liver. Its function is to bring about coagulation of blood. Plasma without fibrinogen is describe as seum and appears as a slight yellowish transparent fluid.
Functions of plasma proteins:-
- These are responsible for oncotic pressure fascilitating exchange of water between blood and tissue.
- Act as buffers maintaining acid base equilibrium or pH of the blood
- Viscosity of plasma and blood is due to the presence of these proteins.
- Gamma globulins from a very effective chemical line of body defence. Their concentration rises during infection by microorganism.
- Alpha and beta globulins transport harmones and certain substance like iodine, copper, iron etc.
- Fibrinogen prevents blood loss
Inorganic Constituents :-
These are present to the extent of 0.9%. The cation present are Na+, Ca++, K+, Mg++ while Cl-, HCO3-, H2PO4-, HPO4- and SO4- are the anions. NaCl is in highest concentrationgiving a saltish taste to the blood.
The inorganic constituents provide medium for the proper function of all tissues. These maintain the osmotic pressure of the blood . Blood calcium is necessary for its coagulation. Excess of electrolytes are excreted by kidneys and colon.
When fibrinogen is removed (as fibrin) the residual plasma is called serum. Putting it in another way, we might term blood minus all cells and fibrin as serum.
Serum=Blood- all cells and fibrin
Formed elements :-
Comprise 45% of the blood and include the erythrocytes, leucocytes and platelets.
Erythrocytes/Red Blood cell/Corpusles (RBCs) :-
RBCs are the most numerous of formed elements. Red colour of blood is due to them. In man they average about 5.7 million/mm3 and in woman, at least a million less.
In rat, they are 9-9.5million/mm3 in the male but are more in female.
A human erythrocyte is a non nucleated cell having the form of a biconcave disc with an average diameter of 7.4/u, a thickness of 2/u and a volume of 90/u3. A single erythrocyte contains 30gm of haemoglobin. Erythrocytes of man and other mammals are flattened biconcave circular discs.
Each RBC is composed of an envelop and a spongy elastic substance called stroma. Inside the meshes of the stroma is present the iron pigment haemoglobin. Haemoglobin is a conjugated protein composed of a protein part globin and non-protein pigment haem. Haem is an iron porphyrin.
Porphyrin +ironïƒ Haem
4 molecules of haem + Globin--ïƒ Haemoglobin.
On combining with oxygen, Hb is converted to oxyhaemoglobin (HbO2). Oxyhaemoglobin after joining out its oxygen is called reduced haemoglobin.
Hb+O2 <====ïƒ¨ HbO2
When haemoglobin is treated with glacial acetic acid it forms haemin crystals.
The erythrocytes develop from the bone marrow of vertebrates and so the bone marrow is described as the cradle of RBC. Then it escapes into the blood stream. The life span of RBC is limited. It is shorter than most of the fixed tissue cells. The life span of circulating human blood is around 120 days and therefore 0.83% of erythrocyte cell mass is renewed everyday in man. As the erythrocyte becomes older and older, the cell membrane becomes more and more fragile and the old RBCs are disintegrated in the spleen. So the spleen is said to be the grave yard for red cells. Once the erythrocytes have undergone fragmentation their haemoglobin is broken down into two important components, namely denatured globulin and iron. The iron is stored in the liver and is made use of in the formation of fresh haemoglobin. The globin part is converted into bilurubin, the yellow pigment and biliverdin, the green one which are used in colouring the bile.
If however, there is a deficiency of haemoglobin in the RBSs or if the number of erythrocytes in blood is less than normal, the oxygen carrying function of the blood is seriously affected. This Hb and/ or erythrocyte deficiency is termed anaemia. There are a number of causes of anaemia. These include blood loss, nutritional deficiency, genetic factors, immunological faults, bone marrow dysfunction, parasitic infections, drugs etc. Some of the anaemias are pernicious, haemolytic, aplastic, iron-deficiency and megaloblastic . Of these, the iron deficiency anaemia is the most common. Depending on whether the RBCs in an anaemic person are larger or smaller than normal, the conditions are respectively known as macrocytic and microcytic anaemia. Some others such as pernicious anaemia and sickle cell anaemia are most dangerous and often lead to death.
2) White blood Corpuscles/ WBCs/Leucocytes :-
The leucocytes differ from the erythrocytes in the following aspects.
- They have no haemoglobin
- They are bigger in size.
- They are nucleated and amoeboid.
- They are much less in number and their life span is longer.
- There are several varieties and they have different functions.
The average total number of WBC is 6000-8000 per cubic mm of blood and the average ratio of WBC to RBC works out to 1:600
Leucocytosis is the term used to describe increase in the total number of white cells in the blood. Leucopenia means the decrease in the white count. Based on their functional aspect leucocytes are termed as phagocytes. They protect the body by attacking the invading microorganisms and devour them. In the new born the WBC count is very high about 20000mm3. Throughout infancy and childhood the count remains high.
There are several varieties of leucocytes. Determination of the different varieties of leucocyte is known as differential count of WBCs. Leucocytes are of two main types
1)Granulocytes:- Granulocytes are produced in the bone marrow and are of three types, namely neutrophils , eosinophils, and basophils. They are cells with granules in the cytoplasm and the nucleus is unlobed. The increase of granulocytes in the blood is called granulocytosis. Dimunution of granulocytes iscalled granulocytopenia and complete disappearance of granulocytes is known as agranulocytosis . They are of the following types.
These are about 79%of the total leucocyte count. A neutrophil has a granular cytoplasm and a multilobed nucleus (2-7lobed). They show amoeboid movement. When they come across the bacteria , they are engulfed by a process termed phagocytosis. During infection and inflammation their number is considerably increased in the circulating blood. These leucocytes are capable of passing out through the capillaries by a process known as diapedesis. As these cells are highly phagocytic and devour foreign particles they are also called scavenger cells. The absolute number of neutrophils is 3000-6000/mm3. They vary in size from 10-12micrometer. The normal life span of these cells is about a week.
b) Eosinophils :-
These are slightly larger (10-15micrometer) than the neutrophils and possess abundant, large, uniform sized refractile granules that stain intensively with eosin or other acid dyes. These are about 1-4% of the total leucocyte count and their absolute number is 150- 400/mm3. The nucleus is two or three lobed. The eosinophils are amoeboid but not phagocytic. The important functions of eosinophils are to bring about destruction and detoxification of toxins of protein origin. The cells are known to increase in number in allergic conditions.
c) Basophils :-
These occur most infrequently in the human blood. Their size range is 8-12micrometer. Basophils are about 0-4% of the total leucocyte count. The prominent nucleus is irregular in shape. It may be elongated and bent like an “S” or “U”. The basophilic cytoplasmic granules are coarse and variable in number, shape and size in different species. Often they obscure the nucleus. The absolute number of basophil is 0-100/mm3. Basophil has some possible role in local anticoagulation and formation of ground substance.
Basophils exhibit a limited degree on amoeboid movement and phagocytic action. There function has not been fully elucidated. Their granules contain hydrolytic enzymes, histamine, heparin etc.
- Agranulocytes :-
Agranolocytes are produced in the lymph-nodes and spleen, and are of two types : lymphocytes and monocytes.
They can undergo mitosis whereas the granulocytes cannot.
- Lymphocytes :-These are about 25-30% of the total leucocyte count and their absolute number is 1500 to 2700/mm3. They have a large nucleus compared to the size of the cytoplasm. The cytoplasm is basophilic without any granules. Lymphocytes are divided into two groups, namely small lymphocytes and large lymphocytes.
The small lymphocytes have a dense heterochromatic nucleus surrounded by a thin rim of slightly basophilic (few ribosomes) cytoplasm.The medium sized cells have a large nucleolus and much euchromatic. The cytoplasm is moderately basophilic (many ribosomes).
In large lymphocytes the nucleus is mostly euchromatic and has 1-2 prominent nuclei. The abundant cytoplasm is intensely basophilic (numerous ribosomes).
Lymphocytes are amoeboid cells which pass very easily through the endothelial walls into the surrounding tissues. They are the longest –lived cells (several months to a few years) among the leucocytes.
Physiologically the lymphocytes belong to two types, the T lymphocytes and the B lymphocytes. These leucocytes are not phagocytic but are very active in the production of highly specific antitoxins and antibodies i.e. in immunological reactions. Several functional populations of T-lymphocytes (efffector Tcells, helper Tcells, suppressor T cells etc) have been identified. Activated T lymphocytes secrete a variety of chemical agents called lymphokines. These are involved in cell-mediated immunity. The B lymphocytes are primarily responsible for humoral immunity. Fully mature B cells produce specific antiobodies or immunoglobins.
These large cells measure 12-15micrometer but may reach 20 micrometer in dry smears. They are very few in number. Monocyte nucleus is ovoid, reniform or horseshoe-shaped, with usually two nucleoli and fine chromatin network. The cytoplasm is more than that in lymphocytes. It contains a distinct Golgi complex near the nuclear indentation, some RER, a moderate number of free ribosomes, prominent mitochondria and glycogen granules. Further, some small vacuoles and several minute azurophilic granules (lysozymes) also occur. From plasmalemma extend many short pseudopodia.
Monocyte have little function in blood. These are amoeboid macrophages which migrate readily through capillaries into the connective tissues where they play a valuable role in phagocytosis and intracellular digestion of worn out cells and invading micro-organisms. They are particularly useful in combating the bacillus of tuberculosis.
Origin of leucocytes :- The leucocytes are produced in the myeloid tissue of the red bone marrow. Lymphocytes also develop in other lymphoid tissues.
Thrombocytes belong to another type of formed elements in the blood. They are oval to spherical in shape and are 2-3micrometer in diameter. They have no nucleus. The number of platelets in human blood varies from 250,000-450,000/mm3. The average life span of platelets is about 5-9 days. They are produced in the megakaryocytes on the bone marrow and are destroyed in the spleen and other reticuloendothelial cells.
Several functions are performed by the platelets. They initiate blood clotting and repair capillary endothelium. Speed of clot reaction or syneresis is directly proportional to the number of platelets present in the blood. They are also concerned with vaso-constriction of blood vessels liberating 5-hydroxytryptamine.