Blood

Blood Composition, Formation and Functions

Blood is a fluid connective tissue which helps in transportation of necessary substances from one part of our body to other.  Total volume of blood in human female is 4-5 litres and in male is 5-6litres. Blood is composed of formed elements (blood corpuscles) which remain embedded in fluid part called plasma.

 

 

Composition of Blood

 

Blood that flows inside the blood vessel (composed of endothelial cells ) is composed of fluid part called plasma and formed elements called blood corpuscles.  Besides these, other  important substances are also found and transported through  blood. Following is a schematic diagram.

From the above figure 1, it is now clear about the constituents of plasma. But what about Formed elements or blood corpuscles)  which exhibit about 45% of the volume of blood? The most abundant blood corpuscles found are` Erythrocytes (RBC).  Other blood corpuscles include Leucocytes (WBC) and Thrombocytes (Platelets).

 

Detailed account of Formed elements ( Blood corpuscles)

Erythrocytes (RBC)   1. Non nucleated, circular, biconcave  dumble shaped cells. In male (4.2-5.8 million RBC) and in female ( 3.6-5.2 million RBC )   2. RBC  are formed in the bone marrow of ribs and vertebrae.   3.  Each disc shaped RBC is 7.5µm in diameter.   4.  Live for 120 days in males and 110 days in females. They disintegrate in spleen and liver.   5. They contain Haemoglobin which carries oxygen.

Leucocytes (WBC)   1. Nucleated clear cells meant mainly for protection against invading micro organism. 2. They are formed in red bone marrow and lymph glands. Life span of human leucocyte is 12 – 13 3. They are mainly of two types: i)Granulocytes: cytoplasm is granular with large irregular nuclei. Produced in bone marrow.  They are further divided into: Eosinophil or Acidophils:  Granular cytoplasm with bilobed nucleus and stains with acid dyes eosin; meant for destruction and detoxication of protein origin toxins. Basophils  Coarse granular cytoplasm with bi or tri lobed nucleus and stains blue Neutrophils  Nucleus polymorphic in nature with granulated cytoplasm with multilobed nucleus  and they take very faint basic stains. In contact with any  bacteria they engulf, digest and destroy them. Agranulocytes : Simple nuclei with agranulated cytoplasm. They are of two types: Lymphocytes Cells with large eccentric nucleus and less cytoplasm.They are commonly found in lymph nodes, thymus, spleen. They produce antibodies.Less motile in nature. There are   B lymphocytes which kills bacteria ; T- lymohocytes  which acts against viruse and Natural killer cells (NK cells) Monocytes: Cells have kidney bean shaped or oval nucleus with large cytoplasm and are motile in nature.  They helps in engulfing bacteria by phagocytosis.

Thrombocytes (Platelets)   1.Flat granular corpuscles formed in the red bone marrow .   2. They contain thromboplastin which helps in clotting of blood.

 

 Mechanism of formation of blood (Hematopoiesis)

 

‘Hemato’ means blood and ‘poiesis’ means to

form. Blood flowing inside a blood vessel

are not synthesized in thes ame place.                                                                

Instead they are formed  from  pleuropotent

(can give rise to any type of blood cell)

hematopoietic stem cells which

originates from bone marrow present

inside a bone. Hematopoietic cells

give rise to cells of two different lineages.

Haematopoietic cell  develops inside the bone marrow of limb bones, vertebrae, sternum or arm bone. They are pleuripotent in nature i.e, they can give rise to any type of blood cell. The immature blood cell is called Blast cell which further divide to form cells of either myeloid or lymphoid progenitor cell. These further will give rise to different types of blood cells. Note here White blood cells or leucocytes have cells of both lymphoid and myeloid lineage.

Mechanism of blood clotting

1.When a blood vesse is damaged, it releases a mixture of lipoprotein and phospholipids called Tissue factor or Thromboplastin.   This in presence of certain factor of blood is converted to Prothombinase or Thrombokinase/

2.  Prothombinase  converts plasma protein Prothombin into active Thrombin. Prothombin needs Vit K in liver for its synthesis.

3.Active enzyme Thrombin can now convert plasma protein Fibrinogen to Fibrin. Fibrin  forms a fibrous network of clot.

4. Formation of thrombus or blood clot with in intact blood vessel is called Thrombosis.

*** Thrombosis occurring with in an artery may block the artery preventing blood and oxygen flow.  When it occurs with one of the coronary arteries supplying heart muscles , it is known as  Coronary Thrombosis, a condition called heart attack.

A thrombosis  in the arteries supplying blood to brain causes cerebral thrombosis, a condition called cerebral stroke.   When a fragmented clot enters the circulating blood

 

                                                                      Function of blood

1. Transport of oxygen from lungs to tissue and carbon dioxide from tissue to respiratory surface
2. Transport of nutrients from intestine to liver and then to whole of the tissue for cellular activities. These include glucose, amino acids, polypeptides, cholesterol, amino acids, minerals, vitamins and water.
3. Waste products like urea, uric acid are eliminated by blood to kidney as urine, lungs, skin for elimination.
4. Endocrine glands produce hormones which are distributed by blood to the vital tissues.
5. Amphoteric property of Plasma proteins helps to maintain the pH of the blood.
6. Blood maintains water balance by exchange of water between tissue fluid and blood.
7. Blood helps in transporting of heat from deeper tissue to surface of the body
8. Blood helps to develop immune system in our body and helps in defence mechanism against invading microorganism..
9. Blood cells like Monocytes helps in phagocytosis; Lymphocytes helps in producing antibodies which kills bacteria and viruses.
10. Blood posses certain coagulating agents like Thrombin and Fibrinogen which helps in coagulation of blood.

 

Some important facts Hematocrit value  is the  percentage of red blood cell in blood. In normal male blood it is 45% while in female it is 40%.  If this value. If this value is high i,e, percentage of RBC is more in compare to plasma, the condition developed is called polycthemia. If hematocrit value is low then the condition developed is called Anaemia. Globulins present in blood are considered as protein antibody and provide immunity to various diseases. They are of α1 , α2 β and γ globulins.  Albumin and globulin present in blood plasma helps to retain water by their osmotic effect. So if less protein is taken accumulation of water in the tissue takes place , a condition called oedema.  Glucosoria is a condition where glucose in the blood comes out through urine. This normally happens when amount of glucose in blood rises beyond 180 mg per 100 ml of blood. Urea is formed when amino acids are catabolised with in the liver. Blood transfer this to kidney for excretion in urine.  Hemolysis is a condition when blood is mixed with hypotonic salt solution then erythrocytes swell and burst and releases all its content.  An abrupt enormous increase in number of leucocyte is called Leukemia, where as leucocytes when damaged are decreased in number , called  Leucopenia.  The process of formation of blood corpuscles of vertebrates from hemopoietic  tissue is called hematopoiesis.  The precursor cells of erytrhocytes are called erythroblast which divide mitotically to form normoblast which give rise to erythrocytes.  Eosinophils takes acidic stains like Eosin, while Basophils attract basic dyes called Haematoxylin. pH of blood is 7. 35 – 7. 45, slightly basic. In invertebrates Hemocyanin (blue pigments) found in crustaceans, in Tunicates vanabin ( bright green colour ) pigments are present. In Giant tube worms haemoglobins also can carry Sulphides which is fatal to other animals. The blood of most annelid worms contains chlorocruorin to transport oxygen. In marine invertebrates like sipunculids, priapulids, brachiopods Hemyrythrin (violet pink) is present.   ·         *** Hemophilia is a genetic illness in which clotting of blood is hampered and results into hemarthrosis or bleeding into joint spaces. ·         Thalessemia is a disorder characterized by abnormal formation of hemoglobin.

 

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