Tuesday, 31 March 2015

Human blood composition and clotting mechanism



Blood is a highly complex fluid that has two parts viz. the cellular part (WBC’s, RBC’s etc.)  and the intercellular part (plasma). The cells are submerged in plasma. The collective name for all blood cells is blood corpuscles. The cellular content of blood is 45% while the plasma content is around 55%. When we say whole blood, we refer to the cellular part of the blood and exclude plasma. We next see the proportions of all the constituents of whole blood.

Constituents of cellular content of blood or whole blood -

RBC’s or red blood corpuscles- these constitute around 45 % of whole blood. WBC’s or white blood cells- these constitute around 1% of the total whole blood volume. Platelets-these constitute around 0.7% o the whole blood.

Constituents of plasma

Water – the water content of plasma is 91 to 92%. Inorganic and organic constituents of blood (solids) - they constitute around 8 to 9 % of blood plasma.Inorganic constituents- they constitute around 0.9% of the solid content of blood plasma. The inorganic constituents of plasma are iron, copper, sodium, potassium, calcium, magnesium etc. Organic constituents- protein and non protein nitrogenous substances form the organic constituents of the solid content of blood plasma.

Proteins like serum globulin, serum albumin, prothrombin, fibrogen form around 7.5% of organic constituents.NPN or non protein nitrogenous substances like urea, uric acid, creatinine, creatine, ammonia, amino acids etc, fats like phospholipids, cholesterol etc, carbohydrates, other substances substances like antibodies, enzymes and coloring matter like bilirubin, carotene etc form the rest of the organic constituents of the solid part of blood plasma.

Haematocrit value of Blood

The plasma content of whole blood is more than the cellular content. Thus, plasma strength varies from 52 to 55% while the cellular strength varies from 45 to 48%. Males have more cellular content than females. Males have around 45% cellular content while females have 40% of it. The ratio of red blood cell or RBC content of whole blood to the plasma content is expressed as haematocrit value and is measured by an instrument called haematocrit.

Specific gravity of blood

The specific gravity of whole blood varies from 1.05to 1.06 at 15 degree Celsius. The average is 1.057 in males and 1.053 in females. Specific gravity of blood rises when water loss occurs from the body, when new fluid flows in serous cavities because of surgical operation or when inflammation occurs in tissues and cells or when there is low water intake. Specific gravity of blood falls when ever haemorrhage occurs, when there is large intake of water or when saline solution is injected into blood.

Origin of the plasma protein content of the blood

The mesenchymal cells produce plasma proteins in the embryo. In adults, liver produces all the major plasma proteins like fibrogen, albumin, prothrombin etc. Albumin is also formed by the reticulo-endothelial cells, by disintegrated red cells and from lymphoid nodules.

Functions of plasma proteins

Plasma proteins serve very important for some functions of the       body. The functions are listed below.They regulate the osmotic pressure of the blood and also regulate the fluid distribution between the cells and blood.They are required for clotting of blood. In the absence of plasma proteins, no blood coagulation will take place and total haemorrhage can occur.They maintain the blood pressure.They maintain the viscosity of bloodThey act as buffers in maintaining the acid base balance of the blood.They are the protein reserve depots of the body.They aid the process of respiration as they form carbamino proteins which carry carbon-di-oxide.They contain antibodies (gamma globulin) in nature which strengthen the immune system of the body.They transport enzymes and hormones, iron, copper etc from one place to another.

Viscosity of blood

Human blood is 5 times more viscous than the pure distilled water. Whole blood or the cellular part of blood is viscous because of the cellular content while plasma is viscous because of plasma proteins. The relative viscosity of plasma is 1.8 and that of whole blood is 4.7. Some pathological conditions like hypercalcemia, hyperglycaemia, diabetes mellitus etc. increase the viscosity of blood. Blood viscosity gets reduced during exercise, fever, malaria and lymphatic leukemia.

Coagulation of blood and its mechanism

When the blood comes out of the body, it loses its fluidity in a few minutes and turns into a semi solid jelly form. This phenomenon is termed as blood clotting or blood coagulation. When left outside for some more time, a straw colored fluid is secreted from the retracting clot which is called serum. The serum does not further coagulate. The RBC’s and WBC’s do not take part in blood coagulation though platelets take some part in it. Blood coagulate by actions of plasma. The RBC and WBC get trapped in the clot meshes and are subsequently removed.  Blood coagulation is a very important property of blood as in absence of it haemorrhage occurs.

The process of blood coagulation takes place with the action of platelets. When blood is shed from the body through any means, the blood platelets get shed on water wettable rough surfaces. The platelets disintegrate and release thromboplastin. The tissues of the damaged area also release the substance. This thromboplastin converts the prothrombin content of the blood into thrombin. Calcium ions aid thromboplastin in the process. The thrombin interacts with fibrogen to form fibrin. The blood clot is thus formed with fibrin formation.

There are 13 factors that are responsible for coagulation of blood.

These are: Fibrogen or Factor 1- this factor is globulin in nature but is a lot bigger than the serum globulin. Its molecular weight is 330000.It converts to fibrin during the process of clotting. Prothrombin or factor 2- it is contained in normal plasma and is protein nature. It has a molecular weight of 62,700. Vitamin K is essential for formation of prothrombin in liver. Thromboplastin or factor 3- it is contained in tissues as well as in blood plasma. Prothrombin is converted into thrombin during blood clot formation.  Thromboplastin and calcium ions aid the process of conversion.

Blood only clots on rough surfaces. That is why blood does not clot inside the body. But if the surface of blood vessels becomes rough because of any reason, body will coagulate inside the body.

1.Calcium or factor 4- calcium acts as a cofactor for blood coagulation.

2.Labile factor or factor 5-this protein is necessary for total conversion of prothrombin into thrombin.

3.Accelerin or factor 6- it is formed form proaccelerin. 

4.Stable factor or proconvertin or factor7- the factor is present in blood plasma. Antihaemophilic factor(AHF) or platelet cofactor1 or factor8- it is contained in blood plasma but disappears with the formation of clot. It aids the formation of intrinsic thromboplastin and also helps in the conversion of prothrombin into thrombin.

5.Christmas factor or platelet cofactor 2 or factor9- this cofactor is essential for formation of internal thromboplastin. 

6.Stuart factor or factor 10- it is similar to factor 8. Plasma thromboplastin antecedent or (PTA) or factor1-  the Hageman factor activates it. PTA helps in forming thrombin. Hageman factor or factor 12- this protein is activated only when blood gets in contact with a rough surface. With its activation the protein splitting enzyme kallikrein is activated which results in the formation of linins in plasma. Kinins cause dilation of blood vessels and increase their vascular permeability.

7.Laki-lorand factor (LLF) or factor 13- this factor, along with calcium, converts the soft fibrin clot to a fibrous and solid form.


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