Coagulation – Cessation of bleeding

What is Coagulation?

Blood coagulation or Hemostasis is the cessation of bleeding from a cut or several vessels. When blood is shed, it loses fluid nature in a few minutes and sets into a semisolid jelly. This phenomenon is called coagulation or clotting. On holding back, the clot retracts to a smaller volume and presses out a clear straw colored fluid called the Serum. Serum does not clot any further.

Thrombosis occurs when the endothelium lining of blood vessels is damaged or removed. These processes include coagulation and Platelet aggregation.

Phases of Coagulation

When the process of coagulation is studied under the ultra microscope, it is seen that minute granules appear at first, often near a clump of disintegrating platelets. The following phases occur in coagulation:

Phase 1: Constriction of the injured blood vessel to diminish blood flow.

Phase 2: Formation of a loose and temporary platelet aggregate at the site of injury. At the site of vessel wall injury, platelet binds to collagen and is activated by thrombin. After the activation platelet changes their shape and in the presence of fibrinogen, aggregate to form the haemostatic plug or thrombus.

Phase 3: Formation of a fibrin mesh that binds to the platelet aggregate, which gives to a more stable haemostatic plug or thrombus.

Phase 4: Partial or complete dissolution of the haemostatic plug or thrombus by plasma.

Coagulation is the property of plasma alone. The red and white blood cells do not take part in it. They only get caught up in the meshes of the clot and are thereby removed. It is due to this fact that the clot has a red colour, and serum is a clear non – cellular fluid.

Blood Coagulation and time related factors

Normal Coagulation Time

Measured according to the method of Lee and White, it is 16-17 minutes in glass tube and 19-60 minutes in siliconised tube.

Bleeding Time

The total length of time required from bleeding starting and ceasing is known as Bleeding Time. Bleeding Time helps to detect vascular defect and platelet disorders. Prolonged bleedings time is generally associated with Thrombocytopenia. Normal average Bleeding Time is 3.25 minutes (2-5 minutes).

Prothrombin Time

When tissue extracts and calcium chloride (CaCl₂) are mixed in optimum amount of blood of normal fibrinogen content, they only factor which has an inadequate concentration of prothrombin can very the coagulation time. If prothrombin is diminished coagulation time increases.

Prothrombin is synthesized in liver in the presence of vitamin K. In clotting mechanism prothrombin is converted to thrombin; which transfer soluble fibrinogen into insoluble fibrin clot. Prothrombin time is generally 11 – 16 sec.

Importance of Coagulation of Blood

The phenomenon of coagulation is of enormous physiological importance. Its purpose is to stop further haemorrhage. When bleeding occurs, the shed blood coagulates and the bleeding vessels become plugged off by the clot. The retraction of the clot compresses the ruptured vessels further and in this way bleeding is stopped.

Mechanism of Coagulation or clotting of blood

As early as 1904, Morawitz described the basic facts about the mechanism of blood clotting. According to him, when blood is shed, the platelets disintegrate and liberate thromboplastin. Certain amount of thromboplastin is also derived from the damaged tissues of the injured locality. Thromboplastin converts prothrombin into thrombin with the help of calcium ions and thrombin interacts with fibrinogen forming fibrin. This is the clot.

After shedding of blood
Thromboplastin + Prothrombin + Calcium ion gives Thrombin
Thrombin + fibrinogen give fibrin (clot)

Since 1940, research work has indicated that the clotting mechanism is a complex process and involves various Coagulation factors.

International Nomenclature of Blood coagulation factors

Blood Clotting Factors

Factor I: Fibrinogen

Fibrinogen is globulin in nature but has much bigger molecule than serum globulin. The molecular weight is about 3, 30,000. It is coagulated at around 56⁰C. It is distinguished from other plasma proteins by its property of clotting.

Factor II: Prothrombin

Prothrombin is a protein found in nature and present in normal plasma. It has molecule weight of about 62,700. It is very labile in aqueous solution and is inactivated by acids at pH 4.8. In oxalated plasma two form of prothrombin are found “A” and “B”. In normal plasma the two forms remain united as calcium compound.

Factor III: Thromboplastin

Thromboplastin is derived from two sources. First source is Intrinsic Thromboplastin which is formed in the plasma due to interaction between different Plasma factors. Second source is Extrinsic Thromboplastin which is formed from different tissue (extract of brain, lungs etc) is also called Tissue Thromboplastin.

Factor IV: Calcium

Ionic calcium greatly helps in blood clotting by acting as a co-factor in the coagulation process. It is essential for the formation of both intrinsic and extrinsic thromboplastin and in conversion of prothrombin into thrombin.

Factor V: Labile factor or Accelerator globulin

Labile factor is necessary for the complete conversion of prothrombin into thrombin by the extrinsic or intrinsic thromboplastin. It is a protein, heat labile and is inactivated within half an hour at 56⁰C or by increasing pH to 10.5. It is present in plasma but is used up during clotting.

Factor VI: Accelerin

Accelerin is the hypothetical activation product of proaccelerin i.e. factor V.

Factor VII: Stable factor or Proconvertin

Proconvertin is present in plasma and is not used up during clotting. It is protein remains associated with prothrombin. It accelerates tissue thromboplastin or extrinsic factor. During blood clotting Proconvertin converted into Convertin.

Factor VIII: Antihaemophilic factor (AHF) or Antihaemophilic globulin (AHG)

Antihaemophilic factor helps in the formation of intrinsic thromboplastin and intrinsic prothrombin conversion. It is present in plasma and disappears when the blood clots. It helps in breakdown of platelet and release of factor I.

Factor IX: Christmas factor

Christmas factor is necessary for intrinsic factor or intrinsic thromboplastin formation.

Factor X: Stuart factor

In 1959 the international nomenclature has been given to this factor. It is stable at room temperature.

Factor XI: Plasma Thromboplastin antecedent (PTA)

This factor is activated by active Hageman factor and ultimately leads to formation of thrombin.

Factor XII: Hagemen or surface factor

Hagemen or surface factor is protein in nature. Inactive form is activated on surface contact. This is turn activates the protein splitting enzyme kallikrein to produce plasma Kinins. The resulting effects are increased vascular permeability and dilatation of blood vessels.

Factor XIII: Fibrin – Stabilizing or Laki – Lorand factor

This form is active along with Ca²⁺ ion convert soft fibrin clot to a solid, fibrous one. Its action also decreases the solubility of the clot in urea solution.

Role of Thrombin

Thrombin is a homogenous glycoprotein of molecular weight 40,000 acting as a proteinase. It splits only four peptide bonds in the conversion of fibrinogen to fibrin and these bonds are all between arginine and glycine residues.

Role of Thrombin in conversion of Fibrinogen to Fibrin

Role of Phospholipid

Phospholipid kephalin (Cephalin) helps in the formation of prothrombinase. It is in the platelet factor 3 in intrinsic system and in extrinsic system in the tissue thromboplastin.

Role of Proteins

Blood clotting factor from V to VIII are plasma proteins, mostly ß – globulins. A few of then are however either α – globulins or ү – globulin.

The needs for contribution of many clotting factors are involved in the coagulation process. But the plasma protein clotting factor usually interacts in pairs. Since all the clotting factors are not supposed to possess enzyme action, yet this conversion of enzymes from an inactive form to an active form in initiated the sequence of action of clotting factor.

The process of intrinsic and extrinsic system has been shown as

Process of formation of Clot

Clot Retraction

Usually blood clot retracts to about half its initial volume within 20 – 24hr. When blood is shed fibrins form a network like structure. The platelets adhere to these fibrin networks and form knots. The fibrin framework then becomes twisted and shortened and clot retraction occurs.

Diseases that occur due to defect in clotting Mechanism

In the liver diseases, e.g. cirrhosis of liver, malignant diseases of the liver, there is diminution of synthesis of prothrombin in the liver. In obstructive jaundice due to absence of bile salts, vitamin K is not absorbed. Due to lack of vitamin K, synthesis of prothrombin and factor VII is decreased. Prothrombin time is prolonged and hemorrhages can occur.

(II) Due to lack of AHG or factor VIII

Haemophilia is a disease found in males but transmitted through females. The coagulation time is abnormally prolonged. There is tendency to bleed severely after trivial injuries. The platelet count remains normal. There is lack of factor VIII or Antihemoglobulin factor (AHG). It is also called bleeder’s disease. Body fails to synthesize this essential globulin due to absence of certain enzyme which is controlled by mutant gene.

(III) Due to lack of Christmas factor of factor IX

Haemophilia C is transmitted as a sex – linked recessive in male caused by lack of factor IX.

Haemophilia C was first found in a patient named “Christmas” and hence named Christmas factor.

Due to lack of factor V, VII and IX a disease causes pseudohaemophilia. Its haemorrhagic condition stimulates haemophilia.

Factors preventing Coagulation

1. By lowering temperature, Coagulation can be prevented.
2. By avoiding contact with water – weltable surface and injured tissues. This prevents Thrombokinase action.
3. Removal of calcium ion.
4. Precipitation of fibrinogen.
5. By adding the substances of biological origin. These are

• Protamines – simple protein found in fish.
• Peptone – When it is injected in vein
• Heparin – Mucoitin – Polysulphuric acid produced by mast cells.
• Hirudin – Leech extract and venom of certain snakes.

Heparin, Hirudin and venom of certain snakes prevent blood clotting by inhibiting action of prothrombin and thrombin fibrinogen reaction.

Factors hastening Coagulation Process

Coagulation process can pace up with following factors:

1. Warmth.
2. Contact with water wettable surface and rough surfaces.
3. Additions of foreign bodies into a sample of blood.
4. Addition of thrombin.
5. Addition of thromboplastin.
6. Vitamin K injection or oral administration in high doses.
7. Addition of calcium chloride, both in vivo and vitro.
8. Adrenaline injection produces constriction of blood vessels and helps in haemostatic mechanism.

References

1. Press release University of Chicago (Oct. 16, 2006), University of Chicago team successfully uses chemistry to predict the dynamics of clotting in human blood, link : news.uchicago.edu/releases/06/images/061016.clotting.jpg
2. Gupta L C, (2003), New Illustrated Medical Dictionary, Publisher: All India Travellers Booksellers (A.I.T.B.S.) Publishers & Distributors.
3. Chatterjee C.C, Human Physiology, Publisher B. Jain Publishers
4. Praful B Godkar, Godkar Darshan P, (2007), Textbook of Medical Laboratory Technology, Publisher: Bhalani Publishing House.