Fibrin is a non-globular, fibrous protein that plays a vital role in forming blood clots. This specific protein is also known as Factor Ia. When protease thrombin acts on fibrinogen it leads it to transform into a polymer, thereby resulting in the formation of fibrin. Along with platelets, the polymerized fibrin forms a blood clot or hemostatic plug at the site of an injury or wound.
When you suffer a cut, you usually bleed. However, the bleeding stops soon. This is because blood has an integrated emergency repair mechanism that blocks any kind of harm to the circulatory system almost immediately. Thereby a temporary patch is created, which makes it possible for additional time to cure the injury.
In fact, when a blood clot forms three mechanisms work simultaneously. Firstly, platelets, which are small pieces of blood cells circulating in the blood stream, assemble at the injury site, thereby giving rise to a frail plug. Next, blood vessels in the neighbourhood of the injury constrict, thereby lessening blood circulation to the affected area. Third and last, there is an assembly of the protein fibrin causing a sturdy network which not only results in the formation of blood clot, but also an impenetrable stumbling block. When these mechanisms are at work simultaneously, they facilitate further blood loss. At the same time, they help in the formation of a tough scab that protects the wound as the healing process is on.
Any break in the blood vessels' lining results in drawing platelets, thereby leading to the formation of a platelet cap. On their surface, these platelets contain thrombin receptors that attach serum thrombin molecules together, which consequently alter soluble fibrinogen present in the serum thrombin into fibrin at the site of the wound. On its part, fibrin results in the formation of elongated strands of sturdy protein that are insoluble and attached to the platelets. Finally, Factor XIII finishes the process of fibrin cross-linking in order to allow it to contract as well as become tougher. After fibrin has been cross-linked it forms a network on top of the platelet plug, thereby completing the process of forming blood clot.
Formation of too much fibrin owing to the commencement the flow of coagulation may result in thrombosis, which is log jam of a blood vessel caused by the binding of red blood cells (erythrocytes), polymerized fibrin, platelets and a number of other elements. Unproductive production of fibrin or its lysis at a premature stage may enhance the chances of suffering a hemorrhages.
On the other hand, any disease or dysfunction of the liver may result in lesser production of fibrinogen, the inactive precursor of fibrin. It may also be responsible for anomalous fibrinogen molecule production, whose activity would be much reduced. Such a condition is known as dysfibrinogenaemia. Incidences of hereditary anomalies in fibrinogen (carried by the gene on chromosome 4) are not only frequent, but also serious in nature. Such inherent fibrinogen abnormalities include conditions like hypodysfibrinogenaemia, dysfibrinogenaemia and hypofibrinogenaemia.
People suffering from reduced, lack of or dysfunctional fibrinogen may be vulnerable to becoming haemophiliacs.
Generally people enduring fibromyalgia have excessive fibrin compared to those who do not have this condition. As a result of fibromyalgia, such people do not possess sufficient enzymatic ability to degrade fibrin or the ingested foods. On the other hand, people with too much fibrin usually have a fibrin accumulation, which builds-up in the form of scar tissue. Such fibrin build-up results in a physical constraint of blood circulation. In fact, fibrin traps the red blood cells in meshes and these ensnared blood cells lose their ability to supply oxygen to muscles. On its part, in order to compensate for the lack of adequate oxygen to the muscles, our body raises the blood pressure. This, in turn, worsens the symptoms related to fibromyalgia.
In normal conditions, fibrin is present in our body as fibrinogen, an inactive precursor of the fibrous protein. Fibrinogen dissolves in water and is present in elevated levels in our blood stream. It remains in the blood stream until it is needed to form clots to protect and heal any wound. When it receives a signal, fibrinogen changes to fibrin, which rushes to the wound site in clusters and forms an elongated fibrous network. Such conversion of fibrinogen into fibrin alters the usual fluidity of blood and makes its jelly-like, which subsequently dries up forming a scab over the wound. However, it is essential that the fibrous fibrin network should only form at the wound site and in no other area of the body, as it is vital for blood to circulate adequately to the other body areas. The flow of specialized proteins to the wound site regulates the formation of blood clot.
Blood coagulation at an injury site is said to be complete in both pathways when fibrin forms an insoluble fibrous network over the affected area. Coagulation of blood denotes the culmination of converting soluble fibrinogen into insoluble fibrous fibrin owing to thrombin's proteolytic activity, which splits two dissimilar peptides from the backbone of fibrinogen. These peptides are known as fibrinopeptide A (FPA) and fibrinopeptide B (FPB) and they are found in two replicas on every fibrinogen molecule. Initially, FPA is split at a relatively rapid pace compared to FPB. Moreover, it has been found that the polymerization reaction is solely subject to the cleaving of the FPA. This results in the assembly of fibrin molecules to turn into protofibrils. Subsequently, these protofibrils become stable following the resultant FPB cleavage.
These protofibrils join in a parallel manner developing into fibrin fibres, which are somewhat twisted. While the fibrin fibres grow laterally, their width is not more than roughly 100 nm. This is because the fibrin molecule stretches the length of the fibre's surface. The process involved in the formation of fibrin is susceptible to the various factors of the environment and any alteration in chemical or physiological conditions will possibly have an effect on the ultimate structure of the network. In fact, there are many factors that have an influence on the structure of fibrin network. The polymer fibres of fibrin can assemble in thicker as well as thinner bundles while they are being formed. Eventually, this factor has an effect on the final network's compactness.
In addition, earlier it was reported that concentration of fibrinogen, thrombin and ion is among the factors that have an influence on the network structure of fibrin. The plasma protein concentration is also said to have a role in the final form of the fibrin network structure. It is commonly believed that an elevated plasma protein concentration also restricts the extension of fibrin fibre spatially. In fact, the interaction of the thinner fibrin fibres with visible light is not as much as that of the thicker fibres. As a result, the network of fibrin fibres made up of thin fibres results in less scattering of light. As a result, the fine network of fibrin fibres appears to be more or less transparent. On the other hand, the thicker fibrin bunch forms a coarse network that is as good as opaque.
The cross linking of the thrombin-activated factor called factor XIII in the coagulation process' concluding phase helps to make the fibrin network stable. The factor XIII is basically a transglutaminase, which connects the fibrin molecules by cross linking them via a process called the ε-(γ-glutamyl) lysin cross links. Factor XIII's physiological functioning is vital, especially people who are prone to bleed owing to a deficiency of factor XIII. In addition, factor XIII also works as a mediator in incorporating fibronectin in the network of fibrin fibres. This process is important, as it enhances the size as well as the density of these fibres.