Haemostasis is a complex phenomenon because the first aspect is keeping the blood fluid and the second is stopping bleeding. Our body is based on three main hemostatic systems, i.e. the vascular, platelet and plasma systems. The main task of hemostasis is to maintain the balance between the coagulation process and fibrinolysis, i.e. the dissolution of blood clots. Imbalance may result in pathological bleeding as well as thromboembolic diseases.
Hemostasis process
Most of us are unaware of the complexity and multi-stage process of hemostasis. We only have the opportunity to observe how it affects our body. We all hurt ourselves. Thanks to haemostasis, after a while the wounded area stops bleeding. This situation does not impress us, but many processes take place in our body.
The first element is vascular hemostasis. When a vessel is damaged, spasm occurs. The walls of blood vessels consist of smooth muscle cells that are able to constrict their lumen. Then the blood flow is restricted, and hence the bleeding is reduced. Vascular endothelial cells are very important and have different functions. Among other things, they are responsible for sending information about a damaged vessel to muscle cells. Their task is also to release molecules that activate and inhibit clotting. These cells are responsible for giving the nerve fibers a pain signal. If the endothelium is damaged, layers of the wall, mostly made of collagen, appear in the lumen of the vessel. During clotting, platelets stick to it.
Then begins the second stage, or platelet haemostasis. In this phase, platelets are responsible for starting the clotting process. Plaques accumulate where the endothelium has been damaged and the exposed collagen binds to them. When the plates are activated, their shape changes, and the stored substances are released from them. These include calcium, magnesium, serotonin, ADP and other factors that trigger the next stage of clotting.
The next phase is plasma haemostasis. Thirteen plasma clotting factors are involved in this process, i.e. protein molecules that constantly circulate in our blood. Interestingly, these molecules have the possibility of cascading activation, i.e. there is a sequence of reactions, which allows for the activation of other, so far passive factors. It is also worth mentioning the so-called extrinsic and intrinsic pathways of the coagulation cascade, in which the factors are different, but the end is common. The result of both of these pathways is fibrin, which is also called stable fibrin (insoluble substance, made of long resistant fibers). The mentioned fibrin is an indispensable element needed in the process of secondary haemostasis.
Secondary haemostasis is the formation of a strong network to strengthen the primary platelet plug. The fixed fibrin clot is the final product of the entire solidification process. Thanks to it, the place of injury is protected against bleeding, and it is also possible to scar the damaged vessel. It is also worth knowing that an indispensable element of hemostasis is the process of fibrinolysis, which consists in dissolving the fibrin. Interestingly, fibrinolysis occurs in our body all the time. Also in places where blood clots are forming at the same time. It is thanks to fibrinolysis that it is possible to control the size of blood clots. If the blood clots were to increase without any restriction, this could cause the vessel lumen to become occluded, thereby blocking the blood flow. Therefore, it can be guessed that the fibrinolysis process is designed to dissolve blood clots in the places of healing wounds and maintain the fluidity of blood under physiological conditions.
The main substance that can dissolve fibrin and clots is plasmin. It is a protein molecule formed during the cascade activation of other factors. The process is considered very complicated. It has many levels at which it is controlled by so-called activators and inhibitors. When plasmin is activated, it is able to break down the fibrin into short, easily dissolving threads. As a result, the clot is broken down into fragments of molecules and cells. Eventually, they are digested by the food cells.