Scheme 1. Vascular-platelet haemostasis
The first stage is temporary (primary and secondary) vasoconstriction - immediately in a few seconds after the injury the primary vasoconstriction occurs due to it the bleeding at the first moment may not happen or bears the limited character. It is caused by the adrenaline or norepinephrine releasing in response to the pain irritation and lasts for about 10-15 sec. Futher, the secondary vasoconstriction occurs because of the platelet activation and the releasing from them in a blood the vasoactive substances - serotonin, adrenaline, thromboxanes.
The second stage is the platelet plug forming because of the adhesion (the binding to the foreign surface) and the aggregation (clumping of the platelets). The adhesion takes place immediately after the injury to the collagen and other adhesion subendothelium proteins. It occurs because of the glycoproteins action by means of which the platelets clump to the collagen fibres and by means of the Willebrand factor as well that one of its active centers usage is bound up to the platelet receptor and the other of its receptors to the collagen or subendothelium. From the adhesive platelets and the injured endothelium as well the ADP (adenosine diphosphate) is released, which is one of the major factors of platelet aggregation. Under the unfluence of ADP the platelets clump, so forming the aggregates. This reaction increasing is due to the platelet activation factor (PAF), thrombin and adrenaline. On this stage the aggregation is reversible and the desaggregation may happen. To complete the platelet plug forming a number of additional mechanisms (they are associated mainly with the platelets) are required. When the signal comes into the platelets the calcium content increases in them and the phospholipase A2 activation occurs. The latter one leads to the arachidonic acid releasing from the platelets membranes that further converts into the very active prostaglandines and thromboxanes. When removing from the platelets they make the aggregation irreversible. As a result the platelet plug or thrombus is formed. But at first it is capable of passing the blood as it is loose. After releasing the actomyosine (thrombostenine) from the platelets during their aggregation the platelet plug is shortened and reinforced. This is the third stage of the vessel-platelet haemostasis – the platelet plug retraction.
Under the normal condition the blood loss stoppage from small vessels lasts from 2 to 4 minutes. Such index in the clinic is known as the bleeding time.
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The arachidonic acid derivates – prostacyclin and thromboxane A2 - play a very important role in the vessel-platelet haemostasis regulation. Prostacyclin is produced by endotheliocytes under the enzyme prostacyclinsynthetase influence. Under the physioilogical conditions prostacyclin predominates over thromboxane – powerful platelet proaggregant. At any endothelium injury in the trauma place the prostacyclin producing disturbs and the thromboxane action begins to be predominate. Thus, the favourable conditions for the platelet aggregation emerge. Some vitamins (A,C,E) and foods (onion, garlic) are the platelet aggregation inhibitors.
Blood coagulation is an enzyme process where both the plasmic and the cell factors participate. Most of the hemocoagulation plasma factors are the proenzymes and their activation occurs due to the limited proteolysis and is accompanied by the peptide inhibitors cleavage. They are designated the Roman figures. There are 13 such factors in plasma.
The platelets play an important role in a blood coagulation process. They contain a lot of (more than 30) different substances which deal with the hemostasis process. Some of them (according to the various literature scientific sources from 5 to 15) are called the platelet (thrombocyte) coagulation factors that are designated with the Arabic numerals.
Materials for auditory self-work
4.1. List of study practical tasks necessary to perform at the practical class.
It’s necessary for work: microscope, aggregatograms, Goryaev’s chamber, covering glasses, plasma rich in platelets, aggregatograph.
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