Lecture 16. Platelets (thrombocytes) physiology. Haemostasis (vascular-platelet and coagulational).

Thrombocytes or blood platelets are formed from red bone marrow giant cells – megacaryocytes. Their diameter is fluctuated in the limits between 2 and 4 mcm, volume is 6-9 mcm³. They are two-membraned, nucleus-free but there are lot’s of different granules in them. They are activated in course of side surface, many (up to 10) processes are appeared in them as a result of which their diameter is increased in 5-10 times. There are integrines on platelet membrane serving as receptors. They participate in thrombocytes interaction one to another and to injured vessel. They have glycoprotein nature expressing fibrinogen, collagen, Willebrandt factor (FW) and other substances.

Platelets contain many granules with great number of biologically active substances. One can differentiate:

1. Alpha-granules – contain more than 30 proteins delt with haemostasis and other reactions:

• platelet factor 4;

• fibrinogene;

• thrombostenin and so on.

2. Dense granules – they contain biologically active substances delt with vascular tone and haemostasis:

• ADP;

• adrenaline;

• serotonin;

• thromboxanes et al.

3. Lysosomal granules:

• kinases;

• enzymes.

Platelet number under norma is:

• in adults - 150-350 x 10⁹/l;

• in new-borns - 200 x 10⁹/l (in average – fluctuations 100-400 x 10⁹/l);

• 7-10^th days – 150-200 x 10⁹/l;

• 14^th day – like in adulthood.

Thrombocytosis - their amount increasing:

1. Physiological:

• pain (noceoceptive);

• stressogenic;

• muscular (in course of physical loading).

2. Pathological:

• spleen pathology;

• spleen removal.

Thrombocytopenia – as a rule, is pathology sign and is observed at:

• radiation disease;

• congenital blood diseases;

• acquired blood diseases.

But in women on course of menstruation period platelets amount may be reduced though they are seldom out of normal limits. But it should be mentioned that even at strong thrombocytopenia reaching up to 50 x 10⁹/l, there is no any bleedings and women mustn’t be under medication at these situations. Only when reaching critical ziphras – 25-30 x 10⁹/l - light bleedings may occur at which treaty measures are essential. It testifies that platelets amount in blood is excessive.

Platelets functions:

1. Participation in vascular-thrombocytic and coagulational haemostasis (properly blood coagulation).

2. Angiotrophyc function – vascular walls feeding (15 per cent of all thrombocytes daily). At this function disorders: vascular wall permeability increasing and resistance decreasing.

3. Protective function:

• phagocytosis;

• contain immunoglobulins;

• lyzozyme source;

• they are essential for reparation;

• cytokines source.

Thrombocytopoiesis regulation

1. Specific:

• thrombocytopoietins;

• interleukines-3,6,7,9,11,13.

2. Non-specific:

1. hormones:

• adrenocorticothropic;

• adrenaline;

2. food products:

• nettle;

• fungus puff-ball;

3. sympathetic nerous system excitement.

Haemostasis – is the reactions complex aimed at the blood loss stopping. In fact the significance of haemostasis system is much more complicated and far exeeds the limits of fighting with blood loss.

The main tasks of the haemostasis are the following: the fluid blood state storage, the transcapillary exchange, the vessel wall resistence regulation and the influence on the reparation processes and so on.

They distinguish the vessel-platelet haemostasis and blood coagulation (clotting). Speaking about the first case the question is about the blood loss stopping from the small vessels with low blood pressure; the second one is connected with the blood loss fighting at the arteries and veins rupture. Such division is rather conditional as both at small and large vessels rupture together with the thrombocyte plug forming the blood coagulation is occured. On the other hand such a division is very suitable for clinical practice because at the vessel-platelet haemostasis disorders the finger skin puncture (or the ear lobe) is accompanied by prolonged coagulation time whereas the bleeding time remains normal (for example at haemophilia because of normal platelet count in hemophiliac). Haemophilia is a wide-spread hereditary pathological state. It is the excessive bleeding caused by a congenital lack of a substance (plasma coagulation factor VIII, IX, X or XI) necessary for blood clotting. Treatment consists of administration of the deficient factor.

Vessel-platelet haemostasis comes to the platelet plug (or thrombus) forming. Conditionally it is divided into three stages. 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 - serotonine, 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 sygnal 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.

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 haemocoagulation plasma factors are the proenzymes and their activation occurs due to the limited protheolysis 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 haemostasis 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 the Ciphers.

In the erythrocytes one can find a number of substances like the platelet ones. They are known as the erythrocyte blood coagulation factors. They have no figure designation. The leukocytes have the coagulation factors called leukocyte factors. For example, monocytes and macrophages upon antigen stimulating synthesize the protein thromboplastine part namely apoprotein III (tissue factor).

Tissue factors the main component of which is thromboplastine play a significant role in a blood coagulation. Thromboplastine or tissue factor consists of the protein part apoprotein III and the phospholipid complex and it is often considered to be a cell membrane fragment. Upon the tissue destruction or endothelium stimulation by means of proinflammatory cytokines or endotoxin the tissue factor can be released in a blood circulation. In various blood circulation regions in the vessels its content differs (e.g. in veins and arteries, lower or upper extremities, on the right or on the left in ones of the same name).