Page 24 - Human anatomy COMPLETELY DONE1

The presence of antibodies in the blood to a certain pathogen creates the body's
immunity to repeated diseases. If the formation of lymphocytes is disrupted, a person
loses protection from infection.
PLATELETS
Platelets are usually called blood plates and are not considered cells. They are
very small, have a flat irregular shape, have no nuclei and live for several days. Platelets
are constantly formed in the red bone marrow as fragments of the cytoplasm of its giant
megakaryocyte cells.
The number of platelets in human blood is 200-400 thousand in mm3.
The main function of platelets is to help stop bleeding. At the site of the wound
of the vessel, the blood coagulates, forming a clot – a blood clot that prevents blood
from flowing out. Blood clotting is the most important protective reaction that protects
the body from blood loss during the destruction of blood vessels.
If the vessel is damaged, the platelets located in this place of the bloodstream are
destroyed. A number of chemicals necessary for blood clotting come out of them.
The basis of the thrombus is the insoluble fibrous protein fibrin, formed from the
fibrinogen protein dissolved in plasma. For the conversion of fibrinogen into fibrin,
substances in the blood are necessary, as well as those formed during the destruction of
platelets, blood vessels and surrounding tissues. Thus, fibrin is formed as a result of a
rather long chain of chemical interactions.
Blood cells are trapped between the fibrin strands. The formation of a blood clot
occurs within 3-8 minutes. The clot gradually thickens and closes the site of damage to
the vessel, thereby stopping the bleeding. Over time, the vessel wall is restored, and the
thrombus resolves.
Calcium salts play an important role in the blood clotting process.
Blood plasma also has the ability to coagulate, since it also contains fibrinogen.

Its coagulation occurs under the same conditions as blood clotting, with the only
difference that plasma coagulates more slowly.
Blood clotting is a complex process during which special biologically active
substances – blood clotting factors (there are more than 10 of them) are isolated from
destroyed platelets, damaged muscle tissues and blood vessels.
Even in the absence of damage to tissues and blood vessels, a small amount of
fibrinogen is converted into fibrin. Usually this process is balanced by the action of
other – anticoagulant factors. In particular, the coagulation of blood inside blood vessels
in the human body is prevented by the biologically active substance heparin, which is
formed in the liver. The anti-clotting factors include gerudine contained in the saliva of
a leech.
The absence of coagulation factors in the blood or a violation of their formation
in the body leads to diseases. One of them is hemophilia. In a patient with hemophilia,
the blood does not clot, and a person can die from bleeding with the most minor vascular
damage.

24