Platelets or Thrombocytes
Definition
Very small non-nucleated discs
2-4 μm. in diameter
Platelets contain substances that promote blood clotting (haemostasis)
Normal platelet count = 2 laks to 4 laks / mm3
Platelet production stimulated by Thrombopoietin produced by the liver and kidneys.
Lifr of a platelet - 8 to 11 days
Destruction by macrophages in the spleen and by Kupffer cells in the liver.
Function
They circulate in the blood and are involved in hemostasis, leading to the formation of blood clots.
When a blood vessel is damaged loss of blod is stopped and healing occurs in a series of overlapping processes in which platelets play a vital part : -
Vasoconstriction : platelets adhere to damaged blood vessel wall
their surfaces become sticky
releases serotonin ( 5 - hydroxytryptamine), which constricts the vessel, reducing blood flow through it.
Platelet plug formation : adherent platelets clump together
attract more platelets
many platelets get attracted to the clumped platelets
a platelet plug is formed quickly to seal the blood vessel damage.
Coagulation :
blood clots in the damaged area
an insoluble thread-like mesh of fibrin is formed which traps blood cells
blood clotting is helped by clotting factors
following clotting prothrombin activator acts on prothrombin and converts it into thrombin.
thrombin converts fibrinogen into fibrin
Fibrinolysis
after the clot has formed the process of removing it and healing the damaged blood vessel begins. the breakdown of the clot or fibrinolysis is the first stage. Plasminogen in the clot is converted into the enzyme plasmin. plasmin initiates the breakdown of
In transfusion medicine
Platelets are either isolated from collected units of whole blood and pooled to make a therapeutic dose or collected by apheresis, sometimes concurrently with plasma or red blood cells. The industry standard is for platelets to be tested for bacteria before transfusion to avoid septic reactions, which can be fatal.
Pooled whole-blood platelets, sometimes called "random" platelets, are made by taking a unit of whole blood that has not been cooled and placing it into a large centrifuge in what is referred to as a "soft spin." This splits the blood into three layers: the plasma, a "buffy coat" layer, which includes the platelets, and the red blood cells. These are expressed into different bags for storage.
Apheresis platelets are collected using a mechanical device that draws blood from the donor and centrifuges the collected blood to separate out the platelets and other components to be collected. The remaining blood is returned to the donor. The advantage to this method is that a single donation provides at least one therapeutic dose, as opposed to the multiple donations for whole-blood platelets. This means that a recipient is not exposed to as many different donors and has less risk of transfusion-transmitted disease and other complications. Sometimes a person such as a cancer patient that requires routine transfusions of platelets will receive repeated donations from a specific donor to further minimize the risk.
Platelets are not cross-matched unless they contain a significant amount of red blood cells (RBCs), which results in a reddish-orange color to the product. This is usually associated with whole-blood platelets, as apheresis methods are more efficient than "soft spin" centrifugation at isolating the specific components of blood. An effort is usually made to issue type specific platelets, but this is not as critical as it is with RBCs.
Platelets collected by either method have a very short shelf life, typically five or seven days depending on the system used. This results in frequent problems with short supply, as testing the donations often requires to use up a full day. Since there are no effective preservative solutions for platelets, they lose potency quickly and are best when fresh.
Platelets, either apheresis or random-donor platelets, can be processed through a volume reduction process. In this process, the platelets are spun in a centrifuge and the excess plasma is removed, leaving 10 to 100 ml of platelet concentrate. Volume-reduced platelets are normally transfused only to neonatal and pediatric patients when a large volume of plasma could overload the child's small circulatory system. The lower volume of plasma also reduces the chances of an adverse transfusion reaction to plasma proteins. Volume reduced platelets have a shelf life of only four hours.
Diseases involving platelets
If the number of platelets is too low, excessive bleeding can occur.
If the number of platelets is too high, blood clots can form (thrombosis), which may obstruct blood vessels and result in such events as a stroke, heart attack, pulmonary embolism or the blockage of blood vessels to other parts of the body
Disease of the platelets is called a thrombocytopathy: -
A low number of platelets (thrombocytopenia)
A decrease in function (thrombasthenia)
An increase in the number of (thrombocytosis).
The disorders that reduce the number of platelets :-
1. Heparin-induced thrombocytopenia (HIT)
2. Thrombotic thrombocytopenic purpura (TTP) that typically cause thromboses, or clots, instead of bleeding.
Duke Method
(To determine bleeding time)
With the Duke method, the patient is pricked with a special needle or lancet, preferably on the earlobe or fingertip, after having been swabbed with alcohol. The prick is about 3-4 mm deep. Then the blood is wiped every 30 seconds with a filter paper. The test ceases when bleeding ceases. The usual time is about 1-3 minutes.
