What Are Platelets?
Platelets are small, disc-shaped cell fragments found in our blood that play a major role in blood clotting and wound healing. They are formed in the bone marrow from large cells called megakaryocytes. Under the influence of the hormone thrombopoietin, stem cells in the bone marrow mature into megakaryocytes, which later break up into thousands of platelets.
Each microliter of blood normally contains 150,000 to 400,000 platelets. These platelets circulate in the bloodstream for about 5 to 9 days before being destroyed by macrophages in the spleen and liver.
The main function of platelets is to prevent blood loss by forming clots and sealing damaged blood vessels.
Role of Platelets in Hemostasis (Blood Clotting)
Whenever a blood vessel is damaged, the body immediately works to stop bleeding through a process called hemostasis, which happens in four key steps:
1. Vasoconstriction
When platelets touch the damaged blood vessel wall, they become sticky and release a chemical called serotonin (5-hydroxytryptamine). This causes the vessel to constrict (narrow), reducing blood flow and loss.
2. Platelet Plug Formation
Platelets stick together and release ADP (adenosine diphosphate), which attracts more platelets to the site. This creates a platelet plug that temporarily seals the injury.
3. Coagulation (Blood Clot Formation)
Next, a stronger fibrin clot forms. This involves several clotting factors that convert prothrombin to thrombin and fibrinogen to fibrin.
There are two main pathways for this:
- Extrinsic Pathway: Triggered by damage to tissues outside the blood.
- Intrinsic Pathway: Triggered by damage inside the blood vessels.
Both lead to the formation of a stable blood clot that prevents further bleeding.
4. Fibrinolysis (Clot Removal)
After the vessel heals, the clot is broken down by an enzyme called plasmin. This process clears out fibrin and restores normal blood flow.
Factors Affecting Blood Coagulation
Coagulation can be accelerated by:
- Slightly higher temperature than normal body temperature
- Injury to blood vessels or platelets
- Slow blood flow
- Snake bites
Coagulation can be slowed down by:
- Low temperature
- Smooth surfaces (no injury)
- Presence of sodium or potassium citrate
If a clot forms inside a blood vessel, it’s called a thrombus.
If a piece of it breaks loose and travels in the bloodstream, it’s called an embolus.
Common Disorders Related to Platelets and Blood Coagulation
1. Thrombocytopenia
This condition occurs when the platelet count falls below 150,000/mm³. It may happen due to:
- Reduced platelet production in bone marrow diseases (like leukemia, pernicious anemia, or radiation exposure).
- Increased platelet destruction in autoimmune conditions.
2. Autoimmune Thrombocytopenic Purpura
Common in children and young adults, this occurs when the immune system destroys platelets, often after a viral infection like measles.
Symptoms include skin hemorrhages (purpura), bruising, bleeding in the urine, or even internal bleeding when platelet counts are extremely low.
3. Secondary Thrombocytopenic Purpura
This can occur due to bone marrow diseases, radiation, or certain drugs like quinine, digoxin, or sulphonamides.
4. Vitamin K Deficiency
Vitamin K is needed by the liver to make clotting factors.
Deficiency can cause bleeding and may result from:
- Poor fat absorption (as Vitamin K is fat-soluble)
- Liver disease
- Prolonged bile duct obstruction
- Antibiotics that kill vitamin K-producing gut bacteria
In newborns, Hemorrhagic Disease of the Newborn can occur when vitamin K levels are too low after birth, especially in premature babies.
5. Disseminated Intravascular Coagulation (DIC)
DIC is a serious condition where blood clots form inside blood vessels throughout the body, using up clotting factors and platelets. This can lead to severe bleeding.
Common causes include:
- Severe infections or septicemia
- Shock
- Complicated childbirth (premature placental separation)
- Severe trauma or burns
- Malignant tumors
Detailed Notes:
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