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Platelets are the megakaryocytes (large cells) found in bone marrow. Their primary function is formation of blood clots to slow down and stop the bleeding and help the wound heal.
- Platelets are also called thrombocytes.
- They are colorless and made in the bone marrow tissue.
- Having too many or lower than normal counts of platelets can cause health problems.
- Lower platelets cause problems in blood clot formation. Bleeding will not be stopped if there are too low platelets in the body.
- If the platelets are present in very high numbers, the blood clots will be formed in the blood vessels and this may block the flow of blood through the vessels.
| Table of Content |
Key terms: Platelets, Coagulation, Blood clotting, Haemostasis
Read More: White blood cells (WBCs)
What is Coagulation?
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Coagulation is simply defined as blood clotting. It is an important process to prevent excessive bleeding during any injuries. Coagulation happens when the platelets and plasma proteins work together to form blood clots.
- Blood clotting is a simple phenomena in which liquid blood becomes thicker (jelly like texture).
- Coagulation process is a part of a haemostasis (a body’s own way to stop bleeding)
- Blood clotting occurs in a stepwise manner.
- Constriction of the blood vessel (Primary haemostasis).
- Formation of a platelet plug (Primary haemostasis).
- Activation of coagulation cascade (Secondary haemostasis).
- Formation of fibrin plug which is the final clot (Secondary haemostasis).
Example of coagulation
When we boil or heat an egg, the yolk and white portion turns into a thick solid. This is because the egg contains protein. The thickening of protein due to high temperature is called coagulation.
Read More: Red blood cells (RBCs)
Role of Platelets in Coagulation
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Circulating platelets are essential mediators to trigger the pathway of coagulation cascade. It activates the primary haemostasis by three processes.
- Activation
Recruitment of more platelets.
- Adhesion
Sticks to each other and forms a plug
- Aggregation
Aggregation and interaction with other factors for the coagulation process.
When any injury in blood vessels takes place, the damaged endothelial cells expose collagen and other factors that are involved in clotting. These factors attract platelets at the site of the wound. Platelets stick to each other and form a platelet plug. The plug will release own cellular contents which further attracts more platelets and activate them. Together these cells will interact with the coagulation factors.
Coagulation factors are the plasma proteins that convert the fibrinogen to fibrin that strengthen the platelet plug formation. Eventually, the platelets help in preventing the blood flow.
Read More: Antibodies and antigens
Haemostasis
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It is the body’s natural way of reacting to injury. Our body stops bleeding after a certain time, even without any special treatment to wounded area. This happens due to the natural ability of preventing excessive bleeding. The body has special mechanism for this process called haemostasis.
- It stops bleeding and allows the body to start healing the wounded site.
- In rare cases, haemostasis does not work as it should. This happenes due to the lower platelet counts that can not clot the blood.
- The term homeostasis is made up of the combination of two words- Haemo, which means blood and stasis, means standing still.
- There are two stages of the hemostasis process: Primary and secondary.
Primary haemostasis
This stage includes platelet clotting. A platelet plug is formed to seal the injury in the blood vessel. Platelet plug is formed when a large number of platelets are recruited to the injured site. Their activation leads to the adhesion and aggregation for clotting. Primary haemostasis involves the constriction of blood vessels.
Secondary haemostasis (coagulation cascade)
Platelet plug is not stable enough to stop bleeding and hence, secondary haemostasis is needed to help the plug. This stage is called coagulation. Coagulation involves blood clotting factors that activate the coagulation cascade to further amplify the clotting process. At last, fibrin is formed (fibrin plug) and together with platelet plug, they form a stable and solid clot.
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Things to Remember
- Haemostasis is the way that the body makes for prevention of excessive bleeding.
- This process involves two stages- Primary and Secondary haemostasis.
- Primary haemostasis involves platelets plug formation and constriction of blood vessels.
- Secondary haemostasis which is also called as coagulation cascade involves formation of fibrin plug via activating the clotting factors.
- Platelets and coagulation factors are the major player of haemostasis which help in blood clotting.
- Lower platelets count are not able to help blood flow prevention and higher platelet numbers may block the blood flow in vessels by forming clots.
Sample Questions
Ques. What are the platelets? (1 mark)
Ans. Platelets are the large cells present in bone marrow. They are also called thrombocytes. These cells are the first player in the blood clotting process.
Ques. What happens if the platelets are not in normal range? (2 marks)
Ans. Platelets are the cells that help in blood clotting and the normal range of these cells is crucial for forming a clot. Having too high or too low platelets in the body can cause serious issues.
Higher count of platelets: It may form the blood clots in vessels and block the flow.
Lower count of platelets: Lower number of platelets will not be able to help the clotting process and eventually the body can not perform haemostasis to stop bleeding from injured sites.
Ques. Define the haemostasis mechanism. (5 marks)
Ans. Haemostasis is the physiological reaction to the injury. It is the mechanism to prevent excessive bleeding from injured tissues. This process involves two stages-Primary and Secondary haemostasis.
Primary haemostasis: It starts with activation of platelets and its recruitment to the injured site for the formation of platelet plug. Once the plug is formed, it will release cellular contents and interact with more platelets as well as with clotting factors. This will continue the secondary hemostasis process with further activation of cascade.
Secondary haemostasis: In this stage, the coagulation will take place and hence it is also called as coagulation cascade phase of haemostasis. The coagulation factors which are the plasma proteins, especially fibrinogen, will convert to fibrin that will make a fibrin plug and together with a less stable platelet plug, it will work to form the clots by making platelet plug stable enough.
Ques. How coagulation is related to haemostasis? (1 mark)
Ans. Coagulation means clotting. In blood coagulation, a blood clot is formed to prevent bleeding. A liquid blood is turned into a gel-like thick substance called a clot and this reaction eventually stops the blood flow via the larger reaction process haemostasis.
Ques. What is the role of platelets in primary haemostasis? (2 marks)
Ans. During primary haemostasis, platelets are recruited at the injured site and form a temporary plug by sticking to each other. They release cellular contents and attract more platelets to the site. Further it will interact with coagulation factors to activate fibrin plug formation process (secondary haemostasis).
Ques. Which among the following is not involved in the blood clotting process? (1 mark)
(a) Thrombocytes
(b) Keratinocytes
(c) Melanocytes
(d) Fibrinogen
(e) b and c both
Ans. e. B and c both
Explanation: Thrombocytes are the platelets that start the clotting process via primary haemostasis while fibrinogen is involved in secondary haemostasis for clotting blood. Keratinocytes and melanocytes are the cells present in epidermis of skin and have no role in the clotting process. Therefore, the correct answer is option e.
Ques. Is coagulation and haemostasis the same? Justify your answer. (3 marks)
Ans. No, coagulation and haemostasis are not the same.
Justification: Coagulation is the part of the larger process haemostasis. It takes place during secondary haemostasis when platelet plug is formed and its time to activate coagulation cascade and formation of fibrin plug to make platelet plug more stable. After that activation, coagulation or clotting will take place and the final clot will appear to prevent bleeding.
Ques. How is the platelet plug formed? Why it needs the fibrin plug to work? (3 marks)
Ans. The platelet plug is formed when the injured site recruits platelets. These cells will stick to each other and form a temporary plug called platelet plug. However, this plug is not stable enough for blood clotting.
The need for fibrin plug is due to the reason that platelet plug is not stable to make clot alone. Fibrin plug will make it more stable and together they will make a final clot.
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