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[A]: When platelets disintegrate, 5-hydr...

[A]: When platelets disintegrate, 5-hydroxy tryptamine and histamine are liberated.
[R]: Speed of clot reaction is directly propor- tional to the number of platelets present.

A

If both A and R are true and R is the correct explanation of A

B

If both A and R are true but R is not the correct explanation of A

C

If A is true and R is false

D

If both A and R are false

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### Step-by-Step Solution: 1. **Understanding Platelets and Their Role**: - Platelets, also known as thrombocytes, are small cell fragments in the blood that play a crucial role in the clotting process. They are essential for preventing blood loss when there is an injury. **Hint**: Recall the function of platelets in the body and their importance in hemostasis (the process of blood clotting). 2. **Disintegration of Platelets**: - When platelets disintegrate or rupture, they release various substances, including 5-hydroxytryptamine (serotonin) and histamine. These substances are involved in the clotting process and help in the constriction of blood vessels. **Hint**: Think about what happens to cells when they break down and what substances might be released that could affect blood clotting. 3. **Role of 5-Hydroxytryptamine and Histamine**: - 5-hydroxytryptamine (serotonin) is a vasoconstrictor, which means it helps to narrow blood vessels, reducing blood flow to the area of injury. Histamine plays a role in inflammatory responses and can also affect blood vessel permeability. **Hint**: Consider how these substances might contribute to the overall process of clot formation and healing. 4. **Clotting Speed and Platelet Count**: - The speed of the clotting reaction is directly proportional to the number of platelets present. This means that the more platelets there are, the faster the clotting process occurs. If there are fewer platelets, the clotting process will be slower. **Hint**: Reflect on the relationship between the quantity of a substance (in this case, platelets) and the speed of a reaction (clotting). 5. **Conclusion**: - Both statements in the question are true: when platelets disintegrate, they release 5-hydroxytryptamine and histamine, and the speed of clotting is directly proportional to the number of platelets present. **Hint**: Summarize the main points to ensure you understand how platelet disintegration and quantity affect clotting.
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