Question

At the site of tissue injury, the activated platelet releases ADP and activates surrounding platelets to form platelet plug, but this process will not continue to activate whole platelets in the body to form a massive ball of platelets because

• A. The adjacent normal endothelial cells physiologically release ‘NO’ which is a platelet inhibitor
• B. There will be plasminogen activators in the plasma
• C. There will be plasminogen activator inhibitors in plasma
• D. There will be a tissue plasminogen activator (tPA) which inhibits the platelets

Answer 1

The Correct Option is A

The question asks why the process of platelet activation does not continue uncontrollably after an initial injury. The correct answer to this question relates to the regulatory mechanisms in place to prevent excessive platelet aggregation. Let's analyze the options:

1. **Option 1:** The adjacent normal endothelial cells physiologically release ‘NO’ which is a platelet inhibitor
   • Endothelial cells release nitric oxide (NO), which is known to be a potent inhibitor of platelet aggregation. NO prevents the excessive clumping of platelets by increasing cyclic GMP levels, which subsequently results in the inhibition of platelet activation and aggregation.
   • This mechanism ensures that only the platelets near the site of injury form a plug, while the platelets elsewhere in circulation are inhibited.
2. **Option 2:** There will be plasminogen activators in the plasma
   • Plasminogen activators play a role in fibrinolysis, which is the breakdown of blood clots. They do not directly inhibit platelet aggregation but rather help in the dissolution of clots once they serve their purpose.
   • This option does not directly relate to the inhibition of platelet activation.
3. **Option 3:** There will be plasminogen activator inhibitors in plasma
   • These inhibitors prevent the activation of plasminogen (and thus delay fibrinolysis), but do not influence platelet activation. Hence, they are not relevant to preventing excessive platelet aggregation.
4. **Option 4:** There will be a tissue plasminogen activator (tPA) which inhibits the platelets
   • tPA is an enzyme involved in breaking down clots by converting plasminogen to plasmin. It does not have any inhibitory effects on platelets and their role in the process of blood clotting.

Therefore, the correct answer is: The adjacent normal endothelial cells physiologically release ‘NO’ which is a platelet inhibitor. This selective and localized control helps maintain blood flow and prevents the formation of unwanted clots, by ensuring that platelet aggregation only happens where it is needed.