OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What triggers the blood clotting process after an injury?

1. Muscle contraction

2. Chemicals released by platelets

3. Increased blood flow

4. White blood cell activation

The correct answer is: Chemicals released by platelets

The blood clotting process is primarily triggered by chemicals released by platelets after an injury occurs. When a blood vessel is damaged, platelets aggregate at the site of the injury and become activated. This activation leads to the release of various chemical signals, including adenosine diphosphate (ADP) and thromboxane A2, which play a crucial role in attracting more platelets to the site and promoting further aggregation. This cascade of events helps to form a temporary "platelet plug" that seals the wound and initiates the more complex process of coagulation, leading to the formation of a stable blood clot through the conversion of fibrinogen to fibrin. In contrast, muscle contraction, while important for movement and overall bodily functions, does not directly influence the clotting process. Increased blood flow might occur in response to injury but does not serve as a trigger for the formation of a clot itself. White blood cell activation is mainly involved in the immune response and does not initiate the clotting cascade. Therefore, the chemicals released by platelets are specifically responsible for starting the intricate series of reactions that culminate in blood clot formation, making this choice the correct answer.