Final answer:
The first response to a blood vessel injury is vascular spasm, where the smooth muscles in the walls of the vessel contract, constricting the flow of blood. The second response is the formation of a platelet plug, where platelets clump together and bind to the exposed collagen and endothelial lining.
Step-by-step explanation:
The first response to a blood vessel injury is vascular spasm. In this response, the smooth muscles in the walls of the vessel contract, constricting the flow of blood and preventing further loss.The released endothelins and the pain receptors in response to the vessel injury trigger this vascular spasm, which can last up to 30 minutes. Next, the formation of a platelet plug occurs where platelets clump together and bind to the exposed collagen and endothelial lining, creating a temporary seal. The platelets also release chemicals that further contribute to hemostasis.