Final answer:
The first response when a blood vessel's lining is damaged is that platelets adhere to the damaged area, beginning the process of clotting and hemostasis.
Step-by-step explanation:
When tissue lining a blood vessel is damaged, the first thing that happens is platelets adhere to the damaged tissue. Coagulation begins almost instantly after an injury to the endothelium lining of a blood vessel. Platelets, or thrombocytes, become activated and change their shape to help them clump together at the site of injury, forming the initial plug. This is augmented by the von Willebrand factor, a glycoprotein that stabilizes the platelet plug. The platelets also release substances that contribute to hemostasis by attracting more platelets and interacting with clotting factors, which convert fibrinogen into fibrin, further strengthening the clot.