Hemostasis is the stoppage of bleeding, which is vitally important when blood vessels are damaged. Following an injury to blood vessels, several actions may help prevent blood loss, including blood vessel spasm, platelet plug formation, and blood coagulation. (a) The vasoconstriction reduces blood loss. (b) Platelets adhere to the exposed collagen fibers of a vessel wall, forming a platelet plug. (c) A clot forms as platelets and erythrocytes become enmeshed in fibrin threads.