Final answer:
The first phase in hemostasis after a blood vessel injury is the vascular spasm, where the vessel's smooth muscle constricts to limit blood flow and reduce blood loss. This is followed by the formation of a platelet plug and coagulation process.
Step-by-step explanation:
When a blood vessel is injured, the first phase in hemostasis to occur is vascular spasm. This immediate reaction involves the constriction of the blood vessel's smooth muscle, limiting the flow of blood through the vessel. This is critical in reducing blood loss. After the vascular spasm, other key phases follow. These include the formation of a platelet plug to temporarily seal the vessel, and then the process of coagulation or blood clotting which stabilizes the plug and concludes the repair of the vessel wall. Notably, platelets not only assist in forming the plug but also release substances that trigger a cascade leading to the synthesis of fibrin. This synthesis can follow an intrinsic or extrinsic pathway but ultimately converges on a common pathway resulting in a stable clot formation reinforced by fibrin strands.