During the vascular phase of hemostasis, what happens to the blood vessels?

• A. They constrict to slow blood flow
• B. They dilate to increase flow
• C. Platelets adhere
• D. Fibrin forms

Answer: (A)

During the vascular phase, damaged blood vessels constrict to slow blood loss. This rapid narrowing of the vessel lumen occurs as smooth muscle in the vessel wall contracts and local mediators like endothelin are released. By decreasing flow at the injury site, this constriction gives the body a better chance to form a stable hemostatic plug and set the stage for the next steps. The sticking and aggregation of platelets and the formation of fibrin happen in subsequent phases, not during this initial vasoconstriction. If the vessels were to dilate, bleeding would worsen; platelets adhering and fibrin formation are part of the later responses that stabilize the clot.