What is typically the first response of the body when a blood vessel is injured?

When a blood vessel is injured, the body’s first response is to initiate hemostasis, which includes the immediate formation of a platelet plug. This process begins with the exposure of the underlying collagen and tissue factors when the vessel wall is damaged. Platelets, which are small cell fragments in the blood, adhere to the exposed area and become activated. Upon activation, they change shape, release chemical signals that attract more platelets, and aggregate to form a temporary "plug" that seals the break in the vessel wall. This platelet plug is crucial for minimizing blood loss and serves as a foundation for later stages of clot formation, such as fibrin clot formation.

Subsequent responses may include the release of inflammatory cytokines to recruit additional immune cells, dilation of the injured vessel to increase blood flow, and eventually the formation of a fibrin clot to provide a more permanent solution to the injury. Thus, while these other processes are important in the overall healing response, the immediate action upon injury is the formation of the platelet plug, making it the first critical step in stem the bleeding.