When a platelet contacts a damaged blood vessel, it is stimulated to release thromboxane a2. thromboxane a2 in turn stimulates vascular spasm and attracts additional platelets to the injured site. in this example thromboxane a2 is acting as a

It is acting as a local signaling agent. It allows for more platelets to be attracted to the damaged area, instead of having them sent all over the body. As a local signal, it is also acting on the behavior of nearby cells, which is seen by the vascular spasm that is being instigated at the site of the damage.

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