Final answer:
Serotonin is the chemical released by platelets that causes vasoconstriction after a blood vessel is damaged.
Step-by-step explanation:
The chemical released by platelets that causes vasoconstriction after a blood vessel is damaged is serotonin. Serotonin maintains vasoconstriction, which helps to reduce blood flow and prevent excessive bleeding from the damaged vessel. It is one of the chemicals released by platelets as part of the clotting process.