Amino acid is to protein as

a physical change is being made.

ok yes, but what is the question? as soon as you ask the actual question i will answer it. : )

these muscles maintain the blood pressure in the vessels. the smooth muscles are innervated by nerves of the autonomic nervous system. when the muscles contract, they cause vasoconstriction of the vessels and increase blood pressure. when the muscles relax, they enable vasodilation of the   blood vessels hence reduce blood pressure.