Which is a part of the theory of evolution by natural selection? apex

Small changes add up over many generations.

Only the best-adapted animals in a population survive.

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.