Which statement best describes a natural resource? a. a machine that generates electricity b. any material supplied by the environment that humans use c. paper money d. physical features of the land, such as mountains and rivers

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.