Animal cells tend to not have a uniform shape. This is because of the lack of which cell structure? chloroplast

nucleus

cell wall

cell membrane

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.