21. In the 1880s, Louis Pasteur developed a method of weakening viruses. The weakened viruses could be injected into healthy individuals.
How is this method effective in fighting viral diseases?

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.  

answer///systemic;