The endocrine system involves communication by chemical messengers called which circulate through the

Your answers would be Hormones and Bloodstream

hope this helps.  

1. "HORMONES"

2. "BLOODSTREAM"

Explanation:

endocrine glands are the ductless glands which secrete chemical messengers called as hormones ,and these circulate through bloodstream to targeted sites where they inhibit or enhance a function occuring.

significant amounts of nitrogen oxides, carbon monoxide, and other pollution

explanation:

passenger vehicles are a major pollution contributor, producing significant amounts of nitrogen oxides, carbon monoxide, and other pollution. in 2013, transportation contributed more than half of the carbon monoxide and nitrogen oxides, and almost a quarter of the hydrocarbons emitted into our air.

an active site is a region that is present on an enzyme where the reactants or the substrate molecules we can say bind and experience a chemical reaction. the substrate is basically a reactant whose concentration is changing and that is converting into a product after binding at the active site of an enzyme