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.

trachea, alveoli, lung capillaries, arteries