Typical orienting reactions include the following the arteries to the brain grow wider allowing more blood to reach it, the heart slows down and arteries to the large muscles become narrower so as to reduce blood supply to them.
It’s from an IELTS study guide.
First, I don't know why two nouns can connect without a preposition. (the following the arteries) As far as I know, The following is used to describe the list of what comes next. e.g. The following is an extract from her diary.
Another, I don't know why present participle (allowing) can come without any connector or comma. I think it might come from "which allows" and then omit "which" but in this way "which" cannot omit because "which" means "the arteries to the brain grow wider" and the sentence doesn't have a connector. In this case, there should be a comma before "allowing".