In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: The blood flow responses seem to be mediated by the release of acetylcholine and VIP within the meninges .
### Output Text:
The blood flow responses seem to be mediated by the release of <span class="chemical">acetylcholine</span> and VIP within the meninges . 