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: Also recorded were one case of pain of the vein , which required treatment , and two cases of hypotension .
### Output Text:
Also recorded were one case of pain of the vein , which required treatment , and two cases of hypotension . 