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: Cimetidine , a histamine 2 ( H2 ) antagonist , produces a decrease in arterial pressure due to vasodilatation , especially in critically ill patients .
### Output Text:
<span class="chemical">Cimetidine</span> , a <span class="chemical">histamine</span> 2 ( H2 ) antagonist , produces a decrease in arterial pressure due to vasodilatation , especially in critically ill patients . 