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: Caffeine did not alter plasma MHPG levels in either the healthy subjects or patients .
### Output Text:
<span class="chemical">Caffeine</span> did not alter plasma <span class="chemical">MHPG</span> levels in either the healthy subjects or patients . 