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 most distinctive aspect of the barium effect was a demonstrated hypersensitivity of the cardiovascular system to sodium pentobarbital .
### Output Text:
The most distinctive aspect of the <span class="chemical">barium</span> effect was a demonstrated hypersensitivity of the cardiovascular system to <span class="chemical">sodium pentobarbital</span> . 