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: In addition , the excitability of the cardiac conduction system was depressed preferentially in the atrioventricular nodal region of hearts from barium - exposed rats .
### Output Text:
In addition , the excitability of the cardiac conduction system was depressed preferentially in the atrioventricular nodal region of hearts from <span class="chemical">barium</span> - exposed rats . 