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 observed effect of NIMO may have been attributable to the preservation of calcium homeostasis during hypotension , because there were no differences in the PbtO ( 2 ) indices among groups .
### Output Text:
The observed effect of <span class="chemical">NIMO</span> may have been attributable to the preservation of <span class="chemical">calcium</span> homeostasis during hypotension , because there were no differences in the PbtO ( 2 ) indices among groups . 