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: Sham - operated rats served as normotensive controls ( 128 + / - 3 mm Hg , n = 8 ) .
### Output Text:
Sham - operated rats served as normotensive controls ( 128 + / - 3 mm Hg , n = 8 ) . 