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: After gastric surgery , rat stomachs were irrigated for 3 h with either simulated gastric juice or normal saline .
### Output Text:
After gastric surgery , rat stomachs were irrigated for 3 h with either simulated gastric juice or normal saline . 