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: We started LMWH ( 25 IU / kg / h ) on postoperative day ( POD ) 1 , switching to UFH ( 5000 U / d ) on POD 2 or 3 .
### Output Text:
We started LMWH ( 25 IU / kg / h ) on postoperative day ( POD ) 1 , switching to <span class="chemical">UFH</span> ( 5000 U / d ) on POD 2 or 3 . 