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: Unfractionated heparin ( UH ) is currently the substitute for selected patients .
### Output Text:
<span class="chemical">Unfractionated heparin</span> ( <span class="chemical">UH</span> ) is currently the substitute for selected patients . 