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: Administration of aprotinin should be avoided in coronary artery bypass graft and high risk patients , whereas administration of tranexamic acid is not recommended in valve surgery .
### Output Text:
Administration of aprotinin should be avoided in coronary artery bypass graft and high risk patients , whereas administration of <span class="chemical">tranexamic acid</span> is not recommended in valve surgery . 