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: BACKGROUND : Our aim was to investigate postoperative complications and mortality after administration of aprotinin compared to tranexamic acid in an unselected , consecutive cohort .
### Output Text:
BACKGROUND : Our aim was to investigate postoperative complications and mortality after administration of aprotinin compared to <span class="chemical">tranexamic acid</span> in an unselected , consecutive cohort . 