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: Despite rapid tubular excretion of estradiol which peaked in less than 1 h , the normal cells did not appear to bind the ligand .
### Output Text:
Despite rapid tubular excretion of <span class="chemical">estradiol</span> which peaked in less than 1 h , the normal cells did not appear to bind the ligand . 