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: This case illustrates that patients receiving ritodrine for preterm labor may risk interactions between the residual betamimetic effects of ritodrine and the effects of anesthetics during cesarean section .
### Output Text:
This case illustrates that patients receiving <span class="chemical">ritodrine</span> for preterm labor may risk interactions between the residual betamimetic effects of <span class="chemical">ritodrine</span> and the effects of anesthetics during cesarean section . 