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: When discharged less than 24 hours later , he was receiving metoprolol and aspirin , with follow - up plans for echocardiography and nuclear imaging to assess perfusion .
### Output Text:
When discharged less than 24 hours later , he was receiving <span class="chemical">metoprolol</span> and <span class="chemical">aspirin</span> , with follow - up plans for echocardiography and nuclear imaging to assess perfusion . 