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: The ischemia probably induced by coronary artery spasm was reversed by nitroglycerin and calcium blocking agents .
### Output Text:
The ischemia probably induced by coronary artery spasm was reversed by <span class="chemical">nitroglycerin</span> and <span class="chemical">calcium</span> blocking agents . 