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: Thus , whereas nitroglycerin preferentially and flow - independently dilates large coronary arteries , cromakalim and pinacidil dilate both large and small coronary arteries and this effect is not dependent upon the simultaneous beta adrenoceptors - mediated rise in myocardial metabolic demand .
### Output Text:
Thus , whereas <span class="chemical">nitroglycerin</span> preferentially and flow - independently dilates large coronary arteries , <span class="chemical">cromakalim</span> and <span class="chemical">pinacidil</span> dilate both large and small coronary arteries and this effect is not dependent upon the simultaneous beta adrenoceptors - mediated rise in myocardial metabolic demand . 