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 study supports a hypothesis that during reperfusion , a short interval of hypertension decreases brain injury and edema ; and that sustained hypertension increases the risk of vasogenic edema .
### Output Text:
This study supports a hypothesis that during reperfusion , a short interval of hypertension decreases brain injury and edema ; and that sustained hypertension increases the risk of vasogenic edema . 