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: We have previously shown that a permanent deficiency in the brain renin - angiotensin system ( RAS ) may increase the sensitivity of the baroreflex control of heart rate .
### Output Text:
We have previously shown that a permanent deficiency in the brain renin - <span class="chemical">angiotensin</span> system ( RAS ) may increase the sensitivity of the baroreflex control of heart rate . 