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: In this patient , renal artery stenosis combined with heart failure and diuretic therapy certainly resulted in a strong activation of the renin - angiotensin system ( RAS ) .
### Output Text:
In this patient , renal artery stenosis combined with heart failure and diuretic therapy certainly resulted in a strong activation of the renin - <span class="chemical">angiotensin</span> system ( RAS ) . 