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: INTRODUCTION : Acute kidney injury ( AKI ) occurs in 7 % of hospitalized and 66 % of Intensive Care Unit ( ICU ) patients .
### Output Text:
INTRODUCTION : Acute kidney injury ( AKI ) occurs in 7 % of hospitalized and 66 % of Intensive Care Unit ( ICU ) patients . 