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: The low incidence of cerebral haemorrhage as a cause of death in patients with Parkinson ' s disease may reflect the hypotensive effect of levodopa and a hypotensive mechanism due to reduced noradrenaline levels in the parkinsonian brain .
### Output Text:
The low incidence of cerebral haemorrhage as a cause of death in patients with Parkinson ' s disease may reflect the hypotensive effect of <span class="chemical">levodopa</span> and a hypotensive mechanism due to reduced <span class="chemical">noradrenaline</span> levels in the parkinsonian brain . 