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: Characteristic of the entity is arterial occlusion by platelet - fibrin thrombi with distal ischemia occurring four to twenty days after the initiation of heparin therapy , preceded by profound thrombocytopenia with platelet counts in the range of 30 , 000 to 40 , 000 per cubic millimeter .
### Output Text:
Characteristic of the entity is arterial occlusion by platelet - fibrin thrombi with distal ischemia occurring four to twenty days after the initiation of <span class="chemical">heparin</span> therapy , preceded by profound thrombocytopenia with platelet counts in the range of 30 , 000 to 40 , 000 per cubic millimeter . 