17677
The micro-circulation - the arterioles , capillaries , and venules - constitutes most of the area of the vascular system and is the site of the transfer of O , glucose , and enzyme substrates into the cells .
 O
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 O