Well, Westles, our "resident Doc" has taken her leave for the forseeable future. Rather than write my own copy I decided to copy this link for you and others. It discusses this topic rather thoroughly.
When I reply to someone wanting instructions on the "how to" of anal penetration, I strongly state that a couple should never chafe the tissue and/or draw blood. The author of this information takes this warning much further stating that even microscopic abrasions can be extremely dangerous.
Hereyago:
http://www.wwu.edu/chw/ask_the_doc/p...1000/0513.html