From what I understand, body parts usually take on the article , "the". ie. "le", "la" and "au". You assume it is the subjects body parts when in discussion.
This is shown in many examples in duolingo: ex. "Il a le visage rouge" = He has a red face. His face is red. ex. "J'ai mal a l'epaule" = My shoulder hurts. I have a sore shoulder.
I assume this has something to do with the literal translation, "J'ai mal a l'epaule" being, "I have a pain to the shoulder".
If I am incorrect or anyone else has something to add, please do.