Pull-ups focus on which muscle region of the body?

Pull-ups is not a healthy way to excerise. There are some cons to this excerise. Regardless, it helps with...
abdominal - defining them, by burning and stretching fats
Calves
Chest 
and Back...
After your workout, your body will see to restoring and replacing muscles, this is where the phrase; "Defined and Refined" comes in. The muslces are then loaded with fibres, which then creates the protein and myofilibs. 

I hope this helped.

Small intestine because that is the organ which absorbs nutrients