One interesting kundalini yoga exercise that I did a lot a long time involved exhaling, holding your breath and then pumping your stomach in and out (while holding your breath) for about the count of 30 or so.

This exercise was suggested to be done in a bath of warm water, but I used to do it everywhere. This exercise can be amazingly difficult sometimes and very easy to do at other times. I did the stomach pump thing to the count of 50 most of the time.

This exercise is said to expell toxins. Supposedly, this exercise was originally done in India by a person while that person was completely wrapped in warm wet towels. The claim made was that after the person finished doing a lot of this, the towels he was wrapped in became dirty and full of the toxins expelled from the body by doing the exercise.

From what I read, there apparently is such as thing as skin respiration, i.e. that we do BREATH through our skin. There is an old James Bond movie in which a woman dies as a result of her being painted gold and she could not breath through her skin.