I think I may have linked this article somewhere in a thread a while back but I thought I would start a thread on it so others can reference it. I found the information in it was useful.
FULL ARTICLEProper respiration has four major benefits for the martial artist. First, through a neuro-physiologic feedback loop, it keeps the mind calm and "grounded." Just try to become extremely angry or hysterical while taking slow, deep, abdominal breaths. It simply cannot be done. In competitive or confrontational situations, some athletes become so anxious and hyperactive that they are bouncing off the walls. This is the sympathetic nervous system in action. An activated sympathetic nervous system releases a flood of adrenaline resulting in the "fight or flight" response. The adrenaline surge is taxing on both the body and the mind wasting enormous energy reserves.