[In the goku-i the first tome talks of Heaven; the I Ching and other ancient texts also expound on Heaven, Earth, and Man in a variety of references. This particular passage in the Tao-te-Ching leads us to understand that references to heaven in the translations is far different from our perception. In this last quote it carries to full circle since breathe is vital to longer life and at the end Heaven is connected to the nostrils for that same breathe.
Breathing is a "key" in life and especially within the fighting/martial art applications. Health and all we are internally start and end with breathe. Breathing can influence the heart and its beating. Longer and deeper breathes can cause the heart to beat slower. Breathes can influence aspects of the body where it causes changes in the brain/mind and the mind then leads the body to react to this change either in a positive way or negative.
Negative effects are when suddenly surprised the breathe becomes shallow and quick causing stress and sometimes fear while a positive effect is you recognize it instinctually thus breathe slower, deeper, diaphragmatically, resulting is changes that reduce the stress and fear and allow for clear thinking processes, etc.
Breathing is connected to all of the body and mind in so many ways that one needs to study this long to achieve a modicum of knowledge but for FMA/MA practitioners all you need to know is when the adrenaline dump hits practice to breathe slower, deeper, and diaphragmatically.]