Part of the complexity and brilliance of your pulmonary system is that your breathing changes speed in accordance with demands being placed on your body/mind at any given second. All this happens automatically (although you have the power to intervene and go from Auto to Manual when it suits you). Cool.
However, when you overburden the pulmonary system you interfere with this exquisite engineering. Your pulmonary system is brilliantly designed to handle occasional assaults but modern man is piling on errors beyond what the body can handle at this point in human evolution: stress, drugs, cigarettes, air pollution, building toxins, inactivity, too much sugar, not enough exercise. . . .
The breath should flow easily and smoothly in and out of the lungs at a speed that is appropriate for who you are, where you are and what you are doing. Notice for a moment if your breathing is steady, even and smooth as a silk ribbon sliding between your fingers. Or is it uneven and raggedy? Are you holding your breath? Does the breath feel as if it is flowing all the way down to your bellybutton or does it stop behind your second shirt button?
Slow steady smooth breathing allows your body time to do triage on the incoming air. Since breathing is the bridge that spans mind, body and spirit, steady breathing has the power to calm your nerves and make your spirit more available.
Be well. Breathe beautifully.