Breath is THE most important tool for healing and cikitsa (therapy). Yoga is a a practice of involving not only the body but also the mind. This link is the BREATH.
T.K.V Desikachar says Breath is the intelligence of the body and becomes our tool for prevention of many different ailments in many different ways.
It is the link between the inner and outer body and the quality of the breath indicates our state of mind. If we are feeling anxious, observe the breath will be short and shallow whilst if we are relaxed and calm, the breath will change to slow and deep.
A tortoise, with slow and deep breath lives almost 100 years whilst a dog with short and fast breath pace has a much shorter life span. It's a direct correlation.
That's why when we are feeling stress, they say - Take a Deep breath……...