(Calude Monet painting, of the bridge, from this site)
The ancient Yoga Chudamani Upanishad (authorship unknown) assures us that "pranayama becomes fire for the fuel of sin, and has always been regarded by yogis as a great bridge for crossing the ocean of the world."
Even Ramana Maharishi, that celebrated sage of Arunchalam, who did not approve of an obsessive yoga practice encouraged breath control as the ideal way to tame a reluctant mind and coax it towards higher spiritual goals. He says: "The source of the breath is the same as that of the mind. Therefore the subsistence of either leads to the other. The practice of stilling the mind through breath control is called yoga."
While this psychic aspect of pranayama interests many, its biological relevance is equally riveting. According to the well-researched book Freedom from Stress by Dr Phil Nurenberger the nasal passages regulate the pressure of air entering and leaving the lungs. This can be measured by the Cottle rhino meter. This pressure (at the nose) sets off reflexes in other parts of the body, including the brain and the autonomous nervous system. This in turn can have important psychological and physiological impact. For one, the nasal lining has a heavy cluster of nerve endings. It has erectile tissue similar to that found in the genital region, which manipulates the flow of breath which in turn affects the inner homeostasis. The roof of the nasal cavity is closest to the master gland pituitary which is constantly managing the homeostat of our internal biological functions like temperature, blood pressure, heart rate, breath rate, blood circulation etc. And since it is also closest to the olfactory nerve, the nasal air flow is also linked to the most primitive part of our limbic system that strongly affect our subconscious and emotional impulses.