This is an excerpt from Yoga Anatomy-3rd Edition by Leslie Kaminoff & Amy Matthews.
There are compelling anatomical reasons why nose breathing is often cited as being healthier than mouth breathing. Various studies use different definitions of nose and mouth breathing. Some researchers label the breath based on where the inhalation occurs (i.e. inhaling through the nose and exhaling through the mouth is still considered to be nose breathing, while the reverse is called mouth breathing). Most yoga sources accept both inhaling and exhaling through our nose as nose breathing.
Air inhaled through our nose is warmed, filtered, moistened, and spun into a coherent vortex by a shell-shaped system of bones, vessels, and tissue within the nasal passageways called the turbinates, also known as nasal conchae, which is Latin for “shell.” Turbinate also refers to a shell shaped like a spinning top or inverted cone (figure 6.31). Although our illustration depicts these structures symmetrically, this is seldom the case in actual bodies. As with spines, most people exhibit some degree of asymmetry in these structures, the most common being deviation in our septum, which makes one nostril more structurally open than the other.
Within our nasal sinuses is secreted the important vasodilator nitric oxide, which relaxes the smooth muscle of blood vessels, causing them to widen.17 This increases blood flow and lowers blood pressure. Compared to mouth breathing, nose breathing transports more nitric oxide (and its benefits) to our lungs and bloodstream (Lundberg et al. 1996). Mouth breathing delivers a greater quantity of air to our lungs; nose breathing delivers better quality.
17. The discovery of nitric oxide as a signaling molecule in the cardiovascular system was so important that it won the 1998 Nobel Prize in Physiology or Medicine for Robert F. Furchgott, Louis J. Ignarro, and Ferid Murad.