Airway resistance is the force working against respiratory movements. Its cause is the friction between the flow of air and the respiratory tubes or airways. The overcoming of airway resistance demands that 75 to 80% of the respiratory muscles of the chest wall, neck and diaphragm work in an harmonious and synchronized manner. Yoga and medicine advocate breathing through the nose by which airway resistance is about twice as high as breathing through the mouth. The airway resistance in many pranayamas is higher than in normal breathing through the nose. A voluntary change in airway resistance seems to be one of the differences between yogic and normal breathing. This is exemplified in the yogic breathing techniques which utilize inner awareness, rhythmicity, muscle relaxation and breath retention. Airways resistance depends on the width of the respiratory airways and increases as they become more narrow. This increased resistance is seen in pranayama at the level of the nose in nadi shodhana and suryabheda; at the level of the mouth in sheetali and sheetkari; or at the level of the glottis in ujjayi and bhramari. This voluntary change of the airways resistance seems to have a number of effects:
These three effects of voluntarily altering the airways resistance need to be discussed in detail. Firstly, higher airway resistance requires a more efficient level of action of the breathing muscles. During exhalation against resistance, the inner intercostal muscles of the chest wall as well as the abdominal muscles are functioning. In the course of inspiration against a higher airway resistance, the external intercostal muscles, diaphragm and scalenus muscles of the neck are being utilized. The optimal function of these muscles is necessary for an economical and effective respiratory process, and is therefore necessary for physical and mental health.
Secondly, the effects on circulation are very important. The negative atmospheric pressure in the chest keeps the lungs expanded. During inhalation with higher airway resistance, as occurs in ujjayi, this negativity is further increased, which causes the sucking up of blood from the great veins into the right part of the heart to be enhanced. Simultaneously, the volume of blood ejected from the heart increases whereas the heart rate decreases. This seems to create more favourable conditions for blood flow through the coronary arteries which nourish the heart muscle itself. Some other circulatory changes during pranayama are not connected with airway resistance, but with the frequency of breathing, for example, the stimulation of circulation during bhastrika, or with the increased content of carbon dioxide in the blood and the dilation of brain vessels occurring during prolonged breath retention.
Thirdly, the practice of pranayama leads to the prolongation of inspiration and exhalation and to an increase in the period of breath retention. Such a manipulation of the breath affects the respiratory centre and it could thus influence the whole brain through the mediation of the reticular formation in the brain stem.
The physiological changes described above cannot totally explain the yogic concept of pranayama. They are only the gross or surface level manifestations, under which more subtle actions and alterations of consciousness take place.