The Energetic Effects of PranayamaBreathing is an exceptional function of the body in that it is ordinarily regulated automatically by the Autonomic Nervous System, but can be consciously modified. Because of this, it can act as a doorway between the conscious and unconscious aspects of the self. Of course, the yogic tradition claims that all functions of the body controlled by the Autonomic Nervous System can, with practice, become volitional—even the beating of the heart. But until the yogi attains that level, practicing control of the breath is the most accessible way to create a bridge.
In order to guide your students along this path, it is helpful to have some understanding of the basic physiological functioning of the breath. Here's how the body is affected by it: As we inhale, the contracting diaphragm (the primary respiratory muscle, which is like the skin of a drum separating the thoracic cavity from the abdominal cavity) descends on the organs below, creating pressure. As a result, the thoracic cavity expands and the abdominal cavity contracts somewhat. As we exhale, the opposite occurs: the diaphragm relaxes and releases upward as the ribcage relaxes inward, allowing for a counter-intuitive spaciousness in the abdomen. This feeling of space in the abdomen can be difficult to feel in an individual with any restriction in natural free breathing, but is easily measurable in infants. During deep prolonged inhalation, a pressure is created in the thoracic cavity that stimulates several effects of the Sympathetic Nervous system (the branch Autonomic Nervous System that creates the "fight or flight response"), the most notable of which are temporary increases in heart rate and blood pressure. Deep prolonged exhalation tends to activate the opposite branch of the Autonomic Nervous System--the Parasympathetic--which again has many effects, including the temporary--but immediate!--drop in both heart-rate and blood pressure.
This is easily felt: Sit quietly for a little while, consciously lengthening your breath as much as you comfortably can and allowing it to round out so that the inhalation flows directly into the exhalation. Once you have established rhythm of long smooth comfortable breathing, place two fingers to the side of your larynx and feel your pulse. If your breath is unforced and long, you should be able to measure the increase in your pulse as you inhale and the decrease as you exhale.
Yoga, in very general terms, is a practice of balancing opposites. Often in our practice and teaching we aim to balance the inhalation and exhalation, which has a neutralizing effect on the currents of the two branches of the Autonomic Nervous System. Depending on the desired effect, however, modifying the focus toward the inhalation or the exhalation will greatly shift the energetic result of a Yoga practice.
The inhalation, though often thought of as an expansive breath, actually creates pressure around the heart, which shifts the system--at least during the breath cycle—into the Sympathetic system. Deep exhalation tends to shift it in the other direction. Thus, in personal practice, if one has a tendency toward anxiety and is trying to release stress, a breath ratio that emphasizes exhalation will be more helpful. On the other hand, in an individual who tends towards depression or lethargy, the same breath ratio will reinforce these difficulties.