Pranayama really means control of the pranic energy. It is achieved through the control of respiration. There are certain functions of the human body which are purely involuntary. The action of the heart, the movements of the stomach and the intestines are involuntarily influenced by emotions, but they are normally beyond voluntary control. We cannot stop or modify them at will. The excretion of urine and faeces on the other hand is partly voluntary and partly involuntary. One gets the urge for passing urine or stool, but we can still control it through willpower. The act of respiration and the process of thinking and emotion are also both voluntary and involuntary. The processes are going on continuously involuntarily, but one can modify them at will as well. This may be the reason for utilising the act of respiration to reach the step which follows pranayama, pratyahara. That one can reach pratyahara through pranayama is a fact which can be experienced through personal sadhana.
Let us consider the other physiological aspects of this yogic breathing of pranayama. Pranayama does not mean inspiring a greater volume of air, or utilising more oxygen. But still, certain physiological figures are worth consideration.
During ordinary, quiet breathing, going on unconsciously at the rate of 14 to 18 breaths per minute, we utilise 500 ml. of air per breath. This is known as the tidal volume. There is no exchange of gases in the trachea and the bronchi which are tubular structures for mere passage of air. Out of the 500 ml. of tidal volume air, about 150 ml. of air is wasted in this dead space. After breathing in quietly, we can still take in a lot of extra air by conscious inspiration. Similarly, after quiet breathing out, we can still force out a lot more air. This is known respectively as inspiratory and expiratory reserve. If one takes a deep breath after forcefully emptying the chest, one can breathe in 4,000 ml. of air. This is called the vital capacity. Out of this, only 150 ml. are lost in the dead space. This is one of the advantages of deep breathing, which is common to all types of deep breathing and not only to yogic breathing alone.
The further advantage of yogic breathing lies m the fact that it is more a vertical breathing than horizontal. The exercises with the chest expander, or the tape measurements of expanded and unexpanded chest, indicate a preference for horizontal expansion of the chest. However, the yogic processes of performing pranayama in a fixed sitting posture, breathing through alternate nostrils or the ujjayi, sheetali or sheetkari way of inspiration, promote vertical breathing. By this vertical breathing all the alveoli of both the lungs open out evenly. This effect is more obvious in the apical, central and basal alveoli. Due to the even expansion of all the alveoli, a vast expanse of alveolar membrane is available for exchange of gases. This surface is about 50 square meters in extent, which is 20 times the entire body surface. The larger the surface available for the process of diffusion, the better is the process. Moreover, if some alveoli remain unopened, they get stuck. There is a collection of secretion in them, and they are prone to disease formation. This is done away with in vertical breathing.
The diaphragmatic muscle between the chest and the abdominal cavities plays a major role in the act of respiration. It has an excursion of one cm. during quiet breathing, but during deep vertical breathing it moves 2-3 cm. The base of the heart and lungs are attached to the upper surface of the diaphragm, while the liver, spleen, stomach and pancreas are immediately under the diaphragm. The circulation in these organs is improved and consequently their performance is stimulated. The lymphatics below the diaphragm are also thoroughly emptied.
As against these advantages of vertical breathing, the horizontal expansion of the chest has certain disadvantages. In horizontal breathing, the alveoli toward the periphery expand more than optimum, while the centrally placed alveoli do not open out properly. This affords a lesser and uneven surface for diffusion of gases. We have seen the consequences of alveoli remaining closed. If the peripheral alveoli open wider than is preferable, they lose their elasticity. They remain in a fixed wide open position, with very little excursion for taking in air. The chest looks big and barrel shaped, but does not move well with respiration. The interalveolar walls may be broken, damaging the capillaries, leading to diseases like emphysema or pulmonale.
Thus, by yogic, total, vertical breathing, we include 4,000 ml. of air per each breath. We offer an even surface of 50 sq. metres for exchange of gases. Due to even expansion of all the alveoli, we not only keep lungs intact in structure and function, we keep them healthy. We can prevent diseases like emphysema, and also gain relief in asthma and chronic bronchitis. The increased amplitude of the movements of the diaphragm improves the functions of stomach, liver, spleen, heart, and the circulation in the lymphatics, thus improving digestion.
Pranayama thus acts directly on the various systems of the body, and affords benefits to them all.