The word nadi literally means 'flow' or 'current'. The ancient texts say that there are seventy two thousand nadis in the psychic body. These are visible as currents of light to a person who has developed psychic vision. In recent times the word nadi has been translated as 'nerve', but actually nadis are composed of astral matter. Like the chakras, they are not actually part of the physical body, although they correspond with the nerves. Nadis are the subtle channels through which the pranic forces flow. Out of the large number of nadis in the psychic body, ten are major and of these, three are most significant. These are ida, pingala and sushumna. The most important of these three is sushumna. All the nadis in the psychic body are subordinate to sushumna, even ida and pingala.
Sushumna nadi is the spiritual channel located at the centre of the spinal cord. It originates from mooladhara chakra at the perineum and terminates at sahasrara, at the crown of the head. Ida nadi emanates from the left side of mooladhara and spirals up the spinal cord, passing through each chakra in turn, forming a criss-cross pathway which terminates at the left side of ajna chakra. Pingala nadi emanates from the right side of mooladhara and passes in an opposite manner to that of ida, terminating at the right side of ajna. Ida and pingala represent the two opposites forces flowing within us. Ida is passive, introvert and feminine; it is also known as the chandra or moon nadi. Pingala, on the other hand, is active, extrovert and masculine and is called the surya or sun nadi.
These pranic currents, ida, pingala and sushumna, operate alternately. The current that is flowing at any particular time may be gauged by noting the flow of breath in the nostrils. When the left nostril has a greater flow of air, then ida nadi is predominant. When the flow is greater in the right nostril, then pingala is predominant. If the flow is equal, then sushumna is predominant. When the right nostril (pingala) flows, there is more vital energy for physical work, digestion of food and so on. The mind is extroverted and the body generates more heat. When the left nostril (ida) is flowing, mental energy is dominant. The mind is introverted and any land of mental work may be undertaken. During sleep ida nadi flows. If pingala flows at night, sleep will be restlessness and disturbed. Likewise, if ida flows while taking food, the digestive process may be slow, causing indigestion. All activities are influenced by the flow of these nadis which alternate approximately every sixty to ninety minutes, although primarily pingala is active by day, ida by night, and sushumna at dawn and dusk, when is the most appropriate time for meditation.
The main aim of hatha yoga is to bring about a balanced flow of prana in ida and pingala nadis. The word hatha is comprised of two beeja mantras ham, representing the sun or solar force, and tham, representing the moon or lunar force. To bring about a balance between these two forces, the body must first be purified by the shatkarmas. The aim of hatha yoga is to balance these two flows so that neither the mental nor physical faculties are dominant. During the twenty-four hour period of the day, the flow of ida should predominate for about twelve hours and the flow of pingala for the other twelve hours. When ida and pingala nadis are purified and balanced, and the mind is controlled, then sushumna, the most important nadi, begins to flow. Sushumna must be flowing for success in meditation. If pingala flows, the body will be restless; if ida flows, the mind will be overactive. When sushumna flows, kundalini awakens and rises through the chakras.
At the physical level, ida and pingala correspond to the two aspects of the autonomic nervous system, the sympathetic and the parasympathetic. Pingala coincides with the sympathetic nervous system, and it speed up the heart, dilate the blood vessels, increase the respiration rate and intensify the efficiency of the eyes, ears and other sense organs. The parasympathetic nerves act directly opposite. Sushumna corresponds to the central nervous system, and spiritual strength - atma shakti. Investigations have shown that when the right nostril is flowing, the left hemisphere of the brain is activated. When the left nostril is flowing, the right hemisphere is activated. This is how the nadis or energy channels control the brain and the events of life and consciousness.
The flow of prana in ida and pingala is completely involuntary and unconscious until controlled by yogic practices.
part of the book Asana Pranayama Mudra Bandha by Satyananda Saraswati