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inhalation (antar kumbhaka) and after exhalation (bahir kumbhaka). There are two types of breath retention: sahita kumbhaka, which is deliberately holding the breath, or kevala kumbhaka, where the breath is suspended spontaneously. Kumbhaka is a part of all pranayama practices. In the Yoga Sutras, Maharshi Patanjali described pranayama as kumbhaka. The aim of all pranayama practices is to achieve kevala kumbhaka, which is equivalent to the state of samadhi. According to the Amritanada Upanishad (v. 13-14):
नोच्छ्वसेन्न च निष्वासेनैव गात्राणि चालयेत् । एवं भावं नियुञ्जीयात् कुम्भकस्येति लक्षणम् ॥ अन्धवत् पश्य रूपाणि शब्दं बधिरवच्छणु ।
काष्ठवत पश्य वै देहं प्रशान्तस्येति लक्षणम् ॥ That is called kumbhaka when there is no expiration or inspiration and the body is motionless, remaining still in one state. Then he sees forms like the blind, hears sounds like the deaf, and feels the body like wood. This is the characteristic of one who has attained much quiescence.
Physiology of kumbhaka During the practice of kumbhaka the oxygen levels in the body fall and the carbon dioxide levels increase, depending on the speed of metabolism and how relaxed or tense one is. The main effect of kumbhaka is to train the nervous system to tolerate higher levels of carbon dioxide in the body before signals from the primitive brain stem force one to take another breath. Many blood capillaries lie dormant in the brain and become active only when more blood is required. Increased carbon dioxide levels stimulate the brain's capillaries to dilate. In this way, more capillaries in the brain are opened up to improve cerebral circulation.
The brain also stores a certain amount of carbon dioxide, which allows for a more efficient oxygen exchange and carrying capacity of the lungs. Often, when one cannot breathe deeply, it means that the brain's concentration of carbon dioxide is
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