“He who knows Prana knows Vedas” is the important declaration of the Srutis. You will find in Vedanta Sutras: “For the same reason, breath is Brahman.” Prana is the sum total of all energy that is manifest in the universe. It is the sum total of all the forces in nature. It is the sum total of all latent forces and powers which are hidden in men and which lie everywhere around us. Heat, light, electricity, magnetism are the manifestations of Prana. All forces, all powers and Prana spring from the fountain or common source, ‘Atman’. All physical forces, all mental forces come under the category ‘Prana’. It is force on every plane of being, from the highest to the lowest. Whatever moves or works or has life, is but an expression or manifestation of Prana. Akasa or ether also is an expression of Prana.
The Prana is related to mind and through mind to will, and through will to the individual soul, and through this to the Supreme Being. If you know how to control the little waves of Prana working through the mind, then the secret of subjugating universal Prana will be known to you. The Yogi who becomes an expert in the knowledge Of this secret, will have no fear from any power, because he has mastery over all the manifestations of powers in the universe. What is commonly known as power of personality is nothing more than the natural capacity of a person to wield his Prana.
Some persons are more successful in life, more influential and fascinating than others. It is all due to the power of this Prana. Such people manipulate everyday, unconsciously of course, the same influence which the Yogi uses consciously by the command of his will. There are others who by chance tumble unaware of this Prana and use it for lower purposes under false names.
This working of Prana is seen in the systolic and diastolic actions of the heart, when it pumps the blood into arteries in the action of inspiration and expiration during the course of breathing; in the digestion of food; in the excretion of urine and faecal matter; in the manufacture of semen, chyle, chyme, gastric juice, bile, intestinal juice, saliva; in closing and opening of the eyelids, in walking, playing, running, talking, thinking, reasoning, feeling and willing. Prana is the link between the astral and physical body. When the slender thread-link Prana is cut off the astral body separates from the physical body. Death takes place. The Prana that was working in the physical body is withdrawn into the astral body.
This Prana remains in a subtle, motionless, unmanifested, undifferentiated state during the cosmic Pralaya. When the vibration is set up, Prana moves and acts upon Akasa, and brings forth the various forms. The macrocosm (Brahmanda) and microcosm (Pindanda) are combinations of Prana (energy) and Akasa (subtle matter). That which moves the steam-engine of a train and a steamer, that which makes the aeroplanes fly in air, that which causes the motion of the breath in lungs, that which is the very life of this breath itself, is Prana. I believe, you have now a comprehensive understanding of the term Prana about which you had a very vague conception in the beginning.
The Science of Pranyama by Sri Swami Sivananda