Sri Krishnamacharya taught several upanishads including prasna upanishad. Therein is a lovely story about prana.
Once all the five important senses became so conceited that each one thought it and only it was the most important aspect of the being. The sight the hearing and others each were blowing their own trumpets. Prana which was busy doing its work incessantly attempted to get into the debate but was rebuffed by the senses. Prana, the life force, while feeling slighted wanted to teach them a lesson. The main prana collected all the five pranas and quietly prepared to leave the body. Suddenly all the organs felt weak and powerless. The eyes could not see, the ears could not hear. They in a flash realized the importance of prana. They all fell at the feet of prana and implored it to not to leave. Prana is also known as saridharaka or upholder of the body. When the prana leaves, the body collapses and hence prana is also called the upholder of the body. Without prana nothing moves nothing works. The pumping heart, the expanding lungs, the contracting stomach and all other similar organs along with the senses come to a halt when prana leaves the body.
This sariradharaka or upholder of the body is prana. Sri Krishnmacharya would unzip (vigraha vakya) of this compound word (samasa) prana as “prakarshena anati gacchati iti pranaH” or that which completely moves the being (all life). So, prana is life force. But according to samkhya this prana is called a samanya vritti or general or innate function of citta or brain. While buddhi (intellect) ahamkara (ego) and manas ( an indriya) have unique functions and their activities are clubbed as cittavrittis, this activity of the brain or citta of maintaining life is called samanya or general or innate function or vritti of citta.
So how does this prana function in maintaining life? It is not like one dimensional electric current which flows and provides energy for the device that uses electricity to operate. Here this prana force or prana vritti is made up of five distinct functions as per the yoga texts (ten according to some) . To maintain life several functions will have to be performed. The living being requires to take air water and food from the outside world. Then after metabolic usage these will have to be expelled from the system and we need an excretory system propelled by the excretory force. Then we have to have a system run by another force whose activity is to circulate the necessary nutrients to all the cells which is the circulatory system. Then we have another subsystem of keeping the body upright and prevent it from collapsing which force also maintains the delicate balance between the intake and discard forces. Then there is this unique system of harmonizing all the junk food thrown into the stomach so that the millions of cells would be able to survive. All these activities of maintaining life are said to be accomplished by the prana, the life force by dividing itself into five different forces which are well known as prana, apana, vyana udana and samana. Sometimes for some it is confusing because the word prana is used both as the main life force and also the one that is responsible for intake function especially intake of air or breathing. Here the breathing word prana is derived from the root ‘ana’ which also means to breathe ( ana svase ) This prana is generally responsible for all the functions of taking what is required from the outside system like inhaling atmospheric air, swallowing food and water. But generally, as breathing is the most important continuous function, Prana is associated with breathing especially inhalation. Then we have apana . The prefix ‘apa’ would mean to discard. so apana would be force charged with the function of throwing out all the waste products from the system. Apana is usually associated with the function of defecation but some would consider apana in a broader sense as the force responsible for exhalation and all other evacuatory functions. Then the prefix ‘vi’ in vyana would indicate vyapana or spreading out so vyana would mean a force responsible to distribute the essentials all over the body to all cells. The prefix ‘sama’ would indicate to harmonize/balance and hence samana is the force that is responsible for digestion by contracting and expanding the stomach and the other parts of the digestive system. Then we have the prefix “ut’ meaning upward and so udana would mean the force for keeping the body erect, balanced including the balance between prana and apana.
Then we have the term “ayama’ meaning to control and or expand. So, pranayama if applied to life force based on ‘ana gatou’ would mean to extend life span. If the term pranayama is applied to ‘ana svase’ or ana to breathe it would mean to extend the span of breath and also bring it under more complete voluntary control. So, pranayama practice is to bring the breath under voluntary control. extend the span of breath and also extend the span of life.