Breathing  and Breath Retention
Prana (Energy or Life Force) is the primary energy that began to vibrate and created the universe.  It is the creative impulse which is found permeating all the universe and is in all beings, objects and particles.  This power is Infinite.

As far as human beings are concerned, Prana  is the energy which makes the heart beat and lungs move; connecting our will with our organs giving them the energy to perform.  Whatever movement of the human structure, is carried out by the power of the Prana.  Therefore, Pranayama is the yama of energy, practiced by controlling the flow of respiration; it's effects can be seen in one's increased physical and mental capabilities.  This joining together and control of the energy results in a flow without obstacles in harmony with the spiritual and mental strengths which transform the Universe.

According to Yoga there are approximately 72,000 nerve channels in the human body, all of which branch out from the spinal column.  Through these channels all the nerve impulses, both conscious and unconscious, circulate.  The three most important are Sushuma, Ida and Pingala.  They flow through the central passages of the spinal column crossing important nerve centers which in Yoga are called Chakras.  The proper harmony and movement of the Chakras and the natural free-flowing Prana are the benefits  of the regular practice of  taking Prana: Pranayama.

As detailed above, Pranayama is the Science of  Breath.  The yogic life is measured by the number of breaths and not by the number of days.  For this reason, slow, deep, rhythmic breathing prolongs the yogi's life.  Rhythmic breathing patterns strengthen the respiratory system and the nervous system.  Once having mastered The Science of Breath, the yogi will control the mind better, eliminating distractions and the mind will become still and quiet.  The object of pranayama is  to still and control the mind and thus be prepared for pratyahara, dhyana and meditation.

PRANA   -    Life Force, Energy which manifests itself through breath
AYAMA   -   To stretch or extend (life) 

Mastering our breath is the primary building block to mastering the rest of our being:    
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Rama Yoga