The following is an excerpt on Pranayama, the formal practice of breath control by Swami Vishnudevenanda.

Pranayama is one of the most important and practices in yogic lifestyle. By practicing pranayama, the yogi is able to control the nervous system of the body, thereby obtain gradual control over prana or the vital force.

To breathe means to live and to live means to breathe. Every living thing depends upon breathing and cessation of breathing is cessation of life itself. From the first cry of the infant to the last gasp of a dying man there is nothing but a series of breaths. Yogis count life not by number of years but number of breaths. We constantly drain our life force or pranic energy by our thinking, willing, acting, etc. Every thought, every act of will, or motion of muscles uses up this life force and in consequence constant replenishing is necessary, which is possible mainly through breathing alone.

Just as oxygen is carried through the blood stream to all parts of the body, building up and replenishing, so is the prana carried to all parts of the nervous system. If we know that Yogis get most of their energy from the air, then the importance of proper breathing is readily understood. Whoever practices breathing regularly and systematically can feel in his own body this great effect of absorption of pranayama.”

The Complete Illustrated Book of Yoga by Swami Vishnu-devananda
Chapter 8
Pranayama or Yogic Breathing
Importance of Pranayama and Rules
Page 228 – 1st., 2nd. and 3rd. paragraphs