The fourth limb of yoga is our breathing practice. The body is based on our breath. Breathing practices encourage a steady flow of energy through the body.
Prana means "life force" and Ayama means "control". The LITTLE BOOK OF YOGA defines pranayama as "aiding the movement of life force through the body by controlling the breath, and is a foundation of yoga practice."
Focusing on your breath can bring calm to your body and your mind. Use breathing exercises as a form of meditation or just to calm yourself during a stressful time.