Odds are you've heard the word Chi or Prana. From China there is Tai Chi and Qi Gong ('qi' is another spelling for 'chi'). India brings us Yoga which involves removing blocks in order to allow Prana to flow.
Both of these terms, Chi and Prana, translate loosely to life force or energy. There isn't an exact translation into English, but life force or energy are very close.
Prana is necessary for the body to function. Yogi Ramacharaka describes the importance of this energy in Science of Breath, "One who has mastered the science of storing away prana, either consciously or unconsciously, often radiates vitality and strength which is felt by those coming in contact with him, and such a person may impart this strength to others, and give them increased vitality and health.... prana [is] carried to all parts of the nervous system, adding strength and vitality.... the supply of prana taken up by the nervous system is exhausted by our thinking, willing, acting, etc., and in consequence constant replenishing is necessary."
How do we replenish our storage of prana? The easiest and most common method is through the simple act of inhaling.
To expand your capacity to bring in air, and thus prana, and thus health and vitality, use lateral stretches to open the small muscles between the ribs. These muscles are called the intercostals.
As an experiment, you may take a tape measure around the rib cage and exhale naturally. Note the measurement. Now inhale comfortably an see how much the rib cage grew. Do this before and after a practice involving side bends--remember to first warm up. Using poses, such as gate (parighasana), held for several breaths is particularly beneficial for increasing your inhales. After your practice, re-measure and see if the inhale can expand further than before the practice. Depending on how intense the asanas and how open you were to begin with will determine the difference you may find.
Today is the first day of the rest of your life!