More about qi
Qi can be seen as the animating force in all life and the universe. At the cellular level, we could say that the ‘energy’ that animates our bodies comes from adenosine-triphosphate (ATP), the molecule created when glucose breaks down in the body. The energy contained in glucose is not used directly by our bodies, but is captured into the terminal phosphate bonds of the nucleic acid molecule ATP. Those phosphate bonds are unstable and easily broken, like a coiled spring of potential energy. As ATP breaks down in reaction with water, our cells use enzymes to transfer its free phosphate groups to other molecules, which power them up to do the activity that keeps us alive:
Without ATP, molecules cannot be made or degraded, cells cannot transport substances across their membrane boundaries, muscles cannot shorten to tug on other structures, and all life processes cease. 
Yet the word ‘breath’ comes from the Latin spiritus, meaning that which is not of the material body. The translation of qi as both energy and breath implies that the life force has a physiological and a spiritual or consciousness component. It also implies that the physiological itself has consciousness.
There are considered to be seven major types of qi, coming from different sources:
Breath or air – zong qi
Food – gu qi
A combination of the above – ying qi (‘nutritive’ qi)
Original, inherited from parents – yuan qi
Internal – nei (inside the body)
External – wai (emanating from the body to the outside)
Protective – wei
Qigong practise is considered to stimulate and regulate the flow of wei qi, through the effects of relaxed and efficient breathing. Wei qi is said to flow just underneath the surface of the skin, allowing healing energies to penetrate the body but protecting it from harmful ones, including pathogens.
The premise of qigong is that qi is responsive, and we can develop our ability to guide it. This is expressed in the Chinese saying yi yi yin qi: ‘use the wisdom-mind (yi) to lead the qi’. My teachers stressed the importance of allowing unforced and open breath, which brings a clearing of the mind, as a way to stimulate a clear qi flow through all the body’s meridians. When we do qigong correctly, this begins to happen naturally. With experience, a practitioner of qigong can use the mind-intent yi to release blockages in this flow.
 Marieb, E. and Hoehn, K. (2010), Human Anatomy & Physiology, 8th Edition, San Francisco, Pearson Education Inc., p56