While the meridian and dantian systems are used by qigong and chinese medicine to define the basic geography of the subtle body, there's also a whole language of different-kinds-of-qi used to speak of various processes within this energetic terrain. One of these forms of qi is called Yuan Qi or Original Qi.
Yuan Qi is stored in an energetic center called Ming Men, which is located between the kidneys, at the level of the second lumbar vertebrae. (If you draw a line from your navel directly back to the spine, this will be the approximate level of Ming Men.) The relationship between the Kidney organ-system and Ming Men is said to be defined by the relationship between the elements of Water and Fire.
The following passage -- drawn from Larre, Schatz & Rochat de al Vallee's Survey Of Traditional Chinese Medicine -- explores the connections between Yuan Qi, the Kidneys and Ming Men, and also how Jing (one of the Three Treasures) fits into the picture:
Within this perspective, for a man jing may be either the basic material of spermatic liquid or the mechanism governing the region in which the spermatic fluid is formed. It is not the gonad. This site of the development of mechanisms of reproduction and sexuality is the region of ming men, located between the two kidneys, at the level of the second lumbar vertebra.
The Nanjing, in the 36th difficult question, says that at the level of ming men "man stores the sperm (jing) and woman secures the organs of gestation." (In this context the organs of gestation mean the processes connected with the ovaries and the whole female genital apparatus.)
Chinese texts say that the relations between kidneys and ming men are those of "the mutual assistance of Water and Fire." The relationship of entrenchment of yin and yang introduces a dialectic cited in Part I, that of Early Heaven and Later Heaven, of the innate and acquired energies. The kidneys receive the hereditary determinants that will assure the unfolding of the individual throughout his or her life, from the time of conception. That is what is meant metaphorically by their relation to Early Heaven. In that work, the kidneys are associated with the ming men, the privileged place of the original breaths (yuan qi) and the place of the conjunction of the original yin and yang.
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There's a lot going on in this passage, and I'll just point out one thing in particular, which has direct relevance for qigong practice. Ming Men -- located in a place that is sometimes referred to as the "roots of the navel" -- is defined here as the "site of the development of mechanisms of reproduction and sexuality" and also as "the place of the conjunction of the original yin and yang."
Many if not most forms of qigong initiate a drawing up of the energy within the sexual organs into the Ming Men area -- the place where that potency has yet to be differentiated into male and female forms, but instead is still in its unified state, characterized by both infinite potential and deep fulfillment.
This process is represented visually in the Neijing Tu: notice the group of Yin-Yang Symbols tucked into the Ming Men area of the diagram -- pointing to this inner-alchemical conjunctio. Very cool! Now the question is -- can we actually experience these things directly, within our own bodyminds .... ?
Of Related Interest: Gender & The Tao