Yuan Qi is said to be Essence that has been transformed into Qi, or Jing in motion. Yuan Qi has it's root in the Kidneys and spread throughout the body by the San Jiao (Triple Burner). It is the foundation of all the Yin and Yang energies of the body. Yuan Qi, like Prenatal Jing, is hereditary, fixed in quantity, but nourished by Postnatal Jing.
Gu Qi is the first stage in the transformation of food. Food is first "Rotted and Ripened" by the stomach and then sent to the Spleen to make Gu Qi, still in unusable form.
Gu Qi is sent from the Middle Burner (housing the Spleen and Stomach) to the Upper Burner (housing the Lungs and Heart), where it combines with air to form Zong Qi.
Part of the Gu Qi from the Middle Burner is also sent to the Lungs, then passes to the Heart, where (with the help of Yuan Qi and Kidney Qi), it is transformed into Blood.
The Spleen sends Gu Qi up to the Lungs, where (with the help of Yuan Qi and Kidney Qi) it combines with air and transforms into Zong Qi.
Also called "Normal" Qi. Zong Qi is transformed into Zhen Qi with the help of Yuan Qi. Zhen Qi is the final stage in the transformation and refinement of Qi. It is the Qi that circulates in the channels and nourishes the organs.
Zhen Qi has two different forms:
Ying Qi nourishes the internal organs and the whole body. It is closely related to Blood, and flows with Blood in the vessels as well in the channels.
It is the Qi that is activated by insertion of an acupuncture needle.
Ying Qi spends two hours in each channel, moving through all twelve channels in a twenty four hour period. During these periods, the specific organs are nourished and maintained by the Ying Qi.
Wei Qi is more Yang than Nutritive Qi. Fast moving, "slippery" and easily motivated.
Primarily on the Exterior (skin and muscles). Travels both inside and outside the channels. Flows primarily in the superficial layers of the body, especially in the Tendino-Muscular meridians.
Deficient Wei Qi can lead to spontaneous sweating (pores not correctly opened and closed, so that the fluids escape).
When an exogenous pathogen (e.g., Wind-Cold) invades the Exterior, the pathogen can block the pores, inhibiting the function of the Wei Qi, and blocking sweating. The treatment is to restore the Lungs' function of dispersing, strengthen the Wei Qi and produce sweating, to expel the pathogen. Sweating therapy is often used in the early stages of a Wind-Cold pathogenic invasion.
Circulation: Wei Qi has a complex circulation pattern, of 50 cycles during a 24 hour period, 25 times in the day and 25 at night.
In the daytime, Wei Qi circulates in the Exterior, but at night it goes into the Interior and circulates in the Yin Organs.
From midnight to noon, the Wei Qi is exteriorizing, and is at its maximum strength on the Exterior at noon.
From noon to midnight, the Wei Qi gradually withdraws into the Interior, to protect the Yin Organs.
This is why one is more apt to catch cold at night rather than in the daytime, since the Wei Qi has withdrawn to the Interior at night. Sleeping under an open window at night, for example, gives exogenous pathogens a better chance for attack than during the daytime, since the Exterior of the body is less well protected.
This is the Qi that is derived from food by the Stomach and Spleen (Postnatal Essence). Central Qi is another way to define Stomach and Spleen Qi, i.e., the Qi of the Middle Jiao (the Center). It is often used to describe the pathological condition where the Spleen Qi is deficient and has caused organ prolapse ("Deficiency of Center Qi").
A general term to describe the various forms of Qi that protect the body from exogenous pathogens. Usually only used when contrasting the strength of the body's Qi with the strength of the invading pathogen.