Qi can be classified into four basic types: Primordial Qi (or Original-Qi) (Yuan-Qi, 
), Pectoral Qi (Zong-Qi, 
), Nutritive Qi (Ying-Qi, 
), and Defensive Qi (Wei-Qi, 
).
(1) Primordial Qi
This is derived from the congenital Essence of one’s parents at the beginning of life. It is the highest-level Qi, the primary motive force for life activities of the human body and also the most essential substance for maintaining and controlling metabolism and life.
The Classic of Difficulties says, “Primary Qi dwells between the two Kidneys below the umbilicus, at the Gate of Vitality” . Primordial Qi’s functional activities are ready at birth. But after birth, Primordial Qi needs further nourishment and enrichment by acquired Essence, which is produced by the Spleen and Stomach. It is distributed to all parts of the body through the circulation of Qi.
(2) Pectoral Qi (Zong-Qi )
Pectoral Qi is stored in the chest, and accumulated in the Shanzhong . Pectoral Qi generation combines natural fresh air inhaled by the lung with essence abstracted from water and food by the Spleen and Stomach, respectively. The Miraculous Pivot says, “If Pectoral Qi does not descend, blood will stagnate in the vessels”. Insufficient Pectoral Qi results in Blood stagnation. It also influences speaking, hearing, and limb movement. Deficient Pectoral Qi leads to palpitations, shortness of breath, weak voice, cold limbs, etc.
(3) Nutritive Qi (Ying-Qi ) and defensive Qi (Wei-Qi )
Both Nutritive and Defensive Qi come from Pectoral Qi and are closely related to each other in production and circulation.
Nutritive Qi is the Qi circulating with blood within the vessels and having nutritive action. Nutritive Qi and Blood are often conceptually combined as “nutritive blood.” Compared with Defensive Qi, Nutritive Qi belongs to Yin, and is also called “Nutritive Yin.” Defensive Qi is the Qi circulating outside the vessels and having a protective function. It belongs to Yang, and is also called “Defensive Yang.”
The most active and energized part of Pectoral Qi becomes Defensive Qi which circulates outside the vessels. Defensive Qi vaporizes to the diaphragm and over the chest and abdomen, and travels between the skin and the flesh.
“Nutritive Qi is extracted from food and water; it regulates the five Yin organs and moistens the six Yang organs; it enters the blood vessels and circulates in the channels and links with the six Yang organs” . Nutritive Qi’s major functions are to produce blood, to flow into the vessels through the Lung, and to nourish the whole body.
Defensive functions include (1) protecting the surface of the body from exo-pathogens, (2) maintaining the body temperature by controlling the pores and adjusting the release of sweat, and (3) nourishing the exterior of the body.
The Miraculous Pivot says, “Defensive Qi warms the muscles, fills up the skin, enters the space between skin and muscles, and opens the pores” . When Defensive Qi is insufficient, exo-pathogens can easily invade the body. There may be lower body temperature, spontaneous sweating, or even sleep disorders, depending on how long Defensive Qi has circulated in the Yang (exterior) portion of the body.
Nutritive Qi and Defensive Qi coordinate with each other to maintain the body’s regular functions.