The nutrient qi and the defensive qi all come from the pectoral qi and are closely related to each other in production and circulation.

The nutrient qi pertains to yin and the defensive qi Pertains to yang. So the nutrient qi and the defensive qi are also called ying-yin (nutrient yin) and wei-yang (defensive yang).

Both the nutrient qi and the defensive qi come from the pectoral qi. The pectoral qi flows to the whole body from the chest in the form of the nutrient qi and the defensive qi. In terms of the production, the nutrient qi and the defensive qi, just like the pectoral qi, is produced by combining the flesh air inhaled by the lung with the food nutrients absorbed and transported by the spleen. The only difference lies in food nutrients. The nutrient qi comes from the "most essential part" of the food nutrients while the defensive qi comes from "the most active and powerful part" of the food nutrients.

   

The nutrient qi and the defensive qi are different in nature. The former, pertaining to yin in nature, flows inside the vessels while the latter, pertaining to yin in nature, flows outside the vessels. That also explains why the nutrient qi distributes in the internal organs while the defensive qi distributes over the surface of the body. Certainly there is the defensive qi in the internal organs and there is the nutrient qi in the superficies. But the nutrient qi mainly flows inside the internal organs and the defensive qi mainly circulates in the superficies.

The nutrient qi and the defensive qi are different from each other in functional characteristics. The functional characteristics of the nutrient qi are marked by transformation into blood to nourish the whole body, especially nourishing the internal organs to maintain their physiological functions. The functional characteristics of the defensive qi are marked by warming and nourishing the body and protecting the body against pathogenic factors, especially warming and nourishing the muscles and superficies as well as regulating sweat so as to protect the body against pathogenic factors. The defensive qi pertains to yang. That is why it can warm the body. But it regulates sweat to excrete excessive yang-heat so as to main rain a constant body temperature.