Defense qi, intrepid and swift, is transformed from the nutrients of food and drinks, and originated from the spleen and the stomach. It reaches the upper energizer and flows swiftly outside the vessels. It is primarily involved in warming and nourishing the interior and the exterior of human body, protecting skin from exogenous pathogenic factors, nourishing interstices, and controlling the opening and closing of sweat pores. Being the primary source of heat, defense qi permeates the whole body, warms the skin, hair, muscles, and zang-fu organs to keep them lustrous and healthy, and maintains constant body temperature. It regulates sweat discharge by controlling the opening and closing of sweat pores.