Fei 
and Pi 
assist each other in Qi generation and water metabolism.
Fei inhales fresh air and distributes water to all parts of the body, while Pi absorbs nutrients by transforming and transporting food and water. Pi supplies Fei with essential substances. “Pi is the origin of acquired Essence, and Fei is the axis of Qi.” Fei distributes and regulates the water passage, assisting Pi in its function of transporting body fluid. Deficient Pi cannot transform phlegm and allows it to accumulate, or may stagnate fluid, leading to cough with excess sputum and dyspnea.