The Stomach, located under the diaphragm and in the epigastrium, connects with the esophagus at its upper opening and with the Small Intestine at its Lower opening. It is the most important of the Yang (Fu) organs. The Basic Questions says, “The Pi and Stomach are the officials in charge of food storage and of our sense of the five flavors” (Chapter 8 in ).
The Stomach has three areas: the upper “Shangwan”, middle “Zhongwan”, and Lower “Xiawan” 
. The Stomach and Pi are externally–internally related through their meridians. The Stomach receives and digests food and water, regulates downward transport with its characteristic descending Qi, and prevents obstructions by the propelling action of Stomach-Qi.
Major physiological functions of the stomach
(1) The stomach controls intake and digestion of food and water
The Stomach takes in, then ferments food and water, or “rots and ripens food.” The Basic Questions says, “The Middle Jiao (Burner) is in the Stomach … regulates the rotting and ripening of food and water” (Chapter 18 in ). The generation of Qi, Blood, and body fluid, and the body’s physiological activities all rely on the nutrition of food and water. The Stomach prepares the basic materials. Pi 
separates and extracts the refined Essence, and transforms Essence further into Qi, Blood, and body fluid to nourish the body. The Basic Questions says, ‘‘The Stomach is the sea of water and grains and the great source of nourishment of the six Yang–Fu and five Yin–Zang). The food digested by the Stomach, now termed “chyme,” is like fermented dough when it is sent down to the Small Intestine.
(2) The stomach controls the descent of Qi
The propelling energy of Stomach-Qi sends digested food down to the Small and Large Intestines. Pi sends up the useful clear material, while the Stomach sends down the turbid digested material.
Stomach dysfunction, seen as distension, will lead to stomachache, poor appetite, halitosis, depression, constipation, nausea, vomiting, hiccups, and so forth.
The Miraculous Pivot says, “The Qi of the organs relies on Stomach-Qi to reach the Fei channel” (Chapter 18 in ), referring to the soft, gentle, harmonious quality of the normal rhythms of Stomach-Qi.
A byproduct of the Stomach’s rotting and ripening process, “dampness,” is sent up to coat the tongue. The color and quality of the coating reflects Stomach pathology. A thick white coating indicates Stomach Cold; a thick yellow coating indicates Heat; no coating indicates Stomach-Qi failure. The Stomach is the origin of the Qi for the entire body. No matter how serious the disease is, if Stomach-Qi remains strong, the prognosis is still hopeful. “If there is Stomach-Qi there is life, if there is no Stomach-Qi there is death.”
(3) The stomach relates to Pi (, spleen system)
Pi and the Stomach are so closely interrelated that they could be seen as two aspects of the same organ system.
Pi and the Stomach are externally–internally related through their meridians. The two viscera jointly digest and absorb nourishment — called “the origin of the acquired base.” Pi –Qi brings food Essence upward; Stomach-Qi delivers food downward. The two actions complement each other. Pi , a Yin Zang-viscus, prefers dryness, dislikes dampness, and is prone to affliction by Heat. The Stomach, a Yang Fu-viscus, prefers moisture, dislikes dryness, and is prone to affliction by Cold. The two viscera cooperate harmoniously in the digestion and absorption of food.
Pi easily suffers from Yang deficiency. If it is too damp, Pi –Qi cannot ascend, and food and fluid cannot be transformed. If the Stomach is too full, Stomach-Qi fails to descend, which affects Pi , leading to abdominal distension, anorexia, or diarrhea. The Stomach tends to suffer from Yin deficiencies. If it is too dry, Stomach-Qi cannot descend and food cannot move down.