Chapter 19 of the Suwen: The Stomach

五 臟 者 ，皆 稟 氣 於 胃 ， 胃 者 五 臟 之 本 也 。

Through the Dao, Yin and Yang came into being and through their relationship, the 10,000 things were born. The relationship between Yin and Yang creates the five essences, or energies, which express themselves in a variety of ways through the material world. They are the five movements, the five tastes, the five notes, etc. In order that we may live, our organs must be connected to their essences. The stomach enables this to happen. The stomach is the root of the five zang because it is a receptive vessel into which the five energies can be placed and distributed.
Hexagram 43 is a picture of a lake or sea that has risen up to heaven and in this way, essences from Heaven can be rained down. The stomach provides a vessel into which the material forms of the energies are placed and as it cooks and heats these forms, they rise up so that the stomach can rain them down onto the fertile soil of the zang organs.
Just as in the metaphor of the Puchang Sea, the stomach must evaporate the most precious of its fluids. The other organs rely on it to do this so that they may be moisturized and nourished by the five flavors. Therefore, the stomach can easily become dry and its own membrane fall victim to the acids created during the process of digestion/cooking.