The Triple Burner, San Jiao, is unusual among the twelve TCM organs in having no single discrete anatomical structure behind it, a point Chinese medical texts themselves acknowledge rather than obscure; classical commentary calls it "the organ with no form," a functional division of the torso into three zones rather than a specific piece of tissue. The Upper Burner, spanning the chest, is described through the metaphor of a mist, governing the diffusion of breath and blood by the Lung and Heart. The Middle Burner, the upper abdomen, is described as a maceration or a churning vat, governing the Spleen and Stomach's work of transforming food into usable substance. The Lower Burner, the lower abdomen and pelvis, is described as a drainage ditch, governing the Kidney, Bladder and Large Intestine's work of separating and eliminating waste.
Because it names a set of processes distributed across the whole torso rather than a single organ, some Western commentators have proposed reading the Triple Burner as an early functional model of the body's water and energy metabolism rather than a mistaken guess at an undiscovered anatomical structure, though this remains an interpretive reading of the classical texts rather than a claim the texts make in those terms themselves.