The coordination between water and fire
The heart is located in the upper jiao (upper aspect of the body) and belongs to yang, while the kidney is located in the lower jiao (upper aspect of the body) and belongs to yin. In terms of the ascent and descent of yin and yang, that which is in the upper area goes downward, while that which is in the lower area goes upward. Therefore, heart-fire descends to the kidney to prevent the kidney-water from becoming too cold, while kidney-water rises to the heart to prevent heart-fire from being overactive. The descending of heart-fire and the ascending of kidney-water maintain a normal balance between the two organs.
Kidney qi includes both kidney yin and kidney yang, and the ascent of kidney yin (water) depends upon the activation of kidney yang. Heart qi includes heart yin and heart yang, while the descent of heart-fire relies on the cooling function of heart yin. Activated by kidney yang, kidney yin (water) rises up to replenish and cool the heart. Cooled by the yin, heart-fire goes downward to assist the kidney. This balancing process is called the “interaction between water and fire”.
Representative formula: Tian Wang Bu Xin Tang
From Fundamentals of Chinese Medicine PMPH- Sun/Eisenstark/Zhang