The heart belongs to fire, and the kidneys belong to water. These two elements, fire and water, mutually restrain and interact with each other to maintain the dynamic balance of physiology, known as "the mutual nourishment of water and fire." If kidney water is insufficient and cannot nourish heart fire, or if heart fire is excessively active and damages kidney yin, this coordination is lost, leading to symptoms such as irritability, insomnia, and seminal emission. This is the pathology described. See the entry on "heart and kidneys failing to communicate."