Yin and yang are in constant flux. If one becomes unbalanced in the body, illness occurs. For example, since yin is cold, an excess of yin can cause illnesses such as insomnia and dry-mouth. Conversely, a yang deficiency can cause cold limbs and a sickly-complexion. Good health then, is maintained by balancing yin and yang.<br>
[[File:Yin and yang chart.jpg]]<br>While the change of yin and yang may seem random, they actual follow a pattern known Xu Xing or the Five Phases, which shows how and when yin will shift to yang and vice versa. Typically, Xu Xing is used to describe the change of seasons and states of elements. For example, Spring giving rise to Early Summer, which turns into Late Summer, which then becomes Fall, then Winter, then Spring again.
[[File:Wu Xing pattern.png|thumb|400px|left|]]<br>In terms of the human body, the organs follow a similar Wu-Xing pattern, called Zang-Fu. Zang refers to organs that are yin in nature. They are the pericardium, heart, liver, spleen, lung, and kidneys. Fu refers to organs that are yang. They are the triple burner, small intestine, large intestine, gall bladder, urinary bladder, and stomach. Each zang has a fu, and every zang-fu pair corresponds to one of the five phases. The diagram below better explains this relationship.
===Five Phases theory===