Meridian tropism refers to that medicinal herbs may often produce their therapeutic effects on some portion of a human body in preference, in other words, their therapeutic action is mainly related to some viscus or channel or some channels in predominance but it may seem to produce fewer effects on or seem not related to the other viscera and channels. Meridian tropism takes the theory of viscera and meridians, and the indication of syndromes as a basis. For instance, Mahuang (Herba Ephedrae) and Xingren (Semen Armeniacae Amarum) effective to syndromes of the disorder of the lung meridian marked by cough and dyspnea are attributed to the lung-meridian; Qingpi (Pericarpium Cirri Reticulatae Viride ) and Xiangfu (Rhizoma Cyperi) indicated for syndromes of the disorder of the liver-meridian marked by distending pain of breast and hypochondrium and hernia pain are attributed to the liver-meridian. 

So generally speaking, what meridian or meridians a medicinal herb is attributed to is just related to the certain meridian or meridians on which the herb may work. If certain medicinal herb can work on several meridians, which means the medicinal herb can be used widely to treat the disorders of these meridians. From the above, we can see that meridian tropism of Chinese medicinal herbs is summerized from the therapeutic effects through a long time of clinical observation, and being practiced repeatedly, gradually develops into a theory.

The theory of meridian tropism plays a certain role in clinical selection of Chinese medicinal herbs according to syndromes, giving a rise of direction and strengthening the therapeutic effects. For instance, medicinal herbs cold in nature have effects of clearing away heat, which also have the differences in tendency towards clearing away heat in the heart, liver, lung, or stomach; those hot in nature can all warm the interior to expel cold, but their effects also have the differences in warming the spleen, stomach, lung or kidney. Therefore, when you prescribe medicinal herbs, you should select those that work on the diseased viscus or meridian or some viscera or meridians in the light of their properties of meridian tropism to achieve desired therapeutic effects. In addition, you can take meridian tropism of Chinese medicinal herbs as a clue to probe their potential effects of some medicinal herbs and to extend their applying range.

The theories such as meridian tropism, four natures and five flavors, lifting, lowering, floating and sinking all explain the properties of Chinese medicinal herbs from various points of view, which jointly constitute their proper-ties and actions. Whereas pathological changes in the same viscus or meridian are different in cold, heat, asthenia or sthenia, and in adverse ascending or descending, and the medicinal herbs which are attributed to the same meridian also have the difference in cold, warm or cool, tonifying or reducing and adverse descending or ascending. For example, Mahuang (Herba Ephedrae), Huangqin ( Radix Scutellariae ) and Shashen ( Radix Adenophorae Strictae) are all attributed to lung-meridian and can all treat cough, but Mahuang (Herba Ephedrae ) pungent and slightly bitter in flavor and warm in nature tends to lifting and floating in actions, so it can disperse the lung to relieve cough and asthma and is indicated for cough and asthma due to exogenous wind and cold. 

Huangqin (Radix Scutellariae) bitter in flavor and cold in nature tends to sinking and lowering in actions, so it can clear away heat, and purge excessive fire and relieve cough and asthma due to lung-heat; Shashen (Radix Adenophorae Strictae) sweet in flavor and cold in nature tends to sinking and lowering in the action, therefore, it can nourish yin to promote secretion of the body fluids and is indicated for cough due to insufficiency of lung-yin. Therefore when you apply medicinal herbs in clinic, you must combine their various properties and effects to give them an all-round consideration so that you can select and apply them correctly and avoid one-sidedness.

But in clinical practice, owing to frequent occurrence of transmission of a disease between zangfu-organs or meridians in addition to the Chinese medicinal herbs only for one viscus or meridian, those for the other viscera or meridians are also usually used together. For instance, when treating lung disorders affects the spleen, both the medicinal herbs for treating lung disorders and invigorating the spleen should be used, which is called supplementing the spleen to nourish the lung. For the same reason, a syndrome with hyperactivity of liver-yang due to insufficiency of kidney-yin should be treated in combination with the medicinal herbs nourishing kidney-yin to have the liver nourished and deficiency of yang calmed, which is the method of nourishing renal yin to tonify liver yin.