The five zang-organs
The term zangxiang first appeared in Huangdi Neiring. Zang, sharing the same pronunciation with viscera in Chinese, refers to internal organs inside the body; xiang means image or phenomenon. When used together, zangxiang, as analyzed above, simply refers to internal organs and the external manifestations of their physiological and pathological states. Obviously zang is the intrinsic base of xiang and xiang is the external manifestations of zang.
The content of zangxiang theory is composed of three parts: the physiological functions and pathological changes of the viscera, the description of which mainly focuses on the physiological functions; the relationships between the five zang-organs and the body, organs and orifices, including the relationships between the five zang-organs and the five constituents (namely vessels, tendons, muscles, skin and bones), the five sensory organs and the nine orifices (namely the tongue, eyes, mouth, nose, ears, external genitals and anus); the relationships between the zang-organs and the fu-organs, including the relationships among the five zang-organs, among the six fu-organs and the relationships between the zang-organs and the fu-organs.
The theory of zangxiang mainly studies the zang-organs and the fu-organs. That is why it is also known as "the theory of zang-fu organs". Zang-fu is a collective term for internal organs which are divided into two major categories, namely the five zang-organs and the six fu-organs. The five zang-organs include the heart, the liver, the spleen, the lung and the kidney; the six fu-organs include the gallbladder, the stomach, the small intestine, the large intestine, the urinary bladder and sanjiao (the triple energizer). Besides, there is another group of tissues and organs, similar to the zang-organs in function and the fu-organs in form, which are termed the extraordinary fu-organs, including the brain, the marrow, the bones, the vessels, the gallbladder and the uterus.
Though all pertain to the internal organs, the five zang-organs and the six fu-organs are different from each other in functions and characteristics. The common function of the five zang-organs is "to store jing-qi (essence)" which is refined food nutrient responsible for maintenance of life activities. The common function of the six fu-organs is "to transport and transform food". That means to receive and digest food, absorbing the nutrients and discharging the waste. That is why TCM has divided the internal organs into two distinct categories, the zang-organs and the fu-organs.
Zang-fu is the core of life activities. The substantial bases responsible for the physiological functions of the five zang-organs and the six fu-organs come from two aspects：one is morphological structure in anatomy and the other is the refined nutrient like qi, blood, yin and yang. Different morphological structure bears certain relationship with the physiological functions of certain internal organs. Take the heart and the stomach for example. Since the heart is directly connected with the vessels, it bears the function of propelling blood to flow in the vessels. Since the stomach is connected with the esophagus and the small intestine, it can receive and digest food. Thus the morphological structure of the internal organs is one of the substantial bases for maintaining their physiological functions. In TCM anatomy was done over two thousand years ago. In Huangdi Neijing and Nanjing there are detailed records about the length of bones as well as the length, size and capacity of the stomach and the intestines. However, TCM pays more attention to the synthetic manifestations of the holistic functions of the body and is more sophisticated in cognizing the intrinsic nature of life from the external manifestations. Zangxiang theory holds that qi, blood, yin and yang are the essential substances for constituting and maintaining the physiological functions of the zang-fu organs. All the pathological changes result from deficiency or disturbance of these substances. Substantially, these refined substances are fine and minute. Though invisible to the eyes, they can still be understood and distinguished according to their physiological functions and pathologic changes. Generally speaking, qi and yang all have the functions of warming, propelling and fixating. However, qi mainly functions to propel and fixate while yang mainly functions to warm. Both blood and yin have the functions of nourishing and moistening. However, blood mainly functions to nourish while yin mainly functions to moisten. Qi, blood, yin and yang differ in qihua (qi-transformation). That is why they bear different physiological functions.
On the whole, zangxiang theory focuses on the study of physiological functions and pathological changes of the zang-fu organs, somatic tissues and organs as well as the mutual relationships among them. So it is the core of the theoretical system of TCM. Though terminologically identical with some of the organs in modern medicine, the zang-fu organs in zangxiang theory are quite different in physiological functions. Because the zangxiang theory was established mainly on the basis of life experience and clinical practice. That is to say that TCM understands the internal organs through observing the physiological and pathological phenomena as well as therapeutic effect. Thus the conceptions of zang-fu organs in TCM are quite beyond the range of anatomical morphology. It is generally believed that the functions of the zang-fu organs in TCM are extensive, not only including part of the functions of the organs with the same names in modern medicine, but also covering certain functions of other related organs. Obviously the zang-fu organs in TCM are not just the conceptions of anatomy, but synthetic functional units.
The five zang-organs
The five zang-organs include the heart, the lung, the liver, the spleen and the kidney. The following is a brief description of the physiological functions of the five zang-organs and their relationships with the body, the organs and the orifices. The latter includes three aspects, i.e. the relationships of the five zang-organs with the five constituents, the organs that the five zang-organs open into and the focal external manifestations of the five zang-organs.
The five constituents include the vessels, the skin, the tendons, the muscles and the bones that are dominated by the five zang-organs respectively.
The orifices, simplification for "the five sensory organs and the nine orifices", include the tongue, the nose, the eyes, the mouth, the ears, the external genitals and the anus. The heart, the lung, the liver and the spleen govern one orifice respectively. But the kidney opens into the ears, the external genitals and the anus.
The focal external manifestations of the five zang-organs refer to five special areas (i. e. the face, the body hair, the nails, the lips and the hair) on the surface of the body on which the essence of the five zang-organs displays.