The zang-xiang theory holds that, although each of the body’s zang-fu organs has its own specific function, they are by no means isolated. The zang and fu organs share specific relationships of connection and correspondence. The two are paired as interior and exterior, linked through the meridians and collaterals. Physiologically, they complement one another; when pathological changes occur, they affect each other. Within this network of zang-fu interrelationships, the five zang organs play the leading and decisive role. Specifically, the heart is paired with the small intestine, the lung with the large intestine, the liver with the gallbladder, the spleen with the stomach, and the kidney with the urinary bladder. Therefore, when a pathological condition arises in one zang or fu organ, the function of its corresponding partner is often taken into consideration. For example, in treating haematuria, the clinician often considers whether the heart fire has transferred heat to the small intestine, and may include medicines that clear heart fire in the prescription. Similarly, in treating constipation, the method of descending and purifying lung qi is often employed to restore the conducting function of the large intestine.

The formation of the zang-xiang theory was built upon the prolonged observation of human physiological and pathological phenomena by ancient physicians. Through the mutual corroboration of repeated diagnosis and treatment outcomes, knowledge of the zang-fu organs’ activity patterns was gradually accumulated. They synthesised a wealth of medical experience, refining and perfecting the theoretical framework of zang-xiang.