Phlegm is a pathological substance caused by disturbance of body fluid. Generally speaking, the thick part is called phlegm while the thin part is called rheum.

Phlegm is either substantial or insubstantial. Substantial phlegm is visible, palpable and audible, such as sputum, scrofula and nodules in the skin and muscles and borborygmus. Insubstantial phlegm is invisible, unpalpable and inaudible. But there are pathological manifestations of phlegm. Substantial phlegm is identical with insubstantial phlegm in nature. When accumulating, it is substantial; when dispersing, it is insubstantial.

Jingui Yaolve (Synopsis of Golden Chamber) classifies rheum into four categories according to their location, namely phlegmatic rheum in the abdomen, suspending rheum in the chest and diaphragm, sustaining rheum in the rib-sides and overflowing rheum in the skin and muscles.

Phlegm, rheum, water and dampness are all pathological substances caused by disturbance of body fluid. But they are different from each other. The one that spreads and appears substantial is dampness; the one that accumulates and appears substantial is rheum; the one that is thin and clear is water; the rheum that is condensed is phlegm. That is to say that all of them result from disturbance of body fluid and can transform into each other.

  

Phlegm and rheum are caused by disturbance of body fluid involving the spleen, the lung, the kidney, the liver and triple energizer. Various pathogenic factors, including the six abnormal climatic factors, internal impairment due to seven emotions, improper diet, overwork and over-rest, can impair the viscera and affect qi-transformation, leading to disturbance of body fluid and the production of phlegm and rheum due to the accumulation of body fluid.

The spleen governs the transportation and transformation of water and dampness, the lung regulates the water passage, the kidney governs water, the liver promotes the metabolism of body fluid and the triple energizer serves as the water passage. The disorder of any of these organs will lead to retention of body fluid and accumulate dampness into phlegm and rheum which then affect the viscera and impair their functions, leading to repeated production of phlegm and rheum. For example, accumulation of phlegm and dampness in the lung causes cough and asthma which are clinically attributed to the deficiency of the spleen. That is why the ancient people believed that "the spleen is the source of phlegm and the lung is the container of phlegm".

The production of phlegm and rheum is also related to cold and heat which are either exogenous or endogenous due to disharmony of visceral yin and yang. If there is cold in the viscera, the activity of qi-transformation will be slowed down and the transformation of body fluid will be difficult, giving rise to the production of phlegm and rheum, especially rheum. If there is heat in the viscera, it will scorch or coagulate body fluid into phlegm. 

If there is heat in the viscera, it scorches body fluid into either rheum or phlegm, frequently condensing into phlegm. If there is cold in the lung, it will retain rheum in the lung and cause frothy and thin sputum, known as "retention of cold rheum in the lung". If there is heat in the lung, it will block phlegm in the lung and cause yellowish, thick and purulent sputum, known as "accumulation of phlegm-heat in the lung".

Phlegm and rheum can flow with qi to anywhere. If phlegm and rheum flow in the meridians, they tend to block the meridians and hinder the flow of qi and blood, leading to numbness and inflexibility of the limbs, or even paralysis. If accumulating in local areas, they frequently cause scrofula, subcutaneous nodules and cold abscess.

Phlegm and rheum result from water and dampness. Retention of phlegm and rheum in the viscera tends to hinder the activity of qi and prevent visceral qi from ascending and descending. Take the lung for example. "The normal function of lung-qi is to depurate and descend. Retention of phlegm and rheum in the lung will lead to chest oppression, cough and dyspnea, etc. Take the stomach for another example. The normal function of the stomach-qi is to descend. Retention of phlegm and rheum in the stomach will cause nausea and vomiting, etc.

If phlegm disturbs the upper and blends lucid yang, it will lead to dizziness; if phlegm confuses the mind or phlegm-fire disturbs the heart, it will cause chest oppression, palpitation, unconsciousness and delirium or mania, etc.

Since phlegm and rheum are caused by various factors, involve many parts of the body and flow to anywhere with qi, the diseases caused by them are various, involving many viscera and tissues and marked by complicated symptoms and constant change. Clinically the diseases caused by phlegm and rheum may involve the five zang-organs and six fu-organs, the body, the sensory organs, the orifices, the four limbs and the skeleton. 

The symptoms are chest oppression, cough, asthma, expectoration, nausea, vomiting, palpitation, dizziness, mania, numbness of the limbs, arthralgia or swelling of joints, subcutaneous swelling or suppuration, edema, ascites and diarrhea, etc. Generally speaking, obstinate disease and disease without evident cause are all related to phlegm and rheum. That is why it was believed in ancient times that "strange diseases are mostly caused by phlegm" and "many diseases are exclusively caused by phlegm".

Since phlegm and rheum can ascend and descend with qi, the disease caused by them is usually marked by constant change and recurrence. Take epilepsy for example. It attacks when phlegm ascends with qi and stops when phlegm descends with qi.