The small intestine is located in the middle of the abdomen, connected with the stomach at the pylorus in the upper and the large intestine at ileocecal junction in the lower. The physiological function of the small intestine is to receive the chyle and separate the lucid from the turbid.

The small intestine receives the chyme from the stomach and keeps for a certain period of time in order to further digest it.

Separate the Lucid 

The lucid refers to food nutrients and the turbid refers to the waste of food. After further digestion and absorption of the nutrients and part of the water, the small intestine transmits the waste to the large intestine. This process is called "to separate the lucid from the turbid" in TCM.

In fact, to receive the chyme and to separate the lucid from the turbid are two aspects in the digesting and absorbing process. There is a close relationship between these two aspects. The former is the condition of the latter and the latter is the result of the former.

TCM emphasizes the functions of the five zang-organs, so the digesting function of the small intestine is attributed to the transporting and transforming function of the spleen. That is why clinically diseases due to disorder of the small intestine in digestion and absorption, such as anorexia, abdominal distension and loose stool, are differentiated as "dysfunction of the spleen" and treated from the aspect of the spleen.

    

The function of the small intestine to absorb water decides the quantity of urine. If the small intestine absorbs water normally and the water is fully absorbed and distributed to all parts of the body, the stool will appear in normal form and the urination will be smooth. If the small intestine is abnormal in absorbing water and the water is kept in the intestines and moves downward with the waste, then the stool will be sloppy and the urine will become scanty. Such a condition is usually differentiated as "dysfunction of the spleen". 

If the small intestine absorbs too much water, the urine will become profuse and the stool will become retained. Such a condition is called "constipation due to spleen deficiency and scanty fluid", indicating the relationship between the small intestine and the spleen. Clinically diarrhea in some cases is treated by the therapy "for promoting urination to consolidate the stool", focusing on promoting urination to reinforce the function of the small intestine to absorb water. The syndrome of "constipation due to spleen deficiency and scanty fluid" can be treated by slowing down the descending activity to reduce the speed of the small intestine in absorbing water.