In modern medical science, the functions of the kidneys are maintaining the balance of water and electrolyte, regulating acid-base concentration, filtering the blood and excreting metabolic wastes as urine. Such functions are reduced to the function of controlling water in the traditional Chinese medical science. The book “Su Wen” (素问), one of the traditional Chinese medical classics, says, “The kidneys are organs of water, and control bodily fluid.” The active substance of the kidneys is called “kidney-Qi” (肾气), and it acts as prerequisite activating catalyst in the metabolism of water.

In the modern Western medical science, a spleen is the largest immune organ and the storage of blood. But in the traditional Chinese medical science, the spleen is closer to the digestive system, and secondly it is related to the circulatory system.

The active substance of a spleen is called “spleen-Qi” (脾气). The functions of a spleen, which also can be called the functions of the spleen-Qi, include: promoting the digestion and absorption of food and water, transmitting upwards nutrient substances and water, keeping the normal positions of viscera, and keeping blood in vessels.