Essence, qi, blood and body fluids are the basic material to form and sustain life activities of the human body. The metabolism including generation, distribution and discharge depends on viscera and meridians. Qi has the function of promotion and warming, while essence, blood, and body fluid have the function of nourishing and moistening, so qi pertains to yang, while essence, blood and body fluids pertains to yin according to the character of yin and yang.
Although they are different in nature, location, function, they are closely related and promote each other during physiological activities, and mutually affect and transmit in disease. Hence, they are closely related.
The relationship between essence and qi
Essence and Qi are closely related, so usually known as essential qi, such as essential qi in kidney, essential qi of food. Essence and qi are different in the yin-yang nature, i. e. ,essence pertains to yin while qi pertains to yang, and they can transform mutually.
1. Essence generating qi
Essence includes innate essence and acquired essence. Both combine and distribute to viscera to nourish qi of viscera and promote generation of qi. For example, essence stored in kidney can generate original qi, and food essence from food can generate nutritive qi. Therefore, essence can generate qi. Sufficient essence can generate sufficient qi to maintain vigorous functions of viscera; deficient essence can lead to deficient qi and hypofunction of viscera.
2. Qi promoting essence generation
Essence generation depends on transforming function of viscera. For example, sufficient qi of the spleen and stomach means normal and active digestion and absorption, so that food can transform into essence needed by the human body. Therefore, qi's circulation is the promotive force for essence generation, i.e., sufficient qi ensures sufficient essence, while deficient qi leads to deficient essence.
Furthermore, qi can consolidate essence. For instance, kidney qi deficiency can lead to hypofunction of consolidation, manifesting as man’s spermatorrhea and woman's diluted leucorrhea.
The relationship between essence and blood
Essence and blood both are generated from food, and supplemented by food essence. Both can promote and transform mutually, so it is said homogeny of blood and essence.
1. Essence generating blood
Essence is an important material basis for blood generation. Food essence transforms into blood through functions of the spleen, stomach, lung and heart, and kidney essence can generate marrow then to generate blood. Therefore, sufficient essence can generate sufficient blood. Either deficient food essence or kidney essence can lead to deficient blood generation manifesting as blood deficiency.
2. Blood generating essence
Essence of the human body is mainly stored in kidney. Kidney essence at first is inherited from parents and supplemented by acquired food essence. During the generation and distribution of kidney essence, blood is an important stage, for it can supplement kidney essence. For example, the liver stores blood, so that liver blood can nourish kidney essence. Therefore, sufficient blood can generate sufficient essence, while deficient blood can lead to deficient essence.
The relationship between qi and blood
Qi is active and pertains to yang while blood is static and pertains to yin. So the relationship between qi and blood can be understood according to the relationship between yin and yang. In TCM the relationship between qi and blood is generalized as "qi is the marshal of blood and blood is the mother of qi". Here "marshal" means governing and "mother" means source and foundation. Since qi pertains to yang, it can govern the circulation of blood; because blood pertains to yin, it is the source for qi~transformation. However, the relationship between qi and blood is not so simple as mentioned above.
(一) The effect of qi on blood
The effect of qi on blood is mainly demonstrated in three aspects, i. e. qi producing blood, qi promoting the circulation of blood and qi controlling blood.
1. Qi producing blood
Qi promotes the production of blood in various ways. In terms of the composition, the nutrient qi is the main component of blood, indicating that the nutrient qi produces blood. In terms of the transformation of blood, the production of blood depends on qi-transformation. The material needed for the production of blood is the food nutrient transformed and absorbed by the spleen and the stomach. The normal functions of the spleen and the stomach are directly related to spleen-qi and stomach-qi. If spleen-qi and stomach-qi are vigorous, blood-producing function will be vigorous too. If spleen-qi and stomach-qi are deficient, blood-producing function will be weakened. In fact the transformation of the food nutrients into blood still needs the transformation of other visceral qi. For example, only when the food nutrients has combined with kidney-essence can the process of transforming the nutrients into blood be accomplished; only when blood transformed from the food nutrients and the kidney-essence has been processed by the transforming activity of heart-qi and lung-qi, especially heart-qi, can blood become red. If the functions of the spleen and the stomach are weak due to qi deficiency or if the transforming activity of visceral qi becomes weak, the normal process of blood transformation will be affected, leading to blood deficiency.
2. Qi promoting the circulation of blood
Blood depends on the propelling function of qi to circulate. That is why TCM holds that "normal flow of qi ensures normal circulation of blood, stagnation of qi leads stasis of blood". In terms of visceral functions, heart-qi is the primary motivation of blood circulation; lung-qi assists the heart to propel blood to circulate; liver-qi promotes the circulation of blood. If qi is too weak to propel blood, blood will flow slowly; if qi activity is obstructed, blood will become stagnant; if qi activity is in disorder, blood will flow abnormally.
3. Qi controlling blood
Qi controlling blood means that qi directs blood to circulate inside the vessels and prevents it from flowing out of the vessels. The kind of qi that can control blood and direct it to flow inside the vessels is spleen-qi. That is why TCM holds that "the spleen commands blood". If qi fails to control blood due to deficiency, blood will flow out of the vessels, leading to bleeding.
（二）The effect of blood on qi
The effect of blood on qi is demonstrated in two aspects: carrying qi and producing qi.
1. Blood carrying qi
Blood pertains to yin and is static, so it keeps on flowing inside. Qi pertains to yang and is active, so it tends to move to the outside. When qi and blood have combined with each other, blood has acquired a motivation to move and qi has obtained a carrier to attach to. That is
why it is said that blood can carry qi. That is to say that only when qi has attached itself to blood can it avoid dispersion and loss. Clinically massive hemorrhage is usually accompanied by loss of qi. Therapeutically, apart from using the therapy for supplementing blood and stopping hemorrhage, other therapeutic methods for supplementing qi and stopping prostration must be resorted to.
2. Blood producing qi
Qi and blood, pertaining to yin and yang respectively, can transform into each other and produce each other. The primordial qi is produced by the congenital essence in the kidney and the food nutrients transformed by the spleen and the stomach. The normal functions of these organs all depend on nutrients provided by blood flowing in the vessels, and so do the other viscera and meridians. Thus the production of qi by blood is accomplished through its provision of nutrients for the viscera and meridians.
The relationship between qi and body fluid
The relationship between qi and body fluid is similar to the relationship between qi and blood, because body fluid is a component of blood. Besides, body fluid exists not only in the vessels, but also in all the tissues and organs in the body. In this sense, the relationship between qi and body fluid differs in some way from the relationship between qi and blood.
（一）The effect of qi on body fluid
The effect of qi on body fluid is demonstrated in three aspects: qi producing body fluid, qi promoting the flow of body fluid and qi controlling body fluid.
1. Qi producing body fluid
Body fluid comes from the water and nutrients of food transformed by the spleen and stomach. The spleen and stomach play an important role in the production of body fluid. If spleen-qi and stomach-qi are sufficient and if the digesting and absorbing functions are normal, the transformation and production of body fluid will be sufficient; if spleen-qi and stomach-qi are deficient and if the digesting and absorbing functions are abnormal, the transformation and production of body fluid will be reduced.
2. Qi promoting the flow of body fluid
The flow of body fluid, including the distribution and excretion, depends on the propelling function of qi. At the early stage body fluid is transported by spleen-qi to the heart and the lung; heart-qi propels body fluid and blood to flow; lung-qi disperses the fluid in the skin and viscera on the one hand, and descends the fluid to the kidney and the bladder on the other; the kidney and the bladder, through qi-transformation, transports the lucid part of the fluid to the heart and the lung on the one hand, and descends the turbid part of the fluid to transform it into urine to be discharged out of the body on the other. Since the flow and metabolism of body fluid all depend on the propelling and transforming functions of qi, the state of qi and the activity of qi directly affect the flow and metabolism of body fluid. If qi deficiency or qi stagnation occurs, the fluid will accumulate and turn into phlegm and edema. That is why it is said in TCM that "normal flow of qi ensures normal flow of water and stagnation of qi leads to stagnation of water."
3. Qi controlling body fluid
Under the propelling and transforming action of qi, the metabolism of the body is demonstrated in two ways: opening and closing. Opening means to excrete the remaining part of fluid out of the body and closing means to keep certain amount of water needed in the body. The way that qi keeps body fluid in the body is called "qi controlling body fluid". The ways to excrete water from the body include urination and sweating. If qi fails to control body fluid due to deficiency, it will lead to abnormal urination and sweating, such as poliguria, incontinence of urine, enuresis and polyhidrosis, etc.
(二)The effect of body fluid on qi
The effect of body fluid on qi is demonstrated in two ways: carrying qi and producing qi.
1. Body fluid carrying qi
Body fluid is the carrier of qi and qi must attach itself to body fluid in order to flow to the whole body. This theory can be understood from two angles. On the one hand, blood produced by body fluid in the vessels can carry the nutrient qi; on the other hand, body fluid flowing in other tissues and organs can carry the defensive qi. If great quantity of body fluid is lost, qi will be exhausted accordingly. This state is called "loss of body fluid followed by exhaustion of qi";. In severe cases, it will become "loss of body fluid followed by loss of qi";.
2. Body fluid producing qi
Body fluid, just like blood, can produce qi. On the one hand, body fluid inside the vessels transforms into blood to nourish the viscera so as to maintain sufficiency of qi in these viscera and the body.
Relationship between blood and body fluids
The relationship between blood and body fluids can be generalized as homogeny of body fluids and blood. First, both are generated from food essence through spleen and stomach's digestion and absorption; also body fluids is consisting part of blood for body fluids generated by the spleen and stomach ascends to the heart and lung to become blood combined with nutrient qi. Second, body fluids and blood can mutually transform. Body fluids outside vessels can permeate into vessels to become a part of blood while body fluids consisting of blood can permeate out vessels to become body fluids. So, both depend on each other and mutually transform.
Pathologically blood and body fluids can inter-affect. For example, excessive bleeding can cause body fluids outside vessels to permeate into vessels to supplement blood volume. Hence, patients of excessive bleeding usually are with symptoms of dry mouth, dry throat, scanty urine,dry skin due to deficient body fluids. Excessive loss of body fluids can cause body fluids inside vessels to permeate outside vessels to supplement body fluids outside vessels, resulting into empty blood vessels or deficient body fluids and dry blood. Hence, patients of severe bleeding should avoid diaphoresis and diuresis; patients of severe body fluids loss such as severe sweating, vomiting,diarrhea, should avoid drugs or formulas to remove blood stasis, so as to avoid further damaging body fluids and blood, which are the clinical application of the theory homogeny of body fluids and blood sharing the same origin.