
In the eyes of TCM, Qi, Blood & Essence (jing) are the essential substances for life activities, all originating from the viscera and flowing constantly inside the body. Ensuring these essential substances are sufficient and circulating throughout our body is very important for our health and wellbeing. Many diseases are caused by the stagnation or lack of these substances.
Qi 气
Among the three, Qi is the most active substance. Qi propels the formation and circulation of Blood and Essence in the whole body. It is translated into “life force,” “Prana” in Hindu philosophy, or “energy” in English, but it carries much more meaning than that! Classic Chinese philosophy believes that the primary state of the universe is Qi, the constant movement of which produces all the things in the universe, including life. Generally speaking, there are five physiological functions of Qi:
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Qi is the motivation of the physiological functions of all the viscera and organs in the body.
For example, kidney-Qi promotes the development of the body and reproduction; transform water and receive lung-Qi; heart-Qi promotes blood circulation; lung-Qi governs respiration and regulates water passage; spleen-Qi promotes digestion and absorption of food and commands the blood; liver-Qi regulates various functions by smoothing the activity of Qi. Weakness of Qi in promotion will lead to hypofunction of the viscera and other organs and cause various deficiency problems.
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Qi warms the body and is the source of heat energy in the body.
The kind of Qi that warms the body is called "yang-Qi 阳气." For example, heart-yang warms and dredges the blood vessels to promote blood circulation; lung-yang warms the skin and muscular interstices and prevents exogenous pathogenic factors from invading the body; spleen-yang warms and transforms food and water, promoting digestion and absorption; liver-yang steams and fumigates Qi, promoting Qi transformation in the five Zang-organs and the six Fu-organs; kidney-yang warms life-gate, stimulates reproduction and transforms water. If the warming function of Qi is weakened, it will lead to stagnation of internal cold, erratic circulation of Qi and blood, and devitalization of the visceral functions.
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Qi protects the body, resisting the invasion of various pathogenic factors and preventing disease.
“Defensive Qi 卫气” defends our body. If the protecting function of Qi is weakened, it mainly leads to decline of body resistance and susceptibility of the body to invasion of pathogenic factors (dampness, fire, cold, dryness, summer-heat, wind), seen as weakened immunity in western understanding.
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Qi can astringe and control liquid substances, such as blood, body fluid and sperm, to prevent them from leaking.
To be specific, Qi fixating blood means that Qi keeps blood to flow inside the vessels and prevents it from flowing out of the vessels; Qi fixating sweat, urine and saliva means that Qi controls the secretion and excretion of these liquids to restrict leaking and prevent loss; Qi fixating sperm means that Qi balances sex function and prevents seminal emission. Besides defecation and location of the viscera are under the influence of the fixating function of Qi. If the fixating function of Qi is weakened, it will lead to loss of blood, body fluid and sperm. The weakness of the Qi fixation may also lead to protracted diarrhea, incontinence of urine and stool as well as proctoptosis (rectal prolapse) and prolapse of the uterus.
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Qi transforms.
Life activities concerned with Qi-transformation can be divided into three categories. Firstly, through food and respiration, the body absorbs nutrients from the external world and transforms them into essence, Qi, blood and body fluid — essential elements to the body. Secondly, inter-promotion among the refined substances (essence, Qi, blood and body fluid, etc.) is the process of automatic regulation, improvement, and balance of life. Thirdly, waste substances and turbid Qi are excreted out of the body in the process of life. If Qi-transformation is weakened, the whole process of life will be in disorder or decline, leading to various diseases. The declination of Qi-transformation even leads to death.
Qi and Directionality
Qi moves in four directions: ascending, descending, exiting, and entering.
This relates to food therapy in that the energy / Qi an ingredient or food carries also moves in four directions that can heal or bring about issues. For example, food with very strong ascending/rising Qi will make a headache worse!

Blood 血
Blood (Xue, 血) in TCM is different from the blood in biomedicine. It is a denser form of Qi and the two are inseparable. Qi gives life and movement to Blood, but Blood nourishes the Organs that produce Qi. Blood production involves the major organs of Kidneys, Lungs, Liver, Stomach, Spleen, and of course, the Heart. After we are born, the spleen and stomach are key providers of blood; if they are weak at absorbing food-nutrients, the production of blood is inevitably reduced.
The physiological functions of blood are to:

Essence 精
Essence, Jing 精 in Chinese, refers to the most basic and precious material that forms and sustains the foundational activities to sustain human life. Jing consists of prenatal essence from our parents and postnatal essence, which is primarily produced from our digestive center. Both kinds of essence are stored in the kidneys to form Kidney Jing.
Therefore, it is essential to take care of our kidneys to preserve Jing as much as possible.
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Prenatal Essence: This is what you get from your parents. It determines your basic body constitution, strength, and vitality. There is only a fixed amount of prenatal essence in all of us and the amount may vary. Those who have developmental problems are believed to inherit insufficient amounts of prenatal essence; those who are born strong and healthy may have won a genetic lottery of good amounts of prenatal essence.
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Postnatal Essence: Postnatal Jing is the complex of essences refined and extracted from your life experiences, from food to activities to sex life. It largely depends on the functions of Stomach and Spleen, but also all other organs. A healthy lifestyle with a balanced diet, a good amount of physical activities, a positive mood, and a moderate amount of sexual activities will supply you with sufficient postnatal essence.
Functions of Essence:
https://tcmwiki.com/wiki/essence
References: https://www.sacredlotus.com/ | https://tcmwiki.com/