The Vital Substances
Tradtional Chinese Medicine (TCM) views the working of the body, mind and spirit through the interaction of the vital substances and they are:
Each has their own characteristics and functions which are fundamental to the energetic and physiological functions of the body as a whole. They also have unique interactions with one another.
Qi 
Qi (pronounced chee) is considered by classical Chinese philosophy to be the primary state of the universe. Its’ division gives rise to Yin and Yang, Heaven and Earth and subsequently to man and the 10’000 things. Through its constant movement, it produces all the things in the universe, including life. Chinese cultural and medical thought considers it to be as fundamental as Yin and Yang. Thus people in ancient China thought that its accumulation would produce life whereas its’ dispersion would put an end to life. Its nature is very active and in constant motion whilst also being extremely fine and invisible.
In the theory of TCM, it is the most essential of the vital substances that makes up the body and maintains life activities. All vital substances, organs and meridians in the body are formed by the motion, transformation and accumulation of qi.
As an important and unique concept of TCM it is defined and sub-categorised into different types according to its function within the body. This includes: Nutrient (Ying), Defensive (Wei), Primordial (Yuan) and Pectoral (Zong). Each of the body’s Zang-Fu organs and their associated meridians also have their own Qi. Disharmony within the body, organs and/or meridians will lead to either Qi Deficiency (Xu), Stangation (Zhi) or Rebellion (Ni).
Blood 
Blood (Xue) in Chinese medicine has a different meaning to that of blood in Western medicine. In terms of the vital substances it is viewed as a form of Qi, this is despite the fact that it is material and very dense. Moreover, Qi and Blood are inseparable. Without Qi, Blood would be an inert fluid.
It is mostly derived from Food-Qi produced by the Spleen. The main functions of blood are that of nourishing and moistening the body and body tissues. Another important function of blood is in providing the material foundation for the mind. As Blood is Yin, it acts to house and anchor the mind.
Aside from its relationship with Qi, Blood also has specific relationships to each of the Five Zang organs, the Essence (Jing) and Body Fluids.
The main types of Blood pathology in Chinese medicine are Blood deficiency, Blood heat and Blood stasis.
Body Fluids 
The next of the vital substances are Body Fluids or Jin Ye. Jin meaning ‘moist’ or ‘saliva’ and indicates anything which is liquid, while Ye meaning ‘fluid’ indicates fluids of living organisms. The Body Fluids origniate from our food and drink. They are then transformed and separated by the Spleen. They are further processed by the Lungs, Bladder and Large Intestine.
The nature of Fluids (Jin) is clear, light and thin or watery, and they circulate with the Wei Qi (Defensive Qi) on the exterior. They generally move quickly and are under the control of the Lungs. Their function is to moisten and partially nourish skin and muscles. They are exuded as sweat and manifest as tears, saliva and mucus. They also comprise the fluid component of Blood and are therefore important in the prevention of Blood stasis.
The Liquids (Ye) are more turbid, heavy and dense. They circulate with the Ying Qi (Nutritive Qi) in the interior and move relatively slowly. They are controlled by the Spleen and Kidneys for their transformation and by the Middle and Lower Jiao (Burner) for their excretion. Their main function is to moisten the joints, spine, brain and bone marrow. They also aid in lubricating the sense organs (eyes, ears, nose and mouth).
They have specific relationships with the Five Zang Organs, Qi and Blood. Pathology of the Body Fluids occurs in two different ways, either through deficiency or accumulation in the form of oedema or phlegm.
Essence
The Essence or Jing is derived through a process of refinement and distillation and is the most precious of the vital substances. In Chinese medical theory, it is used in three different contexts, each with slightly different meanings.
- Pre-Heaven Essence – A blending of the sexual energies of man and woman forming the ‘Pre-Heaven Essence’ of a newly conceived human being. It determines a person’s basic constitution, vitality and strength, and is responsible for making each individual unique. It is inherited from the parents at conception and is generally regarded as fixed, although it is possible to positively influence it in adult life through maintaining a balanced lifestyle and good Qi practices such as Tai Chi or Qi Gong.
- Post-Heaven Essence – This is refined and extracted from food and drink by the Spleen and Stomach after birth. This process ultimately leads to the production of another of the vital substances, Qi.
- Kidney-Essence – This is more specific and plays an important role in human physiology and is derived from both the Pre- and Post-Heaven Essence. Although it is stored in the Kidneys, its fluid nature means it circulates all over the body. It determines growth, reproduction, development, sexual maturation, conception and pregnancy. It is the basis of Kidney-Qi, produces the ‘Marrow’ and is the basis of constitutional strength.