IV. Qi, Blood And Body Fluid
Qi, blood and body fluid are important substances and structures in the body. They sustain the vital activities and they nourish the body, thereby keeping the functions of the tissues, organs and channels in good order. The production and circulation of qi and blood also depends on the health of the tissues and organs that are nourished by these substances.

Qi
Qi is a complex concept; it relates to both substance and function. Clean qi (oxygen), waste qi (carbon dioxide) and qi (nutrients) are generally known as material qi, and the existence of material qi is shown by the functional activity of various organs. The function of an organ depends on the functional qi of that organ; for instance, qi of xin-heart or qi of pi-spleen is the vital energy and functional activity of the xin-heart or pi-spleen. The function of an organ, or its functional qi, cannot exist without material qi, and vice versa.

Zhong qi
Zhong qi is found mainly in the chest. It nourishes the structures and functions of the xin-heart and fei-lung.

Nourishing qi
Nourishing qi circulates in the channels and collaterals, mainly in the viscera

Defensive qi
Defensive qi is in the muscles and skin. It circulates outside the channels, in the subcutaneous tissues, and it defends the body against invasion by pathogens.

The original qi is nourished and maintained by qi derived after birth. These combine to form genuine qi, i.e. the total sum of qi in the healthy body. This contrasts with pathogenic factors that are known as pathogenic qi.

Blood
The nutrients from food are digested by the pi-spleen and stomach and they are then transported to the xin-heart and fei-lung and turned into red (oxygenated) blood by qi. The essence of shen-kidney produces bone marrow, and bone marrow uses the digested food to produce blood.
Qi of shen-kidney promotes digestion by pi-spleen, which in turn strengthens the xin-heart and fei-lung. This interaction therefore promotes haemopoesis.

There is a close relationship between qi and blood. The formation and circulation of blood depends on qi, whereas the formation and distribution of qi, as well as the health of the various organs of the body, is dependent on adequate nourishment from the blood. If the flow of blood 'stagnates' the circulation of qi is 'retarded' and, conversely, if the circulation of qi is 'retarded' then the blood flow 'stagnates'.

Body Fluid
Body fluid is formed from food and drink. It exists in the blood, the tissues, and all the body openings and cavities.

V. The Pathogenesis of Disease
In traditional Chinese Medicine various elements and other factors cause disease. These are known as pathogenic factors or pathogens. Normally the human body is able to resist pathogens and maintain a healthy balance between the body and the environment. This ability is a function of normal qi, especially the defensive qi.

Disease develops because normal qi is unable to resist the onslaught of the pathogenic qi; if pathogenic qi overwhelms normal qi then a functional disturbance of the body results. The major principle of treating a disease in Chinese medicine is to strengthen and protect normal qi and maintain a healthy body. In ancient China a physician was only paid while his patient was healthy, not while his patient was ill!