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Food essence, fluid and fresh air are the three basic substances forming blood. The most nutritious part of food essence is transformed into nutrient qi, which is transported with body fluids to the lungs by spleen qi. In the lungs, nutrient qi, fresh air and body fluid are mixed, and then they perfuse* into the vessels and pass the heart. They are transformed into blood while passing the heart through the transformation function* of heart yang. On the other hand, kidney essence can transform into marrow, and marrow can transform into blood. Therefore, nutrient qi and fresh air are one source and kidney essence is another source for blood transformation.

Diet can be transformed into food essence and body fluids by the spleen. Thus, there is a saying that the spleen and stomach provide the source for blood formation. Kidneys store essence which can be transformed into blood. Liver is responsible for facilitating and regulating qi dynamic, which assists the spleen and stomach in transportation and transformation and assists the kidneys in storing essence.* Therefore, all viscera participate in* blood formation.

Though the heart is the most important viscus* to maintain blood circulation, lungs, liver, and spleen also play certain roles. Heart qi can maintain the heart beating and provide power for blood circulation. The lungs dominate qi, and help the heart in promoting blood circulation. The liver dominates free coursing*, regulates and smooths qi dynamic. When qi moves, the blood flows. The spleen controls blood and maintains it circulating within blood vessels, ensuring no bleeding. Thus, the circulation of blood is accomplished by the coordination of the heart, lungs, spleen, and liver.

There are some other factors that can affect blood circulation, especially qi, blood vessels and blood quality itself.

Vessels provide routes for the blood to flow to associated areas of the body. Blood has viscosity* which affects its circulation. If blood is viscid*, it cannot flow freely, and blood stasis may occur.

Blood flows constantly in the blood vessels. It reaches the viscera and bowels internally, and reaches the skin, sinews, interstices and orifices externally. The main function of the blood is to nourish the whole body.