Blood is transformed from the essence of food via the digestion
and absorption of the spleen and stomach. According to the
Lingshu, "When the middle jiao receives food and water, it
transforms it into red fluid which is called blood." After its
formation, blood circulates incessantly throughout the body to
nourish the zang and fu organs, the skin, the muscles, tendons,
and bones in order to maintain their normal physiological activities.
Blood is the substantial basis for mental activities. Only
when there the qi and blood are abundant can there be high spirits
and clear minds. So in the Suwen it states, "Blood and qi are
the spirits of man." Pathological changes of blood cause symptoms
of palpitation, insomnia, unconsciousness, delirium, etc.