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Blood is a red liquid circulating in the vessels, and is a vital nutrient substance in the body.

Formation and circulation of blood

As the fundamental substances required in blood formation originate from food essence produced by the spleen and stomach, these two organs are regarded as the source of qi and blood. The thirtieth chapter of Miraculous Pivot states: "When the middle jiao receives food essence it will transform it into red fluid which is called blood."

The seventy-first chapter of the same book also says: "Yingqi flows into the vessels to be transformed into blood." Essence and blood may also transform into each other. The book Zhang's General Medicine states: "If blood is not consumed, it turns into essence in the kidney; if essence does not leak out, it is transformed into blood in the liver." Taking food essence and kidney essence as the material basis, blood is formed by the functional activities of zang-fu organs such as the spleen, stomach, heart, lung, liver and kidney.

After being formed, blood normally circulates in the vessels throughout the body, and is acted upon jointly by the heart, liver and spleen. The heart dominates the blood and vessels, and the propelling force of heart qi is the basis of blood circulation. The spleen qi controls blood and prevents extravasation. The liver promotes the free flow of qi, stores blood and regulates its volume. The co-ordination of these three organs ensures continuous blood circulation in the vessels throughout the body.

Dysfunction of any of the three organs may cause abnormal blood circulation. Deficiency of heart qi, for instance, may lead to stagnation of heart blood. Dysfunction of the spleen in controlling blood may lead to bloody stools, uterine or subcutaneous bleeding, and ecchymoses.

Functions of blood

Blood circulates throughout the body, passing through the five zang and six fu organs in the interior, and the skin, muscles, tendons and bones on the exterior. In this way blood nourishes and moistens the various tissues and organs of the body. The Twenty-Second Problem of Classic on Medical Problems generalises this function of blood, saying: "Blood dominates nourishment and moisture." The nourishing and moistening function of blood manifests clearly in the movement of the eye and four limbs.

According to the tenth chapter of Plain Questions, "When the liver receives blood, it gives rise to vision; when the feet receive blood they are capable of walking; when the palms receive blood they are capable of holding; and when the fingers receive blood they are capable of grasping."

The forty-seventh chapter of Miraculous Pivot says: "When the blood is in harmony ... the tendons and bones will be strong and the joints will function smoothly." Insufficiency of blood may impair its nourishing and moistening function, and give rise to symptoms such as impaired vision, dryness of the eyes, motor impairment of the joints, numbness of the four limbs and dryness and itchiness of the skin.

Blood is the material foundation for mental activities. A sufficient blood supply ensures clear consciousness and a vigorous spirit. The twenty-sixth chapter of Plain Questions states: "Qi and blood are the foundation for human mental activities." The thirty-second chapter of Miraculous Pivot says: "Harmonious circulation of blood ensures a vigorous spirit." These quotations explain the close relationship between blood and mental activities. Deficiency of blood, therefore, may produce mental disorders. An example is deficiency of heart or liver blood which may result in mental restlessness, with symptoms such as palpitation, insomnia and dream-disturbed sleep.

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