The axis between kidneys and heart is known as the Root since if communication between both is cut, death ensues.

• Heart houses the Shen and belongs to Fire.
• Kidney houses the Jing and belongs to Water.

Fire - Water polarity is vital for physiology.

• Heart-Fire descends to Kidney to warm them.
• Kidney-Water ascends to Heart to nourish Heart-Yin.
• Kidney is the root of Qi that belongs to Yang (Yang within Yin).
• Heart is the root of Blood that belongs to Yin (Yin within Yang).

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