The meridian begins beneath the fifth toe, crosses the sole obliquely to the hollow below the ball of the foot, curves around the medial malleolus in a loop around the heel, then rises up the inner aspect of the leg and thigh. It enters the spine at the lumbar level and reaches the kidney and bladder. An external branch rises up the abdomen and chest half a finger-width from the midline and ends below the clavicle. An internal branch travels from the kidney to the liver, lung, heart and root of the tongue.
The Kidney stores Essence and commands birth, growth, reproduction and ageing. It governs the bones, produces marrow and fills the brain, described as the sea of marrow. It commands water and, with the Lung, the reception of Qi: chronic breathlessness is often traced back to it. It opens into the ears and manifests in the hair. Fear is the emotion that injures it, and deep willpower, the Zhi, resides there.