Jing is the (material and immaterial) substance most intimately connected to life itself. It is the creator of life and is the foundation for differentiation into Yin and Yang. The Kidneys are sometimes referred to as the "Root of Life" because Jing is the basis for reproduction, growth, and development. It flourishes as we grow and develop, and wanes as we grow old. The Kidneys are said to govern these processes at it stores the Jing.
Jing (Essence) the precious substance has two types:
Prenatal Jing determines our basic constitution, vitality, and strength. It is the basis of sexual life and the formation of sperm and egg. It is the foundation for differentiation into Yin and Yang. Insufficient Jing can cause infertility, impotence, problems relating to growth, reproduction, premature senility, etc.
Postnatal Jing is the refined essence extracted from the food by the internal Organs, particularly the Spleen.
Kidney Jing provides material basis for both Kidney Yin and Kidney Yang.
Jing is also the organic foundation for production of marrow.
Marrow is the common substance of bones, bone marrow, brain ("sea of marrow") and spinal cord.
Kidney Jing produces Marrow, which generates spinal cord and "fills up" brain: therefore Kidney has relationship with brain. Strong Jing nourishes brain and memory/concentration, etc. will be good.
Jing inadequate to nourish the brain: poor memory and concentration, dull thinking. Therefore Kidneys also govern bone marrow and bones (and teeth, which are said to be the outgrowth of bones).
In disease, Kidney Yin and Yang imbalanced (not enough Yang or not enough Yin). Where Yang is deficient, "gate" stays open: profuse and pale urination. Where Yin is deficient, "gate" stays closed: scanty and dark urination.
While the Lungs govern respiration, the Kidneys play an important role in normal breathing. The Kidneys assist the Lungs during inhalation by "Grasping the Qi". In this relationship, the Lungs are called the "Foundation of Qi", and the Kidneys are called the "Root of Qi". When the Kidneys fail to Grasp the Qi, respiratory illnesses can occur, such as chronic asthma.
Ears related to Kidneys: Kidney Jing nourishes ears and enables proper functioning. Weak Jing, weak hearing or tinnitus.
Abundant Kidney Jing nourishes hair: hair then grows well and is healthy and shiny. Weak Kidney Jing: thin brittle, dull, or falling hair, premature graying.
NOTE: Liver Blood also has to do with hair color and shine, but Kidney Jing is instrumental in production of Liver Blood.
The Kidneys control the front orifice, which is the urethra (and spermatic duct in males), and the rear orifice, which is the anus.
These are functionally related to the Kidneys.
Urethra related because Kidney provides the Bladder with the Qi necessary to transform urine (weak Kidney Qi produces leaking of urinary incontinence).
Deficient Kidney Qi or Kidney Jing can cause spermatorrhea or nocturnal emissions.
Anus anatomically related to Large Intestine but functionally related to Kidney. If Kidney Qi is weak, there may be diarrhea or prolapsed anus.
Will Power (Zhi), including concepts of staying power and motivation, depend on Kidneys.