Functions of the Kidney
The kidneys are the root of all pre-natal qi in the body, just as the Spleen is the root of all post-natal qi. Anything that might be considered genetic or hereditary in modern terms stems from the kidneys, as all of our ancestral information is stored there. Thus, the kidneys are also the source of the base yin and yang of the body. Within the kidneys the interdependence of yin and yang are even more pronounced than in the yin and yang of the other organs. It is internally-externally related to the Bladder. Its element is Water.
Stores Essence & Governs Reproduction & Development
Essence (jing), comes in 2 forms: pre and post natal. Post-natal Essence is simply the pure substances made by workings of the Spleen (transformation), Lungs and Heart. Pre-natal Essence is all that is inherited. It, in turn, controls how the baby develops and matures, and makes up the basic constitution of a person. It is also the basis for sexual function and its strength is proportionally related to its ability to reproduce. When pre-natal Essence is strong and abundant: fetus develops properly, constitution is strong, person can reproduce without problem. When pre-natal Essence is weak and deficient: fetus is underdeveloped, can have birth defects, constitution is weak, infertility, physical and mental retardation. The strength of the Essence pushes a person through their lifelong developments of adolescence, puberty, reproduction, aging, menopause, etc. Additionally, it is the root of all yin and yang of the Kidney (which is in turn the root of all yin and yang of the body).
(Note: The "Su Wen" states that development occurs in periods of 7 years for men, and 8 years for women. These ages are milestones and are the norm with which a child's development should be compared.)
Controls Water Metabolism
The kidneys control the water in the lower warmer as the lungs do in the upper warmer. This function helps the kidney to regulate the yin (form) and the yang (function) of the body, by releasing or holding in fluids (processing and urination as in allopathic medicine). The kidneys provide the Qi necessary for the bladder to hold and release fluid and for the large and small intestines to separate pure and impure fluids. Additionally, remember it warms the fluids to send to the Lungs and upper warmer in the form of mist.
Receives & Grasps Qi
The kidneys work hand in hand with the lungs by taking the qi sent down to the lower warmer and holding it. If it cannot do this we have something called counterflow qi where there is qi above, but none below. This can lead to breathing issues, as well as deficiencies of the lower warmer.
Produces Marrow to Fill the Brain & Control the Bones
Get your mind away from the word "marrow" as in where blood cells come from. This marrow is found in both the bones and the brain and spinal cord. The Essence produces the marrow and the marrow in turn makes the bones strong and fills the spinal cord and brain to garner the intelligence and concentration. The brain is often called the "Sea of Marrow" for this reason. Everything involved in aging is governed by the kidneys as you can see. When the kidneys finally go weak, the memory and quality of thought is poor (brain), the bodies reactions and speed are slowed (spine), the teeth crack and fall out (bone), and the bones become brittle.
Opens Into the Ear
The state of hearing is directly related to the strength of the Kidneys.
Manifests in the Hair
The shine, health, and abundance of hair is related to the kidneys as well. Thus when we get old and our essence is weak, our hair stops growing or falls out.
Houses Will Power
No real explanation here. This is all that is said in the Su Wen so assume it means determination and adherence to principles.