It is said that the Kidneys contain the essence (肾藏精) and that the Kidneys govern water (肾主水). There are many other descriptions of the Kidneys, which to me are either of lesser importance or can be derived from the two assertions mentioned above.
Kidneys contain the essence (肾藏精)
Here, essence can be seen as sperm and the testosterone that goes with it. That is why the ancients believed in not letting out your sperm because it takes away your manhood. In fact, testosterone – which is needed to create sperm – is what gives men their male characteristics.
That said, the Kidney is likely not just Testosterone but alot of other sexual hormones, for example DHEA, estrogen and progesterone. The Kidney in Chinese Medicine does a great balancing act with all the sexual hormones in our body.
But that’s not all it is. Androgens are created in the zona reticularis of the adrenals. So adrenals create testosterone. But the Kidneys do not just govern reproduction, they also govern growth and development (生长发育). For this, other hormones like Growth Hormone (GH) are vital. Growth hormone – also called somatotropin – is produced in the pituitary gland. Production of somatotropin as with other sexual hormones decreases with age. In some people, the pituitary secretes insufficient GH from a young age, and so we have dwarfism (呆小症).
The TCM solution here is to pump up on Yin and Yang boosters. Examples include dihuang yinzi (地黄饮子) and baiwei shenqi wan (八味肾气丸).
Kidneys govern water (肾主水)
We can understand this as saying that the Kidney regulates water metabolism within the body. The first place we can see this happening is with the bladder (膀胱). The bladder has an interior-exterior relationship with the Kidneys – it is because of the Kidneys ability to transform qi (气化) that the bladder is able to open and close up in a proper fashion.
When the bladder does not open when it should, we have longbi (癃闭), oliguria (尿少),edema (水肿) etc, we say that the Kidney isn’t qi-transforming enough (气化不主). In this situation, insufficient qi-transformation means not enough pushing power (推动无力), and so urine doesn’t come pumping out like it should. When does this happen?
In another case, once again the Kidney is blamed for insufficent transformation of qi. But this time, while its is accorded the same reason, the lack of qi transformation means lack of power to hold in/contain (固摄无力,制约无权). Possible consequences include polyuria (尿多),urinary incontinence (遗尿 and 尿失禁). When does this happen?
Not Water but Sperm
In addition to urinary incontinence, the Kidneys are blamed for spermatorrhea (遗精,滑精,早泄). These are diseases where not urine but spermatozoa exits the body when it shouldn’t. Once again, we talk about Kidneys not holding in (肾不能固摄). But we skip the bladder, since this is not about urine.
How to treat? We can consider the following methods, of which only one is related to holding in (固摄).
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Less on the kidneys, but more on the prostate as the expression of weak “Kidneys.” This is why happens as men these days age and start having to go to the loo during the night.
BPH in a sense is a form of “weak kidney” in TCM.
Check out the article on BPH.