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The kidneys are composed of the kidney body and the adrenal glands located above them. Modern medicine has gained a deep understanding of the functions of both, which greatly aids in reinterpreting the kidneys in Traditional Chinese Medicine (TCM). The functions of the kidney body revealed by modern medicine are as follows:

• Filtering blood and excreting metabolic waste and toxins in the form of urine.
• Regulating the body's water content, with excess water excreted as urine.
• Releasing hormones (such as renin and prostaglandins) to assist in regulating blood pressure.
• Regulating the balance of various electrolytes (such as sodium, potassium, chloride, calcium, phosphorus, and magnesium).
• Maintaining the acid-base balance of the blood.
• Releasing erythropoietin (EPO) to stimulate the bone marrow to produce red blood cells.
• Synthesizing active vitamin D, promoting intestinal absorption of calcium, and maintaining bone calcium and other chemical balances.

The adrenal glands are endocrine organs that secrete many vital hormones related to survival:

• Adrenaline and noradrenaline: Used to cope with external stress, with effects similar to directly stimulating the sympathetic nervous system throughout the body.
• Glucocorticoids: Promote the breakdown of fats and proteins (mainly from muscles), stimulate gluconeogenesis in the liver, increase glycogen, raise blood sugar levels, and have anti-inflammatory and immunosuppressive effects.
• Mineralocorticoids: Promote the retention of sodium ions and water in the kidneys and intestines, while excreting potassium ions.
• Small amounts of male hormones.

However, TCM believes that the kidneys have even more functions, such as governing growth, development, and reproductive functions. This divergence arises because ancient medical practitioners, limited by the technology of their time, could only infer the functions of the kidneys. As a result, the kidneys in TCM do not solely refer to the two anatomical kidneys but also represent a group of related functions such as reproduction, urination, and endocrine activities. The author prefers to refer to this as the "kidney system" to distinguish it from the physical kidneys.