What is a Diuretic?
Strictly speaking a diuretic is a remedy that increases the secretion & elimination of urine from the body. A diuretic is a substance that will increase the volume of urine produced, in healthy or unhealthy kidneys, by promoting the excretion of salts and water. They are used to reduce edema especially in heart, liver or kidney disorders, to treat high blood pressure, and occasionally to treat glaucoma.
In herbal medicine with its ancient traditions, a diuretic tends to mean a herb that has some sort of beneficial action on the urinary system. Thus, this category may include urinary demulcents and anti-inflammatory remedies. They have a vital role in any good treatment of illness as they will help the body eliminate waste and support the whole process of inner cleansing that is needed. Many of the diaphoretics act as diuretics when taken cold.
How Diuretics Work
If we limit herbal diuretics to the strict sense of the word, there appears to be three broad groups, those that increase kidney blood flow (stimulating), those that reduce the water resorption in the nephrons of the kidney (osmotic), and those that work by increasing cardiac output.
1) Stimulating diuretics work by irritating the kidneys so that they try to flush away the offending substance (eg. Juniperus communis acts in this way). They also include caffeine containing herbs (eg. tea & coffee) that increase the blood flow in the kidneys by effects on the heart or elsewhere in the body. Because there is more blood passing through the kidney, more urine is therefore produced. Constituents that irritate the resorption mechanism in this way are often volatile oils, saponins or alkaloids.
2) Osmotic diuretics work via many different means, but often cause diuresis due to constituents such as mainly mucilage & polysaccharides which cause a nerve reflex reaction through the gut lining (via the vagus) to thin & loosen mucous secretions to be excreted via the kidney. As larger and/or unmetabolized sugars pass into the urine, this change in osmotic pull causes more water to be lost (eg. Agropyron repens & Zea Mays act in this way).
3) Cardiac or peripheral circulatory stimulants work by increasing renal blood flow and hence the glomerular filtration rate. Up to a certain point any excess glomerular filtrate can be reabsorbed from the kidney tubules, but if it is too high then the urine volume will increase. Herbs that increase the cardiac output and/or stimulate peripheral circulation include: