This section is from the book "Materia Medica Pharmacy, Pharmacology And Therapeutics", by W. Hale White. Also available from Amazon: Materia Medica Pharmacy, Pharmacology And Therapeutics..
Alimentary canal. - In the mouth, rhubarb increases the flow of saliva; and in the stomach, in small doses, it, like any other bitter substance, stimulates the flow of gastric juice, and the vascularity and peristaltic movements of the stomach. It is, therefore, a stomachic, and will aid digestion. In large doses it causes purgation, producing in from four to eight hours a liquid motion, colored yellow by the chrysarobin. The resinous constituents of rhubarb are said to increase the flow of bile, but certainly its cholagogue action is not sufficiently powerful to explain completely its purgative properties. It is commonly stated to exaggerate very actively intestinal peristalsis, but there is no adequate proof of this. It is liable to gripe. The purgation is followed by constipation; this is ascribed to the rheotannic acid: if so, it is probably absorbed and subsequently re-excreted into the intestines, otherwise it would be all swept away in the purging.
Kidneys. - The coloring matter is excreted in the urine, and stains it yellow. The urinary flow is slightly increased.
Rhubarb is commonly given to children as a stomachic purgative in indigestion, especially when caused by errors of diet, for it clears away any undigested food, and its stomachic and after-astringent effects are valuable. In the same way it is useful in diarrhoea due to irritation caused by undigested food; here the after-astringency is especially serviceable. A powder of powdered rhubarb and sodium bicarbonate (which conceals the taste) equal parts, with some powdered gentian, or a similar liquid medicine, forms an excellant stomachic for young children. Rhubarb should never be given alone, because of the griping it causes.