Rhubarb (Rheum rhabarbarum) is a perennial plant with large fleshy rhizomes and large leaves with long, thick petioles. It is often grown for its stalks, which are a type of vegetable often found in supermarkets worldwide.
Rhubarb contains high amounts of oxalic acid, as well as anthranoids, especially anthraquinone glycosides, aloeemodin, physcion, tannins (5-10%), pectin, phenolic, and carboxylic acids. The amount of oxalic acid in the plant varies considerably depending on the growth stage, season, and environmental conditions such as drought, cloudy days, or excessive rain.
Oxalic acid combines with calcium in the blood to form insoluble calcium oxalate crystals that may be deposited into the kidneys, resulting in acute renal failure.