What Is Inside a Kidney Cyst?

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Kidney cysts typically do not impair your kidney function and usually do not have any symptoms. Cysts can form anywhere in your kidneys, according to the National Institute of Diabetes and Digestive and Kidney Diseases.

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There are two types of kidney cysts: simple and complex. These cysts are not associated with any genetic or inherited diseases like polycystic kidney disease.

Simple Cysts

Simple kidney cysts are typically filled with either clear or yellow fluid. The sacs, or walls of the cysts, are extremely thin.


A subcategory of simple kidney cysts, a hyperdense cyst, has blood within the walls of the sac, according to the Kidney Cancer Institute.

Complex Cysts

Complex kidney cysts are distinct because of irregularities. These irregularities can appear from their odd outer shape, thick sac walls or enhancements like a blood supply to the cysts, according to Kidney Cancer Institute.


Complex cysts can be filled with calcium, which can be relatively thick. Other substances that can fill the sacs are tissue or solid material.

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