The kidneys are essential organs responsible for blood filtration. Each kidney consists of an outer cortex rich in blood vessels and an inner medulla. Both the cortex and the medulla contain millions of nephrons, the functional units of the kidney which work to filter the blood. According to the National Space Biomedical Research Institute, the nephrons filter around 43 gallons of water daily. Each nephron consists of several small tubules, which are lined with cuboidal epithelial cells. The cuboidal cells within the tubules perform a range of functions in filtering the blood.

Ion Exchange

One major role of the cuboidal cells in the kidney tubules is ion exchange. Ions are components of salt molecules that dissolve in the water in the blood and circulate throughout the body. Table salt, sodium chloride, breaks into sodium and chlorine ions within the bloodstream. Cuboidal cells in the kidneys play a role in regulating the resorption of ions from the blood. Cuboidal cells within the kidney tubules contain proteins called ion pumps, which control the flow of sodium and potassium ions into and out of the cuboidal cells, according to the 2001 textbook "Color Textbook of Histology." The cuboidal cells then pump the ions into other parts of the kidneys for further processing, and absorb other ions from the blood. This ion exchange plays a role in regulating blood pressure.

Sugar Absorption

Another major role of cuboidal cells within the tubules of the kidneys is glucose, or blood sugar, absorption. Sugar from the blood is filtered out during blood filtration, and the glucose pools within the kidney for absorption by the cuboidal cells. The "Color Textbook of Histology" indicates that 100 percent of blood glucose is reabsorbed by the cuboidal cells. Since a failure to absorb sugar would lead to energy loss from a lack of glucose, sugar reabsorption is required to drive the metabolism.

In some cases, such as in poorly controlled diabetes, the extremely high amounts of sugar in the blood put stress on the kidneys so they cannot fully absorb the sugar. As a result, some blood sugar leaks into the urine and helps contribute to the excessive urination experienced by some diabetics.

Water Retention

Another major role of the cuboidal cells in the kidney tubules is water retention and the control over fluid loss. Fluids within the body have a regulated osmotic balance, a regulated amount of salts, sugars or proteins dissolved in the blood at any given time. When salt or sugar exchange occurs within the cuboidal cells in the kidneys, it is coupled with water exchange to maintain the osmotic balance. If if the kidneys absorb too much or too little salt or sugar, the water balance within the body is also thrown off balance.

If there is too much salt in the body, and the cuboidal cells within the kidneys must reabsorb abnormally high levels of sugar, these cells must also absorb abnormally high levels of water to maintain the osmotic balance. The result is an increased in fluid levels in the body, causing water retention and bloating. If glucose is lost to the urine, which may occur in diabetes, then water also accumulates in the urine, leading to increased urination and dehydration. A chronic imbalance in water retention can put stress on the kidneys and lead to other health problems.