What is the primary function of the kidneys?

The primary function of the kidneys is to filter blood and regulate water balance. This vital role is accomplished through the intricate structure of the nephrons, the functional units of the kidneys, which remove waste products and excess substances from the bloodstream.

In addition to filtering waste such as urea, creatinine, and toxins, the kidneys also play a crucial role in maintaining homeostasis by regulating the body's fluid balance and electrolyte levels. They adjust the amount of water reabsorbed during the filtration process based on the body's needs, which is essential for maintaining proper blood pressure, volume, and overall fluid balance.

While the kidneys also produce hormones like erythropoietin (which stimulates red blood cell production) and renin (which helps regulate blood pressure), their primary and most recognized function remains the filtration of blood and the regulation of water and electrolytes. The other options—storage of bile and digestion of proteins—are not functions associated with the kidneys but are primarily the roles of the liver and digestive system, respectively.