What is the primary function of the kidneys?

The primary function of the kidneys is fluid balance regulation. The kidneys play a crucial role in maintaining homeostasis in the body by filtering blood and producing urine, which helps eliminate excess water and waste products. Through processes like filtration, reabsorption, and secretion, the kidneys effectively regulate the volume and composition of body fluids, ensuring that electrolyte levels remain balanced and that waste is properly removed from the bloodstream.

While hormone production is also a function of the kidneys, particularly in the secretion of erythropoietin to stimulate red blood cell production and renin for blood pressure regulation, fluid balance is their most fundamental and vital role. Oxygen transport is primarily carried out by the red blood cells and hemoglobin in the bloodstream, while nutrient absorption occurs predominantly in the digestive system, making those options less relevant to the primary function of the kidneys.