- The kidneys play a variety of important homeostatic functions largely through their capacity to regulate the concentration of small solutes within the blood. These small solutes include metabolic waste products and xenobiotics which the kidneys excrete within the urine. Additionally, the kidneys are responsible for the relatively stable concentrations of key electrolytes within the blood by matching their excretion in the urine with their dietary intake. The kidneys also play a key role in maintaining long term arterial blood pressure, blood osmolarity, and blood volume by regulating the amount of salt and water excretion in the urine. Finally, the kidneys assist in maintaining the pH of the extracellular fluid by regulating excretion and synthesis of acidic and basic molecules. Beyond their role in regulating small solutes, the kidneys also carry out a number of important endocrine functions which are reviewed elsewhere. In this section we focus on the kidneys' capacity to excrete waste products, maintain stable concentrations of key electrolytes, and regulate blood osmolarity and volume.