What is the primary function of the kidneys?

The primary function of the kidneys is to filter waste products from the blood and regulate fluid balance. This is achieved through a complex system of nephrons, the functional units of the kidney, which remove waste products (like urea and creatinine) from the bloodstream and excrete them in urine. Additionally, the kidneys play a crucial role in maintaining the body's electrolyte balance and regulating the volume of blood, which is essential for fluid balance.

The kidneys also help regulate blood pressure indirectly through the renin-angiotensin-aldosterone system, but their main role is focused on the removal of toxins and fluid regulation. By filtering the blood and managing the excretion of waste, the kidneys help maintain homeostasis in the body.

While the production of red blood cells, control of blood pressure, and synthesis of hormones are important functions associated with the kidneys, they are secondary or supportive roles compared to the primary function of waste filtration and fluid balance regulation.