What is the primary function of the kidneys?

The primary function of the kidneys is to remove unwanted salts and chemical waste from the blood. This crucial process is known as filtration, where the kidneys filter blood to eliminate waste products generated from metabolism and adjust the balance of electrolytes, ensuring that the body maintains homeostasis. They also play a key role in regulating fluid balance, blood pressure, and the acid-base balance of the body.

The kidneys achieve this through a complex system of nephrons, which are the functional units that filter the blood. As blood passes through these nephrons, waste products such as urea and creatinine, as well as excess salts, are selectively removed and excreted in the urine, while essential substances are reabsorbed back into the bloodstream.

Other options do not accurately reflect the primary role of the kidneys. For instance, while the kidneys can play a role in hormone regulation (such as producing erythropoietin), that function is secondary to their primary role of waste removal. Similarly, the lungs are responsible for filtering out toxins, and chemical energy production is primarily the role of metabolic processes in cells, not the kidneys.