What is the primary function of the kidneys in the urinary system?

The primary function of the kidneys in the urinary system is to filter waste from the blood. This process of filtration is essential for maintaining homeostasis in the body, as it removes metabolic waste products, toxins, and excess substances from the bloodstream. The kidneys are crucial in regulating electrolyte balance, blood pressure, and fluid balance. Through the formation of urine, they ensure that waste materials are expelled from the body while conserving necessary nutrients and maintaining the body's overall chemical balance.

While other functions are important, such as storing urine, transporting urine, or releasing hormones, they do not represent the primary role of the kidneys. The bladder is responsible for storing urine, the ureters transport urine to the bladder, and hormones such as erythropoietin and renin are released as part of the kidneys' regulatory functions but are secondary to their main role in waste filtration. Thus, the kidneys' ability to filter waste is the cornerstone of their function in the urinary system.