What is the primary function of the kidneys?

The primary function of the kidneys is to filter waste from the blood and produce urine. This process is essential for maintaining the body's overall homeostasis. The kidneys remove excess substances, toxins, and metabolic waste products such as urea and creatinine from the bloodstream. As blood flows through the kidneys, they perform filtration, reabsorption, and secretion, which leads to the formation of urine that is ultimately excreted from the body. This function not only aids in the elimination of waste but also helps in regulating the levels of various substances in the blood, thus contributing to overall fluid and electrolyte balance, yet the main role remains the filtration and urine production.

While the kidneys also have secondary functions, such as regulating blood pressure through the renin-angiotensin-aldosterone system and producing hormones like erythropoietin for red blood cell production, these are not the primary roles. Rather, they support other physiological processes that are crucial for maintaining health. Hence, the filtering of waste and urine production is the most fundamental role of the kidneys.