What is the primary role of the kidneys in the body?

The primary role of the kidneys is to filter waste products from the blood. This function is essential for maintaining the body's homeostasis by eliminating toxins and excess substances through urine production. The kidneys receive blood from the renal arteries, where they filter out waste products such as urea, creatinine, and excess ions like sodium and potassium. This filtration process helps to control the body's fluid balance, electrolyte levels, and acid-base balance.

In addition to waste filtration, the kidneys play a crucial role in regulating blood pressure, stimulating red blood cell production through erythropoietin, and maintaining calcium balance through vitamin D activation. Given the complexity and importance of these functions, the kidneys are vital organs that contribute significantly to overall health.

The other options, while related to various body functions, do not accurately represent the primary role of the kidneys. Hormone production is indeed a function of other organs and, while the kidneys do produce certain hormones, their main contribution is waste filtration. Protein synthesis is primarily the responsibility of the liver and other tissues, and circulation of oxygenated blood is managed by the cardiovascular system, particularly the heart and blood vessels.