Which organ is primarily responsible for filtering and excreting waste from the blood?

The kidneys are the primary organs responsible for filtering and excreting waste from the blood. They play a crucial role in maintaining homeostasis by regulating the body's fluid balance, electrolyte levels, and blood pressure. The kidneys filter about 50 gallons of blood daily, removing waste products such as urea, creatinine, and excess ions, which are then excreted from the body in the form of urine. This function is vital for preventing the accumulation of toxins in the body and ensuring overall health.

The liver, while important for detoxification and metabolism, primarily processes nutrients and produces bile rather than serving as the main organ for blood filtration and waste excretion. The lungs are primarily involved in gas exchange, taking in oxygen and expelling carbon dioxide, rather than filtering blood waste. The spleen functions in the filtering of blood related to immune response and recycling of iron from red blood cells, but it does not play a direct role in waste excretion. Therefore, the kidneys are rightly identified as the correct answer for their specialized function in filtering and excreting waste.