The organ responsible for filtering blood in humans is the kidney. Humans have two kidneys, located on either side of the spine, which play a critical role in the body's renal system. The kidneys filter waste products and excess substances from the blood, producing urine in the process. They help maintain the body's balance of electrolytes, regulate blood pressure, and ensure proper hydration levels. By removing toxins and waste, the kidneys are essential for overall health. Any impairment in kidney function can lead to significant health issues, underscoring their importance in maintaining bodily functions.