The organ responsible for filtering blood in the human body is the kidney. The kidneys are crucial in maintaining homeostasis by regulating the composition of blood plasma. They filter out waste products, excess salts, and toxins from the blood, excreting them as urine. Additionally, the kidneys help regulate blood pressure, electrolyte balance, and the production of red blood cells through the secretion of hormones like erythropoietin. Each healthy adult typically has two kidneys located on either side of the spine, essential for various bodily functions.