The organ primarily responsible for filtering blood is the kidneys. They play a crucial role in maintaining homeostasis by regulating the volume and composition of bodily fluids. Kidneys filter out waste products and excess substances from the blood, which are then excreted as urine. Additionally, they help manage electrolyte balance, blood pressure, and the production of red blood cells by releasing hormones. Each kidney contains about a million filtering units called nephrons, which perform these vital functions to keep the body in optimal health.