The organ primarily responsible for filtering blood in the human body is the kidney. Each kidney contains approximately a million functional units called nephrons, which filter waste products, excess salts, and water from the bloodstream. This filtration process forms urine, which is expelled from the body, thus maintaining homeostasis. Additionally, kidneys play a crucial role in regulating blood pressure, electrolyte balance, and red blood cell production through the secretion of hormones like erythropoietin.