The organ primarily responsible for filtering blood in the human body is the kidneys. There are two kidneys located on either side of the spine just below the rib cage. They serve crucial functions, including the filtration of blood to remove waste products and excess substances, which are expelled as urine. The kidneys also help regulate blood pressure, maintain electrolyte balance, and control red blood cell production. Maintaining healthy kidneys is vital for overall health, as they play a critical role in the body's homeostasis.