Key Stage 3 (KS3) Biology Practice Test

The kidneys are primarily responsible for filtering blood and eliminating waste products from the body. They perform a crucial role in homeostasis by regulating the balance of electrolytes, maintaining fluid balance, and removing metabolic waste such as urea and creatinine through the formation of urine. As blood passes through the kidneys, waste materials and excess substances are filtered out, while essential nutrients and molecules are reabsorbed into the bloodstream as needed. This process is vital for maintaining the body's overall health and preventing the accumulation of harmful substances.

The other organs listed have different functions: the heart is responsible for pumping blood throughout the body, the stomach aids in digestion by breaking down food, and the small intestine absorbs nutrients from digested food. None of these play a direct role in waste elimination from the blood like the kidneys do.