The elimination by an organism of waste products that result from metabolic processes. In plants, waste is minimal and is eliminated primarily by diffusion to the outside environment. Animals have specific organs of excretion. In vertebrates, the kidney filters blood, conserving water and producing urea and other waste products in the form of urine. The urine is then passed through the ureters to the bladder and discharged through the urethra. The skin and lungs, which eliminate carbon dioxide, are also excretory organs.