<P> The large intestine's main function is to transport food particles through the body and expel the indigestible parts at the other end, but it also collects waste from throughout the body . The typical brown colour of mammal waste is due to bilirubin, a breakdown product of normal heme catabolism . The lower part of the large intestine also extracts any remaining usable water and then removes solid waste . At about 10 feet long in humans, it transports the wastes through the tubes to be excreted . </P> <P> The liver detoxifies and breaks down chemicals, poisons and other toxins that enter the body . For example, the liver transforms ammonia (which is poisonous) into urea in fish, amphibians and mammals, and into uric acid in birds and reptiles . Urea is filtered by the kidney into urine or through the gills in fish and tadpoles . Uric acid is paste - like and expelled as a semi-solid waste (the "white" in bird excrements). The liver also produces bile, and the body uses bile to break down fats into usable fats and unusable waste . </P> <P> Invertebrates lack a liver, but most terrestrial groups, like insects, possesses a number of blind guts that serve the similar functions . Marine invertebrates do not need the ammonia conversion of the liver, as they can usually expel ammonia directly by diffusion through the skin . </P> <P> Sweat glands in the skin secrete a fluid waste called sweat or perspiration; however, its primary functions are temperature control and pheromone release . Therefore, its role as a part of the excretory system is minimal . Sweating also maintains the level of salt in the body . </P>

Where does diffusion occur in the excretory system