The
excretory system is a passive biological system that removes excess, unnecessary materials from the body fluids of an
organism, so as to help maintain internal chemical
homeostasis and prevent damage to the body. The dual function of excretory systems is the elimination of the
waste products of
metabolism and to drain the body of used up and broken down components in a liquid and gaseous state. In humans and other
amniotes (
mammals,
birds and
reptiles) most of these substances leave the body as
urine and to some degree exhalation, mammals also expel them through
sweating.