<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2009) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2009) (Learn how and when to remove this template message) </Td> </Tr> <P> Metabolic wastes or excretes are substances left over from metabolic processes (such as cellular respiration) which cannot be used by the organism (they are surplus or toxic), and must therefore be excreted . This includes nitrogen compounds, water, CO, phosphates, sulfates, etc . Animals treat these compounds as excretes . Plants have chemical "machinery" which transforms some of them (primarily the nitrogen compounds) into useful substances, and it has been shown by Brian J. Ford that abscised leaves also carry wastes away from the parent plant . In this way, Ford argues that the shed leaf acts as an excretory (an organ carrying away excretory products). </P>

Where are most nitrogen compounds excreted from humans