Excretory System Read Kraus pages 248 to 250. List the many different?functions of the skin: There are four different functions for the skin. The skin acts as a protective organ, meaning that the skin provides defense from mechanical injury and bacterial infections. How does the liver assist with excretion of nitrogenous?waste? The liver assists with the excretion of nitrogen waste. When humans eat food containing a surplus in protein, an excess of amino acid forms. Amino acid contains nitrogen. If the protein is not used to repair the body or help it grow, it is changed in the liver into nitrogenous waste and is secreted. This is why doctors warn about the excess consumption of foods like meat, fish, and eggs.