All animals excrete liquid waste and eliminate solid waste. What is eaten and digested must be eliminated from the body. As part of the digestive system the removal of waste is is essential to maintain a healthy organism no matter how simple or complex.
The kidneys filter liquid and collect the waste, which becomes urine. This moves into the bladder where it is channeled through the urethra and out of the body. It works closely with the circulatory and endocrine systems.
The solid matter leaves the small intestines and moves to the large intestines controlled by the connecting ileocecal valve. There the moist waste enters the colon where the liquid is squeezed out of the waste and recirculates throughout the body. The solid matter then proceeds to the rectum where it leaves the body by way of the anus.
A healthy lower digestive tract is essential to prevent illness. When these systems slow the toxins stay in the body. This weakens the immune system and causes illness. One of the reasons is because the byproduct of the digestion of amino acids and protein is ammonia. It is a highly toxic chemical and must be removed from the body
Excretion occurs through several areas of the body. In dogs, the saliva acts as sweat glands do in humans. The lungs give out carbon dioxide which is toxic to the body. The liver converts toxins into harmless chemicals.
These systems work in conjunction with the environment and help to maintain the food chain for grazing animals. Earthworms process hard clay into fertile soil. Manure is an excellent fertilizer keeping soil healthy and able to feed the plants that feed the animals.
Milwaukee area residence can find more detailed information about the alimentation and excretion in animals at the Milwaukee Public Museum, the Milwaukee Public Library and at the Milwaukee County Zoo.