The Excretory System

by Karthik Gurumurthy

All animals produce waste. Waste is all the material that is not useful to the body, and needs to be removed.

Carbon dioxide made by cells is waste. Food that could not be digested is waste. Even some of the chemicals produced by the body becomes waste once they are no longer needed. Imagine if you never threw away your trash at home.  It would pile up, get in the way, and start to smell. If an animal could not get rid of its body’s waste, its body would not function properly.

All animals have excretory systems that find, gather, and remove waste from their bodies. Waste materials leave cells through the cell membrane. Then special organs and structures collect and remove it. Solid waste, called feces, is usually a combination of undigested food, old cells, bacteria, water and leftover chemicals from the digestion process. Animals with two-way digestive systems get rid of feces through the same hole through which they take in food. Animals with one-way digestive systems get rid of waste through the anus, located at the opposite end of their body from the mouth.

Waste gases are produced from respiration and digestion. Carbon dioxide leaves the body through the breathing organs and structures- lungs, gills, spiracles, and pores. Gases from digestion leave through the mouth (burping) and anus (farting).

Waste also leaves the body through the skin of some animals. When animals sweat, the moisture they produce is not just water, but also contains salt and liquid waste from the body. The liquid waste, called urea, is made in the liver and transported in the blood. Sweating serves two purposes in many animals. It helps to control body temperature and is a way for waste to leave the body.