Amphibians: Excretory System Chapter 27 / 4/14/15


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Like all animals, amphibians need a way to remove waste materials from their body so it doesn't build up and become toxic. The amphibian excretory system is similar to other vertebrates, but read this lesson to learn more.
The Purpose of an Excretory System

Amphibians are vertebrates (animals with a backbone) that begin life as aquatic organisms. When they undergo metamorphosis they develop lungs, and mature amphibians may live in water, on land, or a combination of the two.

Like any vertebrate, amphibians have an excretory system. What is the purpose of an excretory system? Well, the body produces waste products through cellular respiration, and these wastes need to be excreted from the body. If wastes aren't removed from an organism, they will become toxic, so the body works to efficiently eliminate it. Eliminating waste also allows the body to maintain the proper balance of salts and other nutrients.

Excretory System in Amphibians

The most common type of waste produced through metabolic processes is nitrogenous wastes. Nitrogenous wastes form from the natural breakdown of proteins in the body. Amphibians have two kidneys (just like humans), and those kidneys filter wastes out of the blood and combine them with water to form urine. Urine then travels from the kidneys via the ureters to the bladder, and then out through the cloaca. The cloaca (or vent) is an opening used for the excretory, intestinal, and reproductive tracts of amphibians. Urine leaves the body through the cloaca when the bladder is full.

Nitrogenous wastes can take one of three forms: ammonia, uric acid, and urea. Due to different life stages or strategies, different groups of amphibians produce different forms. Let's go over these waste forms and how they're dealt with by different groups.


Ammonia is water soluble and is the most toxic form of nitrogenous waste. It becomes harmful if not diluted quickly. Ammonia changes the pH in cells to rise to dangerous alkaline levels if too much of it remains in the body.

Juveniles (like frog and toad tadpoles) and aquatic species of amphibians excrete their nitrogenous wastes as ammonium because they aren't worried about conserving water (because they live in it!) and the ammonia is quickly taken away by the current.


With a little energy investment, ammonia can be turn into urea by enzymes in the liver. Urea is a less toxic form of nitrogenous waste. It requires less water to excrete, doesn't affect the pH the same was ammonia can, and so is safer to have in the body. Terrestrial amphibians have less access to water than the aquatic ones, so the majority of them produce urea.

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