Excretion in Animals: Method, Types & Excretory Organs

Excretion is referred to as the process that contributes towards the removal of waste from the body. Maintenance of a steady-state or constant internal environment, called homeostasis. Excretion plays an important role in maintaining a healthy body. Animals accumulate uric acid, carbon dioxide, ammonia, urea, water, and ions like K+, Cl-, Na+, sulphate, phosphate, etc., either by metabolic activities or by other means like excess ingestion. Animals must regulate water and salt balance in the body and eliminate nitrogenous wastes. It is known to us that all the functions within the body are interrelated to each other. This article will focus on discussion the excretory system within animals.

What is Excretion?

Elimination of nitrogenous metabolites and other metabolic waste products from the body is known as excretion.
Nitrogenous wastes are mostly removed from the body in the form of excretory products.

Excretion of Nitrogenous Wastes in Animals

1. Ammonotelism: The elimination of ammonia as the chief nitrogenous waste material is termed ammonotelism. Ammonia is formed by the oxidative deamination of amino acids. Ammonia is extremely poisonous and readily dissoluble in water. Hence it should be removed from the body quickly and in the form of a very dilute solution. In many invertebrates, ammonia is excreted through the surface of the body. In fishes, most of the ammonia is lost as NH₂+ across the epithelium of the gills, and kidneys excrete only minor amounts of nitrogenous wastes.

Examples of Ammonotelic Animals: Sycon, Hydra, Fasciola, Taenia, Nereis, Hirundinaria, Ascaris, Pila, Labeo, prawn, crocodiles, etc.

Fig: Hydra

2. Ureotelism: The elimination of urea as the chief nitrogenous waste material is termed ureotelism. Urea is less toxic than ammonia. Although ammonia excretion works fine in aquatic situations, it is unsuitable for terrestrial situations. As terrestrial animals cannot afford to spend more water for excretion, they excrete less toxic nitrogenous wastes like urea and uric acid. Ammonia produced by metabolism is converted into urea by the ornithine cycle in the liver of these animals and released into the blood, which is filtered and excreted out by the kidneys.

Examples of Ureotelic Animals: Since water intake is limited, many terrestrial amphibians, earthworms, mammals, and marine fishes mainly excrete urea and are called ureotelic animals. 

Fig: Earthworm

3. Uricotelism: The elimination of uric acid as the principal nitrogenous waste, in the form of pellets or paste with a minimum loss of water. Bird droppings, or guano, are a mixture of white uric acid and brown feces. Uric acid is relatively non-toxic and does not readily dissolve in water. It, therefore, can be excreted as a semisolid paste with very little water loss.

Examples of Uricotelic Animals: Lizards, snakes, terrestrial insects, birds, etc.

Fig: Lizard

4. Aminotelism: The elimination of amino acids as the chief nitrogenous waste material is termed aminotelism.

Examples of Aminotelic Animals: Echinoderms and molluscs are the best examples of aminotelic.

Fig: Molluscs

5. Guanotelism: Spiders and Scorpio convert ammonia into guanine. It is excreted in the form of crystals. It is also an adaptation to check water loss.

Examples of Guanotelic Animals: Spiders and Scorpio.

Fig: Scorpio

Other Nitrogenous Wastes

1. Trimethylamine Oxide (TMO): TMO is found in marine bony fishes (teleosts), marine molluscs, and marine crustaceans. It has a fishy smell. It is non-toxic and produced from ammonia. In marine cartilaginous fishes, it protects the body proteins from the damaging effect of urea.
2. Allantoin: Uric acid formed during purine metabolism is converted into allantoin with the help of the uricase enzyme in mammals (except man, apes).
3. Creatine: Creatine is produced from glycine and arginine in the liver and kidney. It can be phosphorylated by creatine kinase to form phosphocreatine. In normal urine, creatine is absent. But in newborn infants, pregnant and lactating females, urine contains creatine.
4. Creatinine: Creatine is mainly stored in muscles, but about 1.5-2% of the body’s creatine is converted to creatinine daily. It is excreted along with urine. Increased level of creatinine in blood indicates kidney damage.
5. Hippuric Acid: In mammals, dietary benzoic acid is removed by its combination with glycine to form hippuric acid.
6. Ornithuric Acid: In birds, benzoic acid combines with ornithine to form ornithuric acid.

Excretory Organs in Animals

1. Freshwater protozoans have contractile vacuoles for excretion and osmoregulation.
2. Renette cells and glands with canals help in excretion through excretory pores in nematodes.
3. Chloragogen cells of earthworms are useful for excretion.
4. The organ of Bojanus is the kidney (metanephridium) of molluscs.
5. Keber’s organ (pericardial gland) is the excretory organ in some molluscs, for example, Unio.
6. The proboscis gland is the excretory structure of hemichordates: for example- Balanoglossus.
7. The neural gland is the excretory structure of Urochordates. Example- Herdmania, Ascidia.
8. Hatschek’s nephridium is an excretory organ present in Branchiostoma.
9. Mesonephric kidneys are the functional adult kidneys in cyclostomes, fishes, and amphibians.
10. Metanephric kidneys are the functional adult kidneys in reptiles, birds, and mammals (amniotes).

Importance of Excretion

1. Excretory products like urea, ammonia are toxic and poisons the living cells present inside the body; hence, excretion is important to maintain a healthy body.
2. Excretion is important to regulate blood concentration.
3. Removal of sweat through the skin maintains the body temperature cool.

Difference Between Excretion in Plants and Animals

Summary

All animals eat various foods to get energy, and their by-products of metabolism should be removed from the body. The process of removal of waste matter from the body is called excretion. There are various modes of excretion in animals. Ammonia, urea, uric acid, amino acids, guanine, etc., are the five major excretory products of animals. There are various excretory organs that widely vary among animal groups. However, they are generally built on a complex network of tubules that provide a large surface area for the exchange of water and solutes, including nitrogenous wastes. In most invertebrates, these structures are simple tubular forms whereas vertebrates have complex tubular organs called kidneys.

Frequently Asked Questions (FAQs) on Excretion in Animals

Frequently asked questions related to excretion in animals is listed as follows:

Q.1. Why is excretion important for animals and plants?
Ans: Excretion is very important for both animals and plants because of the removal of waste from the body by absorbing necessary nutrients from the food consumed to keep the body clean and healthy. Plants need to excrete oxygen and carbon dioxide, whereas animals need to excrete urea, uric acid, ammonia, etc., which are highly toxic if retains in the body.

Q.2. What is excretion with examples?
Ans: The process of expelling the amount of waste matter from our body is called excretion. Urine is an example of excretion.

Q.3. What are the waste products of animals?
Ans: Urea, uric acid, amino acid, ammonia, etc., are the waste products of animals.

Q.4. What are the organs of excretion?
Ans: Excretion takes place through various organs in different animals. Skin, lungs, kidneys, etc., are the organs of excretion.

Q.5. Is Dung an animal?
Ans: Dung is not an animal. It is a waste matter excreted by cows, chickens, etc.

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