Modes of nitrogen excretion

Everyday we need to egest waste from our body in the form of poop the same we need to excrete some hormones, vitamins from our body as well, that goes in the form of urine. There is a difference between egestion and excretion. I have a different post for it, but today we are only going to talk about different modes of excretion.

Related article: Difference between excretion and egestion

What is excretion ?

As I said in the above paragraph that we need to remove certain waste from the body apart from just pooping that is known as the metabolic waste which needs to be excreted in order to maintain the composition of the body fluids and tissues.

Excretion means to eliminate or to remove these waste from your body which might also cause harm if not excreted. How are these waste products formed in the first place to be eliminated ? Well, these waste products are formed due to breakdown of glucose i.e. called as catabolism, amino acids, glycerol and fatty acids.

There are many wastes apart from acids like nitrogenous waste, inorganic salts, hormones and vitamins. Such wastes are known as non-volatile wastes. The organs that excrete such wastes is known as the excretory organ.

Modes of nitrogen excretion:

There are various modes by which nitrogenous waste can be excreted from the body. Here are the types of nitrogen excretion which takes please in living organisms.

1. Ammonotelism:

The excretory product which is in the form of ammonia is ammonotelism and the animals that excrete ammonia are known as ammonotelic animals. Ammonia is highly soluble in water that makes it highly toxic, hence it cannot be kept in the body for a longer time and needs to be excreted as it is formed.

To eliminate ammonia from the body a lot water is required i.e. about 300-500ml of water is needed to eliminate one gram of ammonia from the body. Due to the need of a lot of water you can see ammonotelism in aquatic invertebrates, bony fishes, tadpole larva of frog.

Ammonia is excreted through skin, gills and kidney. It is excreted by simple diffusion so its concentration in the body is kept low.

2. Ureotelism:

The excretory product which is in the form of urea is ureotelism and the animals that excrete urea are known as uricotelic animals. Examples of ureotelic animal are, terrestrial animals like frog, turtles, toads, mammals and marine fishes

As you know some animals can't excrete ammonia due to the need of high amount of water. This ammonia is converted to a less toxic form which is urea. Ammonia combines with carbon dioxide to form urea. It is formed in the liver by ornithine cycle

This does not require more water to be excreted so moderate amount of water is required i.e. 50ml of water for one gram of urea. Urea can be stored in the body for sometime before excretion and it also excreted at a lower rate. It is soluble in water and stored in a dissolved form called urine.

3. Uricotelism: 

You might now get confused between ureotelism and uricotelism but these both are different. Ureotelism is excretion of urea and uricotelism is excretion of uric acid which is less toxic, harmless and insoluble in water that means it can be stored for a longer time in he body. 

Uric acid is excreted by animals like land snails, reptiles, birds, terrestrial insects, lizards and snakes. As uric acid is insoluble in water it is very beneficial for the desert animals. In humans and other mammals, uric acid is formed by the breakdown of purine and pryrimidine nitrogen bases of nucleic acid. 

To synthesize uric acid from ammonia requires a lot of energy. It takes place in the liver by inosinic pathway. Uric acid is eliminated in the form of solid pallets or a thick paste which does not require a lot of water but a little amount of water is enough. About 10ml of water is required for one gram of uric acid. 

4. Aminotelism: 

The excretory product in the form of amino acid is aminotelism. Again, it will be difficult and confusing between ammonotelism and aminotelism but the trick here is that ammonotelism has two "m" whereas in aminotelism there is one "m" and amino instantly reminds you of amino acid.

Amino acid is excreted by certain invertebrates like some mollusces and some echinoderms and the animals that excrete amino acid are known as amnotelic animals.

5. Guanotelism:

The excretory product in the form of guanine is guanotelism. Guanine is excreted by spiders and the animals that excrete guanine are known as guanotelic animals. 

Why is it important to excrete nitrogenous wastes ?

There is not much to think about this but a very simple and easy reason that why should nitrogenous waste be excreted from the body. 

If you read the above paragraphs you would know that the by-products that is created during the metabolic activities can cause harm to the body tissues or maybe organs, that is why it has to be excreted out as soon as it is formed. 

Nitrogenous waste tends to form ammonia, but due to the need of high amount of water to excrete ammonia this is not possible in terrestrial animals like humans, birds, etc.. To excrete ammonia it has to be converted to a less toxic form which is either urea or uric acid. 

The more ammonia is stored in the body, the pH of the body fluids will start to rise damaging the surrounding tissues or organs.

Some other nitrogenous wastes:

Apart from the nitrogenous wastes mentioned above here are some other nitrogenous wastes you might need to know maybe for your general knowledge. The following are some other nitrogenous waste:

1. Creatine and creatinine:

Our muscle cells have molecules of creatine phosphate in it, which are highly energetic molecule that stores the bioenergy like ATP

It is excreted from the body through the urine but for that creatine has to be converted to creatinine first. 

2. Trim Ethylamine Oxide (TMO):

This type of nitrogenous waste is excreted by marine Teleost fishes which is the largest infraclass in the class of Actinopterygii. 

They excrete a large amount of trim ethylamine oxide as their nitrogen waste product. Some amount of this is also stored in their body for osmoregulation. 

3. Ornithuric acid:

This type of nitrogenous waste is excreted in small amounts by birds. Ornithuric acid is formed by combining benzoic acid with amino acid ornithine. 

4. Hippuric acid:

This a less toxic form of nitrogen waste which is formed by combining benzoic acid and glycine. 

5. Bilirubin and biliverdin:

These are the bile pigments which are formed in the liver by breakdown of haemoglobin of used RBC's which is excreted though bile. 

In jaundice, our skin gets yellow is due to the increase in the level of bilirubin in the blood. 

6. Allantoin:

Allantoin is formed by oxidation reaction of uric acid catalyzed by the enzyme uricase. Higher primates including man do not have the enzyme uricase. This type of nitrogen waste is excreted by the embryos of the amniotes. 

According to Google, "Amniotes are a clade of tetrapod vertebrates comprising he reptiles, birds and mammals". When the fetus is in it's early stage the excretory matter is stored in the form of allantois.

Human excretory system:

The human excretory system consists of the following parts:

1. A pair of kidneys:

The kidneys are dark red and bean shaped. They are about 10 cm long, 5 cm wide and 4 cm thick in size. The kidneys are placed just under the diaphragm in the abdominal cavity, where it is attached to the dorsal body wall. 


The right kidney is slightly positioned lower than the left kidney. It is convex and on the lateral margin. The normal weight of the kidneys in healthy adult males is 150 gms and in healthy adult females it is 135 gms. Our kidneys have ten lakh structural and functional units called the nephrons or uriniferous tubules.

LS of kidney

The LS of kidney shows a medial notch called the hilus from where the blood vessels, nerves and ureters are attached which leaves the hilus. The ureters leave with urine. The kidneys are covered by semi-liquid fatty tissue which is called adipose capsule and the outer covering of this capsule is made up of tough fibrous connective tissue called renal fascia.

The LS shows two regions, the outer renal cortex and the inner renal medulla. Kidney is also called as retro-peritoneal as it has peritoneal covering font he anterior surface that means in front of it.

2. A pair of ureters:

As I said in the above paragraph the ureters are a pair of narrow ducts that come out from the hilus carrying urine which opens up in the bladder by lateral angle


Each ureter is about 40 cm long. The function of ureters is to bring down the urine from the kidneys to the urinary bladder by peristalic contractions. The ureters are composed of transitional epithelium.

3. A single unpaired urinary bladder:

The urinary bladder lies in the pelvic region of the body. It is a pear shaped, muscular and a sac-like structure that stores the urine temporarily. The bladder can hold up to 500ml to 1L of urine. It seems to be absent in the birds. 

 Urinary bladder
Urinary bladder

The inner lining of the bladder is composed of transitional epithelium just like the ureters. The muscular layer is well developed. It is called the detrusor muscle which consists of three layers. The inner and outer layers of longitudinal fibres and middle layer of circular fibre.

There are valves present in the bladder that prevent the backwards flow of the urine. The opening of the bladder is guarded by sphincter muscle which is under voluntary control called urethral sphincter which helps in micturation (urination). 

4. Urethra in males and vestibules in females:

The urethra is a canal like muscular structure that comes out from the neck of the bladder and leads outside. Not like urethra is not present in the females, it is just that in females it is shorter compared to males as it does not have to pass through a penis instead it opens in the vestibule of the vagina. 

Urethra in males and vestibule in females
Urethra in males and vestibule in females

In females the length of the urethra is 4 cm and in males the length of the urethra is 20 cm. In females, the only function is to carry urine from the urinary bladder, but in males the urethra carries urine as well as semen at the time of ejaculation. 

The urethra in females opens by urethral orific in the vestibules of the vagina by vaginal aperture and in males it opens at the tip of the penis by urinogenital aperture

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