Excretion: Meaning and Types

For the survival, growth, and repair of organisms, they consume food, the majority of which is digested and used up by the body for energy production. The remaining undigested leftover part of food is eliminated from the body in the form of faeces through the digestive system. Nitrogenous materials (urea), pigments (bilirubin), CO2, inorganic salts, excess water, are some of the examples of metabolic waste products.

The major nitrogenous wastes excreted by animals are ammonia, urea, and uric acid which are accumulated in them through metabolic activities or other ways such as excessive intake. These must be excreted from the body because if they accumulate up in the body, they can be harmful to one’s health.

Excretion definition

The process through which metabolic wastes are removed from an organism is known as excretion.  Excretion occurs at exchange boundaries such as the plasma membrane of unicellular organisms and or excretory tubules (flame cell, nephridia, malpighian tubules, nephrons etc..) of multicellular organisms. Excretion is carried out by the kidneys, lungs, and skin in humans. The most common excretory products are 

• Ammonia
• Urea
• Uric acid
• Guanine
• Carbon-di-oxide
• Water
• Bilirubin 

Some other excretory products are:

• Trimethylamine oxide (TMAO)
• Allantoin
• Hippuric acid
• Creatine
• Creatinine 

Modes of excretion

Based on the type of excretory product, there are five modes of excretion in animals. They are

• • Ammonotelism (excretory product – Ammonia)

• Ureotelism (excretory product – Urea)

• • Uricotelism (excretory product – Uric acid)

• Aminotelism (excretory product – Amino acids)
• Guanotelism (excretory product – Guanine)

Ammonotelism

Animals that excrete ammonia as their nitrogenous waste are called ammonotelic animals and the process is called ammonotelism. Ammonia is the first metabolic waste product produced by the metabolism of proteins. 

This mode of excretion is seen in all aquatic invertebrates, many bony fishes, aquatic amphibians, and aquatic insects. Ammonia is the most toxic nitrogenous waste and requires larger quantities of water for its elimination. So, more water is lost from the body. Hence, this mode is suitable for aquatic organisms which have continuous access to water. 

Since ammonia is so easily soluble,  it is expelled as ammonium ions through diffusion through body surfaces or through gill surfaces (in fish). If not expelled, it produces ammonium hydroxide (NH4OH) due to its solubility, which has an alkaline caustic activity and can injure cells directly.

Ureotelism 

Animals that excrete urea like mammals, many terrestrial amphibians, and marine fish are called Ureotelic animals and the process of eliminating urea as the excretory product is called as Ureotelism. Humans are ureotelic organisms. 

The liver of these animals converts ammonia produced by metabolism into urea, which is then released into the bloodstream and filtered and expelled via kidneys. This excretion of urea by kidneys in mammals is called the urea cycle or ornithine cycle.

Uricotelism

Uric acid is the excretory product in some animals like reptiles, birds, land snails and insects. These animals are called uricotelic animals as their excretory product is uric acid and this process is called as Uricotelism. This uric acid is removed in the form of pellets or paste. Uric acid is least toxic with minimum water loss. The excrements produce pasty white suspensions because uric acid is not easily soluble in water.

Ammonotelism

The process of excreting amino acids is called ammonotelism and the animals which show ammonotelism are called ammonotelic animals. For example, some molluscs and echinoderms.

Guanotelism

Method of excretion of guanine which is converted from ammonia is called Guanotelism and the animals such as spiders, birds, earthworms and some reptiles are called Guanotelic animals. Guanine is water insoluble and therefore it does not require water for excretion.

Other excretory products

CO2 and Water

They are formed by the metabolism of carbohydrates and fats and are easy to remove.

They are effectively eliminated by the lungs (in the form of exhaled air), skin (in the form of sweat), and kidneys (by urinary excretion in the form of urine).

Bilirubin

Bile pigments such as bilirubin (formed by the breakdown of RBCs), drugs etc. are removed in the liver.

TMAO

TMAO is found in the tissues of marine crustaceans and fish, where it inhibits proteins from being damaged by water pressure, which can cause the animal to die.

Allantoin

It is given out by the processing of purines and pyrimidines.

Hippuric acid

In mammals, it is given out by the conversion of benzoic acid which is excreted through urine.

Creatine

It is excreted by the foetus during pregnancy.

Creatinine

It is a compound present in muscles which when present in excess is excreted in urine.