EXCRETION IN ANIMALS (IN VERTEBRATES AND INVERTEBRATES) - BIOLOGY - 2025

The excretion in animals is an organic process by which these living beings eliminate metabolic wastes, products of different vital functions performed in your body.

Embryonic development shows that the excretory system of vertebrate animals originates in a series of tubules, which open on the inside of the body. Later Bowman's capsule develops, forming a diverticulum of each tubule. These ducts and tubules can fulfill a reproductive function, which is why they are often called the urogenital system.

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In invertebrate animals, the excretory organs have a very diverse evolutionary origin. This does not imply that each species has developed a particular excretory system.

The term excretory should not be confused with that of secretion. The glands secrete substances so that they fulfill a specific function in the body.

While urine, as a chemical compound excreted, is composed of toxic elements, such as ammonia, which if kept in the body would significantly affect its functioning.

In vertebrates (process)

Source: By BruceBlaus, from Wikimedia Commons In vertebrate animals, excretion takes place primarily in the lungs, kidneys, and skin.

-Mammals

The kidney is a compact organ where two regions are distinguished: the area of ​​the cortex and the medullary. The functional unit is the nephron, a tubular structure made up of four regions. It arises in the cortex as a vesicle, called Bowman's capsule.

This capsule continues with the proximal convoluted tubule and loop of Henle. This enters the medulla and exits the cortex again, forming the distal convoluted tubule. A collecting tubule, which is open in several nephrons, forms the pelvis of the kidney. From this, the ureter connects to the urinary bladder.

Urine leaves the body through the urethra, which is connected to the animal's bladder.

Urine production

Urine originates in the nephrons, through three processes: filtration, reabsorption and excretion.

Glomerular filtration

It takes place in the nephrons, specifically in the glomeruli. When the blood reaches these, it is subjected to strong pressure that allows water, glucose, vitamins, amino acids, sodium, urea, among other salts to be extracted from it.

The liquid originating from this process is equivalent to about 20% of the total volume of the animal's plasma.

Tubular reabsorption

Because the body cannot lose all the amount of fluid that was initially filtered, the reabsorption process occurs. There, the filtered fluid passes to the proximal convoluted tubule, where glucose, amino acids, potassium, among others, are reabsorbed.

This process continues to occur in the loop of Henle and in the part known as the distal convoluted tubule. These tubules are impervious to urea.

Excretion

In the distal convoluted tubule some substances, such as potassium and hydrogen, are excreted into the bladder. When it is full, the nervous system receives a signal, thus activating the process of urine output outside the animal's body.

-Birds and reptiles

The main product excreted in these animals is uric acid. The daily urine volume of birds is low, because their glomeruli are small. The urine produced in birds does not go to the bladder, but to the cloaca. This is the terminal portion of the food system.

Their kidneys do not have the ability to produce concentrated urine, which is compensated by the salt glands that birds have. These organs are the modified lacrimal glands, which are responsible for eliminating excess salt that may have in your body.

For this, the animals produce a substance with high levels of sodium chloride, excreted through the nostrils.

-Amphibians

These animals excrete nitrogen in the form of urea. On land, they could lose water quickly through evaporation. This is because their skin is permeable to water.

Urine is stored in the bladder, which provides a reserve of water that can be used by the animal on land, if needed.

-Fishes

Water can enter the fish's body by osmosis. The salts that compose it are filtered. To compensate for this, the large glomeruli in the kidney produce copious amounts of urine, equivalent to 20% of your body.

Nitrogen is excreted in the form of ammonia. This exits in two ways: through the urine and through the gills. If there is any excess salt in the body, the body eliminates them through the rectal gland.

In invertebrates (process)

Source: By Putaringonit, from Wikimedia Commons

The contractile vacuoles of protozoa

Some protozoa have an internal sac-like organelle. This vacuole is enlarged thanks to the accumulation of liquid that is discharged to the outside.

The nephrids of annelids, nemerteans, flatworms and rotifers

Annelids have two nephridia in each segment. The nephridium is shaped like a very long and very thin tubule. One of its ends opens towards the body and the other towards the outside. However, in some annelids this culminates in a cellular grouping known as solenocytes.

Body fluid enters the nephridium through the nephridiostom. During their journey through the tubule, the salts are reabsorbed. Finally, the urine passes into an enlarged portion of the tubule, known as the bladder, and from there to the outside through the nephridiopore.

The kidney glands of mollusks

This is a wide tube opening, which runs from the pericardium, which surrounds the heart, to the outside of the animal. As aquatic animals, mollusks excrete nitrogen as ammonia.

Urine is formed through a filtration process, which can occur from the walls of the heart to the pericardium. It could also occur from the glandular blood vessels.

The coxal glands of aquatic arthropods

These are a pair of tubular organs that open basally. In higher species this organ opens at the base of the antennae. Each of these is made up of a bent tubule, forming a small sac, called a coelomic sac.

This opens into a much larger region called the labyrinth, culminating in the bladder. In crayfish there is no bladder, but a channel, which is a narrow area of ​​the tubule. Urine is formed by blood filtration, through the coelomic sac

Malpighian tubules of insects

These tubules can vary in number, and species can be found with two of these and others have more than 100. These end in the body cavity, opening to the alimentary canal. Because of this, urine passes through the rectum before leaving the body.

In that part of the intestine, the composition of this substance that will be excreted changes substantially. It becomes more acidic and the urate turns into insoluble uric acid. Water is reabsorbed, together with other soluble products of digestion.

The excretory system in insects is regulated by hormones. In Rhodnius, the distention of the body after digestion causes some cells of the nervous system to release a hormone. These act on the Malpighian tubules, facilitating the flow of primary urine

References

1. Wikipedia (2018).Excretion. Recovered from en.wikipedia.org.
2. Encyclopedia Britannica (2918). Excretion. Recovered from britannica.com.
3. László Rosivall (2018). Comparative aspects of renal excretion in vertebrates. Department of Pathophysiology, Faculty of Medicine, Semmelweis University, Hungary. Recovered from eolss.net.
4. Dirk Weihrauch, Garett JP Allen (2018). Ammonia excretion in aquatic invertebrates: new insights and questions. Journal of Experimental Biology. Recovered from jeb.biologists.org.
5. Gaurab Karki (2017). Excretion in Animals; significance of excretion, modes and types of excretory wastes in different animals. Online biology notes. Recovered from onlinebiologynotes.com.