GILLS: CHARACTERISTICS, FUNCTIONS, TYPES AND IMPORTANCE - BIOLOGY - 2025

The gills or gills are the respiratory organs of aquatic animals, they have the function of carrying out the exchange of oxygen between the individual and the environment. They appear from very simple forms in invertebrates, to complex structures evolved in vertebrates, made up of thousands of specialized lamellae located inside a gill cavity ventilated by a continuous flow of water.

Cells demand energy to function, this energy is obtained from the breakdown of sugars and other substances in the metabolic process called cellular respiration. In most species, oxygen in the air is used for energy and carbon dioxide is expelled as waste.

Branchial arches of a European pike (Esox lucius). By User: Uwe Gille, from Wikimedia Commons The way in which organisms carry out the exchange of gases with their environment is influenced by both the shape of the body and the environment where it lives.

Aquatic environments have less oxygen than terrestrial environments and the diffusion of oxygen is slower than in air. The amount of dissolved oxygen in water decreases as the temperature increases and the current decreases.

Less evolved species do not require specialized respiratory structures to fulfill their basic functions. However, in larger ones, it is vital to have more complex exchange systems, so that they can adequately cover their metabolic needs.

The gills are found in invertebrates and vertebrates, they can be thread-shaped, laminar or arborescent, endowed with numerous capillary vessels, and we also observe them internally or externally.

There are animals that live in the littoral area, such as mollusks and crabs, which are capable of actively breathing with their gills in the water and in the air, as long as they are kept moist. Unlike other aquatic organisms, which suffocate when leaving the water despite the abundance of available oxygen.

General characteristics

The amount of oxygen present in air is approximately 21%, while in water it is only 1% dissolved. This variation forced aquatic organisms to create structures such as gills, intended exclusively for the extraction of oxygen.

The gills can be so efficient that they achieve oxygen extraction rates of 80%, three times higher than that of human lungs from air.

Variety of aquatic organisms

These respiratory organs developed in a huge variety of aquatic organisms, we can find different types of gills in mollusks, worms, crustaceans, echinoderms, fish and even in reptiles in certain phases of their life cycle.

Variety of shapes

As a consequence, they vary greatly in shape, size, location and origin, resulting in specific adaptations in each species.

For the more evolved aquatic animals, the increase in size and mobility determined a higher oxygen demand. One of the solutions to this problem was to increase the area of ​​the gills.

Fish, for example, have a high number of folds that are kept separated from each other by the water. This gives them a large gas exchange surface, which allows them to reach their maximum efficiency.

Sensitive organs

The gills are very sensitive organs, susceptible to physical injury and diseases caused by parasites, bacteria and fungi. For this reason, the less evolved gills are generally considered to be external.

Injuries

In bony fish, the gills in the face of high concentrations of chemical pollutants such as heavy metals, suspended solids and other toxic substances, suffer morphological damage or injuries called edema.

These cause necrosis of the gill tissue, and in severe cases they can even cause the death of the organism due to the alteration of respiration.

Due to this characteristic, fish gills are frequently used by scientists as important biomarkers of contamination in aquatic environments.

Features

The main function of the gills, for both invertebrate and vertebrate organisms, is to carry out the process of gas exchange of the individual with the aquatic environment.

Because the availability of oxygen is lower in water, aquatic animals must work harder to capture a certain volume of oxygen, which represents an interesting situation, since it means that much of the oxygen obtained will be used in the search for new oxygen.

Man uses 1 to 2% of their metabolism when at rest to ventilate the lungs, whereas fish at rest require approximately 10 to 20% to ventilate the gills.

The gills can also develop secondary functions in certain species, for example, in some mollusks these were modified to contribute to the capture of food, since they are organs that continuously filter water.

In different crustaceans and fish, they also carry out the osmotic regulation of the concentration of substances available in the environment in relation to the body, finding cases to which they are responsible for excreting toxic elements.

In each type of aquatic organism, the gills have a particular function, which depends on the degree of evolution and the complexity of the respiratory system.

How do they work?

In general, the gills function as filters that trap the oxygen O ₂ found in the water, essential to fulfill its vital functions, and expel the waste carbon dioxide CO ₂ that is present in the body.

To achieve this filtration a constant flow of water is required, which can be produced by movements of the external gills in worms, by movements of the individual as carried out by sharks, or by the pumping of the seals in bony fish.

Gas exchange occurs through contact diffusion between water and the blood fluid contained in the gills.

The most efficient system is called counter-current flow, where the blood flowing through the branchial capillaries comes into contact with oxygen-rich water. A concentration gradient is produced that allows oxygen to enter through the gill plates and diffuse into the bloodstream, at the same time as carbon dioxide diffuses outside.

If the flow of water and blood were in the same direction, the same oxygen uptake rates would not be achieved, because the concentrations of this gas would quickly equalize along the branchial membranes.

Types (external and internal)

The gills can appear in the external or internal part of the organism. This differentiation is mainly a consequence of the degree of evolution, the type of habitat where it develops and the particular characteristics of each species.

External gills

The external gills are observed mainly in little evolved species of invertebrates, and temporarily in the first stages of development of reptiles, since they lose them after undergoing metamorphosis.

Mexican axolotl (Ambystoma mexicanum). By Alexander Baranov from Montpellier, France (.), Via Wikimedia Commons These types of gills have certain disadvantages, firstly because they are delicate appendages, they are prone to abrasions and attract predators. In organisms that have movement, they hinder their locomotion.

Being in direct contact with the external environment, they are usually very susceptible and can be easily affected by adverse environmental factors, such as poor water quality, or by the presence of toxic substances.

If the gills are damaged, it is very likely that bacterial, parasitic or fungal infections will occur, which depending on the severity can lead to death.

Internal gills

The internal gills, because they are more efficient than the external ones, occur in larger aquatic organisms, but they have different levels of specialization depending on how evolved the species is.

These are generally located in chambers that protect them, but they need currents that allow them to have constant contact with the outside environment to comply with the exchange of gases.

The fish also developed calcareous caps called gills that serve to protect the gills, act as gates that restrict the flow of water, and also pump the water.

Importance

The gills are essential for the survival of aquatic organisms, because they fulfill an indispensable role for the growth of cells.

In addition to breathing and being an essential part of the circulatory system, they can contribute to the feeding of certain mollusks, function as excretory systems of toxic substances and be regulators of different ions in organisms as evolved as fish.

Scientific studies show that individuals who have suffered damage to the branchial respiratory system, have slower development and are smaller in size, are more prone to infection and sometimes serious injury, which can lead to death.

The gills have achieved adaptations to the most diverse habitats and environmental conditions, allowing the establishment of life in practically anoxic ecosystems.

The level of specialization of the gills is directly related to the evolutionary phase of the species, and they are definitely the most efficient way to obtain oxygen in aquatic systems.

References

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