There are two types of breathing methods for fish: air and water. These differences arose and improved in the process of evolution, under the influence of various external factors. If the fish have only a water type of breathing, then this process is carried out with the help of the skin and gills. In fish with an air type, the respiratory process is carried out with the help of the gill organs, swimming bladder, intestines and through the skin. The main respiratory organs, of course, are the gills, and the rest are auxiliary. However, auxiliary or additional organs do not always play a secondary role, most often they are the most important.
Varieties of fish breathing
Cartilage and bone fish have a different structure of gill covers. So, the first have partitions in the gill slots, which ensures the opening of the gills outward with separate holes. These partitions are covered with gill lobes, which, in turn, are covered by a network of blood vessels. Such a structure of gill covers is clearly visible on the example of stingrays and sharks.
At the same time, in bony species, these partitions are reduced as unnecessary, since the gill covers are movable by themselves. Gill arches of fish serve as a support, on which gill petals are located.
Gill Functions Branchial arches
The most important function of the gills is, of course, gas exchange. With their help, oxygen is absorbed from the water, and carbon dioxide (carbon dioxide) is released into it. But few know that gills also help fish exchange water-salt substances. So, after processing, urea, ammonia is discharged into the environment, salt exchange occurs between water and the fish organism, and this primarily concerns sodium ions.
In the process of evolution and modification of subgroups of fish, the gill apparatus also changed. So, in bony fishes, gills look like scallops, in cartilaginous ones they consist of plates, and cyclostomes have a bag-like shape of gills. Depending on the structure of the respiratory apparatus, the structure as well as the function of the gill arch of the fish is different.
Structure
The gills are located on the sides of the corresponding cavities of bony fish and are protected by covers. Each gill consists of five arcs. Four branchial arches are fully formed, and one is rudimentary. From the outside, the gill arch is more convex; to the sides of the arches, gill lobes, based on cartilage rays, extend. Gill arches serve as a support for attaching the petals, which are supported by their base on their base, and the free edges diverge inward and outward at an acute angle. On the gill petals themselves are the so-called secondary plates, which are located across the petal (or petals, as they are also called). On the gills there is a huge number of petals; various fish can have from 14 to 35 per millimeter, with a height of not more than 200 microns. They are so small that their width does not reach 20 microns.
The main function of gill arches
Gill arches of vertebrates serve as a filtering mechanism with the help of gill stamens located on an arc that faces the mouth of fish. This makes it possible to hold in your mouth suspensions located in the water column and various nutrient microorganisms.
Depending on what the fish eats, the gill stamens have also changed; they are based on bone plates. So, if the fish is a predator, then its stamens are located less frequently and are lower, and in fish that eat exclusively plankton living in the water column, the gill stamens are high and denser. In those fish that are omnivorous, the stamens have a middle location between predators and planktonophages.
Circulatory system of the pulmonary circulation
The gills of the fish have a bright pink color due to the large amount of blood enriched with oxygen. This is due to the intense process of blood circulation. Blood, which must be enriched with oxygen (venous), is collected from the whole organism of the fish and enters the gill arches through the abdominal aorta. The abdominal aorta branches into two bronchial arteries, followed by the gill arterial arch, which, in turn, is divided into a large number of lobe arteries enveloping the gill lobes located along the inner edge of the cartilage rays. But this is not the limit. The petal arteries themselves are divided into a huge number of capillaries, wrapping a thick mesh around the inside and outside of the petals. The diameter of the capillaries is so small that it is equal to the size of the red blood cell itself, which carries oxygen through the blood. Thus, branchial arches serve as a support for stamens providing gas exchange.
On the other hand of the petals, all the marginal arterioles merge into a single vessel that flows into a vein that carries blood, which, in turn, passes into the bronchial and then into the spinal aorta.
If we examine in more detail the branchial arches of fish and conduct histological examination, it is best to study a longitudinal section. So not only the stamens and petals will be visible, but also the respiratory folds, which are a barrier between the aquatic environment and the blood.
These folds are lined with just one layer of the epithelium, and inside - with capillaries supported by pylar cells (supporting). The barrier of capillaries and respiratory cells is very vulnerable to environmental influences. If there are impurities of toxic substances in the water, these walls swell, exfoliate, and they thicken. This is fraught with serious consequences, since the process of gas exchange in the blood is complicated, which ultimately leads to hypoxia.
Gas exchange in fish
The production of oxygen by fish occurs through passive gas exchange. The main condition for enriching blood with oxygen is a constant flow of water in the gills, and for this it is necessary that the gill arch and the entire apparatus maintain their structure, then the function of the gill arches in the fish will not be impaired. The diffuse surface must also maintain its integrity in order to properly enrich hemoglobin with oxygen.
For passive gas exchange, the blood in the capillaries of the fish moves in the opposite direction to the blood flow in the gills. This feature contributes to the almost complete extraction of oxygen from water and its enrichment of blood. In some individuals, the enrichment rate of blood relative to the composition of oxygen in water is 80%. The flow of water through the gills occurs due to pumping it through the gill cavity, while the main function is performed by the movement of the oral apparatus and gill covers.
What determines the respiratory rate of fish?
Due to the characteristic features, you can calculate the respiratory rate of fish, which depends on the movement of the gill covers. The oxygen concentration in the water and the carbon dioxide content in the blood affect the respiratory rate of the fish. Moreover, these aquatic animals are more sensitive to a low oxygen concentration than to a large amount of carbon dioxide in the blood. The temperature of the water, pH, and many other factors also affect respiratory rate.
Fish have a specific ability to extract foreign matter from the surface of the gill arches and from their cavities. This ability is called a cough. The gill covers are periodically covered, and with the help of the backward movement of water, all suspensions located on the gills are washed out by a current of water. Such a manifestation in fish is most often observed if the water is contaminated with suspensions or toxic substances.
Additional functions of gills
In addition to the main, respiratory, gills perform osmoregulatory and excretory functions. Fish are ammoniothelial organisms, in fact, like all animals that live in water. This means that the final decomposition product of nitrogen in the body is ammonia. It is thanks to the gills that it is released from the body of fish in the form of ammonium ions, while purifying the body. In addition to oxygen, salts, low-molecular compounds, as well as a large number of inorganic ions in the water column, enter the blood through the gills as a result of passive diffusion. In addition to the gills, the absorption of these substances is carried out using special structures.
This number includes specific chloride cells that perform an osmoregulatory function. They are able to move chlorine and sodium ions, while moving in the direction opposite to the large diffusion gradient.
The movement of chlorine ions depends on the habitat of the fish. So, in freshwater individuals, monovalent ions are transferred by chloride cells from water to the blood, replacing those that were lost as a result of the functioning of the excretory system of fish. But in marine fish, the process is carried out in the opposite direction: the release occurs from the blood into the environment.
If the concentration of harmful chemical elements is markedly increased in water, then the auxiliary osmoregulation function of the gills may be impaired. As a result, not the amount of substances that is necessary, but a much higher concentration enters the bloodstream, which can adversely affect the condition of the animals. This specificity is not always negative. So, knowing such a feature of gills, you can deal with many fish diseases by introducing medications and vaccines directly into the water.
Skin breathing of various fish
Absolutely all fish have the ability to skin respiration. But only to what extent it is developed depends on a large number of factors: this is age, environmental conditions, and many others. So, if the fish lives in clean running water, the percentage of skin respiration is insignificant and amounts to only 2-10%, while the respiratory function of the embryo is carried out exclusively through the skin, as well as the vascular system of the gall sac.
Intestinal respiration
Depending on the habitat, the way the fish breathe changes. So, tropical catfish and loach fish actively breathe through the intestines. When swallowed, air enters there and, with the help of a dense network of blood vessels, enters the bloodstream. This method began to develop in fish in connection with specific environmental conditions. Water in their bodies of water, due to high temperatures, has a low oxygen concentration, which is exacerbated by turbidity and lack of flow. As a result of evolutionary transformations, fish in such reservoirs learned to survive using oxygen from the air.
Additional swim bladder function
The swim bladder is designed for hydrostatic regulation. This is its main function. However, in some fish species, the swim bladder is adapted for breathing. It is used as a reservoir for air.
Types of swim bladder structure
Depending on the anatomical structure of the swimming bladder, all fish species are divided into:
- open bubble;
- closed bubble.
The first group is the most numerous and is the main, while the group of closed-bubble fish is very small. Perch, mullet, cod, stickleback, etc. belong to it. In open-bubble fish, according to the name, the swimming bladder is open for communication with the main intestinal stream, and in closed-bubble fish, respectively, no.
Cyprinids also have a specific structure of the swimming bladder. It is divided into rear and front cameras, which are connected by a narrow and short channel. The walls of the anterior chamber of the bladder consist of two shells, the outer and the inner, while the outer chamber is absent in the rear chamber.
A swimming bladder is lined with one row of squamous epithelium, after which there is a row of loose connective, muscle and a layer of vascular tissue. The swimming bladder has a mother-of-pearl reflection characteristic of it, which is provided by a special dense connective tissue having a fibrous structure. To ensure the strength of the bladder outside, both chambers are covered with an elastic serous membrane.
Labyrinth organ
In a small number of tropical fish, such a specific organ as the labyrinth and suprajugal is developed. This species includes macropods, gourami, cockerels and snakeheads. Formations can be observed in the form of a change in the pharynx, which is transformed into a suprajugal organ, or a gill cavity (the so-called labyrinth organ) protrudes. Their main purpose is the possibility of obtaining oxygen from the air.