The main function of the excretory system of any living creature, including fish, is the removal of metabolic products from the body and maintaining the water-salt balance in the blood and tissues. Of course, the excretory system of fish has a simpler structure than, for example, the human one. The fulfillment of functions takes place along a certain chain, for the understanding of which it is necessary to study the structure of the system as a whole and the work of its organs separately.
Structure: which organs form the excretory system of fish
For the removal from the body of unnecessary, and often poisonous substances, these representatives of the aquatic fauna, like humans, are responsible for paired kidneys, which are a complex system of small tubular tubules. The latter open into the common excretory duct. The bladder in most fish comes out in a separate hole.
The metabolic products formed in the kidneys through the ducts predominantly enter the bladder.
Buds of fry
Understanding which organs form the excretory system of fish, we can conclude that the key role in its functioning belongs to the kidneys.
In the evolutionary chain, fish are far from the first place. Biologists attribute them to the class of lower vertebrates. By the complexity of the structure of their organs, waterfowl are inferior to both amphibians and reptiles. In higher vertebrates, including humans, the pelvic kidneys. In fish, they are trunk.
The degree of complexity of the structure of the kidneys in any living creatures is determined by:
- the number of tubules;
- the presence and structure of ciliary funnels.
In some representatives of the fauna, the kidneys are laid in the upper part and consist of 6-7 tubules. The ciliary funnel, which acts as a filter, in such organisms opens at one end into the ureter and the other into the body cavity. It is this structure that characterizes the kidneys of fry and some adult fish. These include eelpout, atherin, gobies and others. In other species of fish, a primitive kidney is gradually transformed into a lymphoid hematopoietic organ.
Kidneys of adult fish
In fry, in most cases, the kidney is located in the upper part of the body. In adult fish, this paired organ fills the space between the swim bladder and the spine. As already mentioned, the kidneys of these representatives of the water element belong to the body class and look like ribbon-like cords of maroon color.
The main functional element of the kidney of an adult fish is the nephron. The latter in turn consists of:
- excretory tubules;
- Malpigiev's little body.
The Malpighian body in fish is formed by a capillary glomerulus and Shumlyansky-Bowman capsules, which are microscopic cups with double walls. The urinary tubules extending from them open into the collective. The latter, in turn, merge into larger ones and fall into the ureters.
Atrial funnels in the kidneys of most fish are absent, except for some species. Such functional elements, for example, are found in sturgeons and some cartilaginous.
Building Examples
The kidneys are rather complex in structure organs of the excretory system of fish. It is customary to distinguish three main departments:
- front (head kidney);
- middle;
- rear.
The kidney departments of different fish species may have a different shape. To consider the structure of this body specifically for each class in one small article, unfortunately, is rather difficult. Therefore, as an example, we will understand how the kidney of carp, pike and perch looks. In cyprinids, the right and left kidneys are located separately. Below they are connected into an unpaired tape. The well-developed middle section is greatly expanded and in the form of a tape covers the swimming bladder.
At the perch and pike, the kidneys have a slightly different structure: the middle sections are located separately, and the front and rear are connected.
Bladder
The structure of the excretory system of fish is quite complex. The bladder is present in most varieties of these representatives of the aquatic fauna.
The main classes of fish in nature, as you know, there are only two:
The difference between them, first of all, is the structure of the skeleton. In the first case, it consists of cartilage, in the second, respectively, of bones. The class of cartilaginous fish is represented in nature by approximately 730 species. There are much more bony representatives of the aquatic fauna: about 20 thousand species.
The excretory system of fish (bone and cartilage) has an uneven structure. The former have a bladder, while the latter do not. Of course, the absence of this organ in cartilaginous fish does not at all mean that their BC is imperfect. It performs its functions just fine.
The excretory system of cartilaginous fish includes organs, the structure of which maximally prevents the uncontrolled flow of urine into the environment. Such representatives of the fauna usually emit “liquid waste” into the water very little.
Rectal gland of fish
As already mentioned, the excretory system of fish is responsible not only for the removal of metabolic products, but also for the maintenance in the body of a normal level of water-salt balance. In fish, this function is performed by the rectal gland, which is a finger-shaped outgrowth that extends from the dorsal part of the rectum. The glandular cells of the rectal gland secrete a special secret containing a large amount of NaCl. First of all, this organ removes excess salt from the body from food or sea water.
In addition to maintaining salt balance, the rectal gland of fish performs another very important function. During the breeding season, the secreted mucus stretches after the fish, attracting a characteristic smell of individuals of the opposite sex.
Salt balance
The osmotic pressure of all such representatives of the fauna (both marine and freshwater) is significantly different from environmental indicators. The only exception to this rule are mixins. The concentration of salts in their body is the same as in seawater.
In cartilaginous fish belonging to the group of isosmotic, the pressure is the same as in myxines and coincides with the pressure of water. But the concentration of salts in this case is an order of magnitude lower than in the external environment. The balance of pressure in the fish body is provided by a high content of urea in the blood. Concentration and removal of chloride ions and sodium ions from the body is performed by the rectal gland.
The excretory system of bone fish is well adapted to adjust salt balance. The pressure of such representatives of the fauna is regulated a little differently. Such fish do not belong to the isosmotic class. Therefore, in the process of evolution, they developed special mechanisms that control and regulate the salt content in the blood.
So, marine bone fish constantly losing water under the influence of osmotic pressure, are forced to drink very often to compensate for losses. Sea water in their bodies is constantly filtered from salts. The latter are excreted in two ways:
- calcium cations with chloride ions are ejected through gill membranes;
- magnesium cations with sulfate anions are excreted by the kidneys.
In bone freshwater fish, unlike marine fish, the concentration of salts in the body is lower than in the external environment. Representatives of the fauna are equalized by the capture of ions from water through gill membranes. In addition, a large amount of urea is produced in the body of such cold-blooded animals.
Urine Composition
As we found out, the excretory system of fish (cartilage and bone) is somewhat different in structure. The urine of these representatives of the fauna is not identical in composition. The main component of liquid secretions of bone fish is ammonia - a substance that is toxic even in minimal concentrations. In cartilage, this is urea.
Metabolic products are delivered to the kidneys of fish, which are essentially filtrators, with blood flow. The latter is previously fed into the vascular glomeruli. It is in them that the filtration process takes place, as a result of which primary urine is formed. The vessels removed from the glomeruli entangle the excretory tubules. Joining together, they form the posterior cardinal veins.
In the middle part of the tubules (in the kidneys), secondary (final) urine forms. Here, among other things, the absorption of substances necessary for the body. This may be, for example, glucose, water, amino acids.
Pronephric channel
The excretory system of fish is represented by the pronephric canal - the main outlet duct of the main kidney. In cartilaginous fish, it consists of two parts: wolves and müller channels. The latter is available only to females. In males, it is atrophied.
In the wolf fry, the channel is designed to perform the functions of the vas deferens. In the male of the cartilaginous variety, a separate ureter is formed as it grows, which opens in the urogenital sinus. The latter, in turn, is connected to the cesspool. In adults, the wolf channel is transformed into the vas deferens.
Features of the excretory system of fish of bone species are, first of all, the absence of cloaca and the disconnection of the excretory and reproductive systems. Wolf channels in such representatives of the fauna are combined into an unpaired stream. The latter is located along the wall of the abdominal cavity of the fish in the back, forming a bladder on the way.