The main function of the excretory system of any living creature, including fish, is to remove metabolic products from the body and maintain the water-s alt balance in the blood and tissues. Of course, the excretory system of fish has a simpler structure than, for example, the human one. The execution of functions occurs along a certain chain, to understand which one should study the structure of the system as a whole and the work of its organs separately.
Structure: what organs form the excretory system of fish
For the removal of unnecessary, and often toxic substances from the body, these representatives of the aquatic fauna, like humans, are responsible for paired kidneys, which are a complex system of small wire tubules. The latter open into the common excretory duct. The bladder in most fish comes out separately.hole.
The metabolic products formed in the kidneys mainly enter the bladder through the ducts.
Fry kidneys
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 classify them as lower vertebrates. In terms of the complexity of the structure of organs, waterfowl are inferior to both amphibians and reptiles. In higher vertebrates, including humans, the kidneys are pelvic. In fish, they are trunk.
The degree of complexity of the structure of the kidneys in any living creature is determined by:
- number of tubules;
- the presence and structure of ciliated funnels.
In some representatives of the fauna, the kidneys are laid in the upper part and consist of 6-7 tubules. The ciliated funnel, which acts as a filter, in such organisms, one end opens into the ureter, the other into the body cavity. It is this structure that characterizes the kidneys of fry and some adult fish. These include the eelpout, smelt, gobies and others. In other species of fish, the primitive kidney gradually transforms into a lymphoid hematopoietic organ.
Adult fish kidneys
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 kidneysThese representatives of the water element belong to the class of the trunk and look like ribbon-like strands of maroon color.
The main functional element of the adult fish kidney is the nephron. The latter in turn consists of:
- excretory tubules;
- Malpighian bodies.
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 collecting ducts. The latter, in turn, merge into larger ones and fall into the ureters.
Flickering funnels in the kidneys of most fish are absent, except for some species. Such functional elements, for example, are found in sturgeon and some cartilage.
Build examples
Kidneys are rather complex organs of the excretory system of fish. It is customary to distinguish three main departments:
- anterior (head kidney);
- medium;
- rear.
Departments of the kidneys of different types of fish may have a different shape. It is, unfortunately, quite difficult to consider the structure of this organ specifically for each class in one short article. Therefore, as an example, let's figure out what a kidney of carp, pike and perch looks like. 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 wraps around the swim bladder in the form of a ribbon.
The kidneys of perch and pike have a slightly different structure: the middle sections are located separately, and the front and back 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.
There are only two main classes of fish in nature:
- cartilaginous;
- bone.
The difference between them, first of all, lies in 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 about 730 species. There are much more bone representatives of aquatic fauna: about 20 thousand varieties.
The excretory system of fish (bone and cartilage) has a different structure. The former have a bladder, while the latter do not. Of course, the absence of this organ in cartilaginous fish does not mean that their VS is imperfect. She performs her 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 release very little "liquid waste" into the water.
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 maintaining a normal level of water-s alt balance in the body. In fish, this function is performedrectal 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 s alt from the body from food or sea water.
In addition to maintaining the s alt balance, the rectal gland of fish performs another very important function. During the breeding season, the secreted mucus trails after the fish, attracting the characteristic smell of individuals of the opposite sex.
S alt balance
The osmotic pressure of all such representatives of the fauna (both marine and freshwater) is significantly different from the environment. Mixins are the only exception to this rule. The concentration of s alts in their body is the same as in sea water.
In cartilaginous fish belonging to the isoosmotic group, the pressure is the same as that of hagfish and coincides with the pressure of water. But the concentration of s alts is an order of magnitude lower than in the external environment. The pressure balance in the fish body is provided by a high content of urea in the blood. The concentration and removal of chloride ions and sodium ions from the body is performed by the rectal gland.
The excretory system of bony fish is well adapted to adjust the s alt 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 have developed special mechanisms that control and regulate the s alt content in the blood.
Thus, marine bony fish constantly losing water under the influence of osmotic pressure, to compensate for the loss are forced to drink very often. Sea water in their body is constantly filtered from s alts. The latter are excreted from the body in two ways:
- calcium cations with chloride ions are ejected through gill membranes;
- magnesium cations with sulfate anions are excreted by the kidneys.
In bony freshwater fish, unlike marine fish, the concentration of s alts in the body is lower than in the external environment. Representatives of the fauna equalize the pressure by capturing ions from the water through the gill membranes. In addition, large amounts of urea are produced in the body of such cold-blooded animals.
Composition of urine
As we found out, the structure of the excretory system of fish (cartilaginous and bone) is somewhat different. The composition of the urine of these representatives of the fauna is also different. The main component of the liquid secretions of bony 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 filter feeders, with the bloodstream. The latter is preliminarily supplied to the vascular glomeruli. It is in them that the filtration process takes place, as a result of which primary urine is formed. Vessels derived from the glomeruli entangle the excretory tubules. Joining together, they form the posterior cardinal veins.
In the middle part of the tubules (inkidneys) is the formation of secondary (final) urine. Here, among other things, the absorption of substances necessary for the body occurs. It can be, for example, glucose, water, amino acids.
Pronephric canal
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: wolf and muller canals. The latter is present only in females. In males, it is atrophied.
In wolf fry, the canal is designed to perform the functions of the vas deferens. In the male cartilaginous variety, as they grow older, a separate ureter is formed, which opens into the urogenital sinus. The latter, in turn, is connected to the cloaca. In adults, the Wolf canal transforms into the vas deferens.
Features of the excretory system of fish of bone species are, first of all, the absence of a cloaca and the separation of the excretory and reproductive systems. Wolf channels in such representatives of the fauna are combined into an unpaired stream. The latter at the same time is located on the wall of the abdominal cavity of the fish at the back, forming the bladder on the way.
