The swim bladder is an organ of the bony fish . It helps the fish to adjust its specific weight to that of the surrounding water so that it can float in the water. It is formed from a protrusion of the foregut and represents a further development of the fish lung, with a function change from a respiratory organ to a hydrostatic organ.
The swim bladder also serves to stabilize the position, because in an upright position, the center of mass lies below the volume center because of the dorsal position of the swim bladder (i.e. at the top of the body) .
Most bony fish have a swim bladder. An exception is for example, bullhead , so she moves usually only on the ground. Fish that do not have a swim bladder and still do not belong to the bottom fish have to create buoyancy through constant swimming, for example the sharks , or they reduce their bones and store fat, such as the sunfish ( mola ).
Filling the swim bladder
There are two mechanisms for filling the swim bladder:
- by swallowing air that enters the swim bladder via the intestines (in physostomes )
- Via the blood vessels that carry the gas released from the gills . Fish that use this mechanism are called physocists ; they are usually found in deeper waters. Many physocists, however, initially live as physostomes as young animals, i. H. The swim bladder is initially filled by swallowing air.
The following processes occur in the miraculous network of the swim bladder: Passive diffusion of oxygen from arterial (O 2 -rich) capillaries to venous (O 2- poor) capillaries prevents O 2 from being transferred from the swim bladder into the blood or becomes it returned again. The principle behind this is the countercurrent principle , caused by a vertex that lies on the swim bladder (in the so-called red body), comparable to a curved tube through which O 2 can diffuse. The high O 2 content of the swim bladder is generated in the swim bladder by the obligate anaerobic ( glycolytic ) metabolism in the epithelial cells of the gas gland . The lactate formed here by glycolysis has two effects: the salting out effect and the Bohr effect . The lactate reduces the O 2 solubility of the blood, the pH value is lowered by the lactate. Due to the two effects an O produced 2 - Gradient , fills the swim bladder.
Emptying the swim bladder
There are two different ways to empty the swim bladder:
- The physostomes use the ductus pneumaticus , a connection between the swim bladder and the gill intestine (and thus a connection to the outside world) to release the oxygen. The physostomic sturgeon have neither the oval nor the red body and can nevertheless (slowly) regulate the swim bladder volume without swallowing or releasing air.
- The physocists use the oval, an area of the swim bladder with a high blood supply, to reabsorb gas into the bloodstream. The surface of the oval and the associated oxygen absorption is controlled by muscles.
Movement in water is much more energy-consuming and laborious than in air due to its approx. 800 times higher density and approx. 55 times higher viscosity . The density of animal tissues is greater than that of water, so that aquatic animals have to swim almost constantly in order not to sink to the bottom. An organism with the density of water, on the other hand, could float motionless in the water and thus save kinetic energy. It is therefore not surprising that various tissues or organs can be found in aquatic animals that are characterized by a particularly low density and thus bring the entire organism closer to weightlessness. In order to keep the volume and density of the swim bladder constant, a submerged fish must secrete gas into it . When ascending, however, it has to release gas from the bubble in order not to lose density due to the increasing bubble volume and to shoot to the surface.
Physically similar acts a freediving person who only takes a lot of air with him for deep diving in the lungs and a scuba diver who adjusts the buoyancy of his buoyancy compensator when the depth changes by supplying or deflating air and exhales to be on the safe side when surfacing.
The swim bladder as a sound-forming organ
The swim bladder can also be used to make sounds via internal or external muscles.
- Inner (intrinsic) drum muscles (tone muscle or sonic muscle) are located in the swim bladder wall of toadfish and gurnard
- External (extrinsic) drum muscles (at least one attachment to structures outside the swim bladder)
The swim bladder as a respiratory organ
The swim bladder can also serve as a respiratory organ (primary or secondary), which enables these fish to survive even in periods of drought or when the oxygen content of the water is low. This is e.g. B. with arapaima or the bonefoot ( Lepisosteus ) the case.
Human use of swim bladders
In some Asian cultures, the swim bladders of larger fish are considered a delicacy. In China they are called maw花 膠 / 鱼鳔 and are served in soups or stews.
Swim bladders are also used in the food industry as a source of collagen , e.g. B. used as a fining agent for wines. They are the main raw material for making isinglass . This glue found z. B. has been used in the manufacture of composite bows since the Bronze Age.
In the past, swim bladders were used to make condoms .
- Physics in context: "The life of animals in the river" (accessed April 20, 2020)
- Learning with everyday ideas: swim bladder (accessed April 20, 2020)
- Swimming cystitis in carp (accessed April 20, 2020)
- Archimedes and the (whale) fish experiments on the physics of swimming in water (accessed April 20, 2020)
- The swim bladder (accessed April 20, 2020)
- Hans-Albrecht Freye: Zoology . 9th edition. Fischer Verlag, Jena 1991, ISBN 3-334-00235-7
- Bernd Pelster: The swim bladder as a hydrostatic organ. In: Biology in Our Time. 23rd year 1993, No. 4
- Tanja Schulz-Mirbach et al .: Relationship between Swim Bladder Morphology and Hearing Abilities - A Case Study on Asian and African Cichlids. In: PLoS ONE. Volume 7, No. 8, 2012, p. E42292, doi: 10.1371 / journal.pone.0042292 (full text freely accessible)