The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes. Gills are very efficient at collecting free oxygen from water, they have to be because water typically contains only a fraction of free oxygen of that found in air.
Air typically contains 280 mg/l of oxygen while fresh water typically only contains 5 - 10 mg/l.
Most fishes will be satisfied with oxygen levels of 5 or 6 mg/l of free oxygen in their environment but if this level drops the fish will be seen gasping at the surface in an attempt to find the most oxygen.
Some fish like the labyrinth fishes have evolved to live in water with low levels of oxygen by breathing atmospheric oxygen as a supplement to that found in water.
Some fish such as sharks and tuna use ram ventilation because they cannot pump water through their gills and they must keep moving with their mouth open so that water enters their mouth and flows over the fill filaments in order to be able to breathe. The vast majority of fishes breathe by branchial pump where water is actively pumped over their gills regardless of whether the fish are in motion. Using this method means that the fish can rest and not suffocate but because this method uses energy it also means that 10 to 15% of the oxygen extracted from the water is used simply to pump the gills when the fish is completely resting.
To complicate things further some fish have evolved to extract oxygen from the atmosphere but they do not use their gills to accomplish this, instead they use an organ called a labyrinth, modified, vascularized gas bladders or in the case of the lung fishes one or two lungs depending on the species.
Some newly hatched fry have undeveloped gills when they first hatch. These fry are able to use a process called cutaneous respiration where they absorb enough oxygen through their skin to meet their needs until their gills develop.
Fig 1. A view of the gills location with the operculum (gill cover) removed
Fig 2. One of the gill arches with all its components
Fig 3. One of the gill filaments and lamella in great close up
Fig 4. Blood flow through the gill filaments
The gills themselves consist of a series of gill arches which have gill filaments on one side and gill rakers on the other. It is the gill filaments which are used for respiration. The gill arches are covered from the outside by the fishes operculum which acts as a protector and is used to pump the water over the gills in branchial respiration. Each branchial arch in the teleost is elaborated into multiple filaments, which are further subdivided into thousands of lamellae and this is where gas exchange occurs. The water flows in the opposite direction to the blood so that maximum exchange can take place.
As shown in Fig 3, the lamella have permeable walls of epithelial cells with a central channel where red blood cells flow in the opposite direction to the water flow as shown in Fig 4. Fluids can flow through the permeable walls of the lamella quite easily and this allows the fish to get rid of CO2 and NH3 and replace them with oxygen which is then transported throughout the body of the fish. These cells mediate NaCl extrusion in marine fishes and NaCl uptake in freshwater fishes. These transport steps also provide pathways for the extrusion of ammonia and acid vs. base equivalents
The fish does have some control over this by using hormones which make the walls of epithelial cells slightly more or less permeable which also helps to regulate osmosis. Due to the nature of the lamella and their need to be very permeable to allow reparation means that this is where most of the fluid enters the fishes body due to osmotic pressure.
Unlike terrestrial vertebrates the bony fish excrete nitrogenous waste through their gills in the form of ammonia (NH3). Mammals detoxify ammonia by conversion to urea via the ornithine cycle in the liver. Birds and reptiles convert ammonia to uric acid for excretion. The full process of how they do this is still poorly understood. Nitrogenous waste is formed in the fishes blood from the metabolism of amino acids (protein). Fish which have a diet which is high in protein such as predators will excrete more ammonia than a herbivore species does. The ammonia will be excreted at a greater rate about 1 to 3 hours after the fish have been fed it will then slowly fall back to its basal level.
pH has a dramatic effect on fishes gills. As the pH falls the gills become more permeable. If the pH is higher than the species of fish is adapted for it will place the fish under a great deal of stress. The increased permeability makes it difficult for fish to hold on to electrolytes and they could lose to much sodium and chloride ions as a result. Fish which have evolved to live in acidic water have gills which can cope with the conditions and they are able to extract sodium and chloride ions from water with a very low ionic content.
Fishes which come from other environments do not have this specialised ability and some fish lose control of their Osmoregularity system as a result. A very common mistake is to keep black mollies with various tetras in a community tank with slightly acidic conditions. Black mollies cannot cope with this and quite often end up suffering from pop eye or dropsy as a result.
High pH presents a different set of problems to the fish. Because ammonia comes in two forms NH3 and NH4 with only NH3 being permeable through the gills and with NH4 increasing with pH then a high pH can lead to the fish having excess ammonia in it's body and being unable to get rid of it. Fish which are adapted to live at high pH values can sometimes use alternative methods of getting rid of nitrogenous waste by producing urea. It was thought at one time that only the elasmobranchs secreted nitrogenous waste this way but recent studies have shown a number of teleost's have this ability too.
Oreochromis alcalicus grahami, come from Lake Magadi in Africa where the pH is typically 9.6 to 10. All nitrogenous waste from this species is excreted as urea and none via the gills.
pH is therefore extremely important to fish and they all have a natural range and they won't thrive outside that range. This can be problematic because fish which have been bred in captivity for many generations will have a very different pH tolerance to their wild counterparts particularly if the species originally comes from a more extreme environment. For example, todays fancy discus fish which live in fairly neutral conditions would not last long if exposed to a pH below 4 which wild discus can be exposed to in some locations at certain times of the year.
Most fish can cope with slightly lower levels of oxygen than the ideal for a short time. They do this by increasing the flow of water over their gills, reducing their heart rate which together maximises the uptake of oxygen. In more extreme or prolonged periods of hypoxia extra red blood cells will be released from the spleen.
If the hypoxia is temperature related then this gives the fish extra problems because as the temp rises the oxygen level falls and the fishes need of oxygen increases. Fishes which have experienced (if they survive) this will show reduced growth rates immediately after the event.
Fishes which have been subjected to nitrite poisoning (new tank syndrome) will have lower hemoglobin levels and as a result they will have a much lower tolerance of hypoxic condition than a healthy fish.
Osmoregulation - is the active regulation of the osmotic pressure of an organism's fluids to maintain the homeostasis of the organism's water content; that is, it keeps the organism's fluids from becoming too diluted or too concentrated
Ram ventilation - the production of respiratory flow in some fish in which the mouth is opened during swimming, such that water flows through the mouth and across the gills
Branchial ventilation - is used to force water through the gills by alternately contracting and expanding the mouth and gill cavities
Opercula - the bony gill covers
Cutaneous respiration - oxygen passes right thru the skin into the skin cells and then into the blood stream and Carbon Dioxide goes the other way
Gill filaments - are the soft, red, fleshy part of the gills, through which oxygen is taken into the blood
Epithelial cells - Are specialised cells which provide a site for net movement of salts and water down their respective gradients. Specialized cells in the gill epithelium are joined by tight junctions of variable depth and express a variety of transporters and channels. These cells mediate NaCl extrusion in marine fishes and NaCl uptake in freshwater fishes. These transport steps also provide pathways for the extrusion of ammonia and acid vs. base equivalents.
Wikipedia - Osmoregulation.
Encyclopedia.com - Ram ventilation.
www.zi.ku.dk - Branchial pump.
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Eddy, F. Brian; Handy, Richard D. (2012-05-03). Ecological and Environmental Physiology of Fishes (p. 125). Oxford University Press. Kindle Edition.