Category: Biology | Physiology

Respiration in Fishes

Respiration is a physiological process by which organisms exchange gases (especially intake of oxygen and releases of carbon-di-oxide),with the environment. This process is also known as external respiration. On the other hand, internal or cellular respiration means the oxidation of complex organic substances (carbohydrates or fats) within cells and tissues to form carbon dioxide and water with release of energy. (BdFISH Dictionary)

Respiration refers a physiological process by which oxygen is taken up by the blood and at the same time a waste product of metabolism namely carbon-di-oxide is given off.

Respiration is an important heat and energy yielding process. It includes the metabolic processes by which an organism uses oxygen to produce energy and it is mediated by multi-enzymatic system.

The respiration is an oxidative process in which energy is released due to a series of chemical reaction. Following chemical reaction occurs during respiration-

C6H12O6 +6O2 =6CO2+12H2O+ATP+673Kcal

Respiration takes place in the lungs in air-breathing vertebrates and through the gills in fishes and many invertebrates.

Types of respiration:

Respiration may be divided into two major types; such as-

  • External respiration
  • Internal respiration

External respiration: The respiration in which gaseous exchange (O2 & CO2) taking place between blood and water (or air) through the medium of respiratory organ is known as external respiration.

Internal respiration: The respiration in which the essential transfer of gasses between blood and tissues or cells of the body and brings about release of energy is known as internal respiration.

Respiratory mechanism in bony fishes: In bony fishes the main respiratory organ (gill) is covered by the operculum. In all bony fishes water is propelled over the gills by suction pressure. The whole respiratory mechanism of fishes are described by some consecutive steps in below-

Step.1: At the beginning of respiration, operculum is closed frequently and mouth is opened by the action of sternohyoid and elevator muscle of palatine. At the same time, the brancchiostegal rays are spread and lowered and the mouth cavity enlarged to create negative water pressure in it. Water is thus drawn/enter into the mouth.

Step.2: After a slight time lag, the space between the gills and the operculum is enlarged as the gill covers are abducted anteriorly. Though the opercular skin flaps are still closed posteriorly by the outside water pressure. The pressure deficit in the gill cavity and the water flows over the gill.

Step.3: The buccal and opercular cavities begin to reduce while the oral valves prevent the flow of water out of the mouth and the mouth cavity begins to function as a pressure pump instead of suction pump.

Step.4: In this stage, closed operculum with the opercular flaps has reached its furthest state of abduction and water is accumulating outside the gills. At this point the opercula are quickly brought towards the body, the gill flaps open and the water is expelled, being prevented from flowing backwards by excess pressure in the buccal-cavity as compared to the epibranchial cavity.

Role of blood as a carrier of O2: Generally O2 diffuses very slowly from one liquid into another. Thus fishes, like other vertebrates, have evolved in their red blood cells gas-carrying device of high efficiency. Hemoglobin of blood is a respiratory pigment of fishes and other vertebrates found in the red blood cells (R.B.C). Hemoglobin consists of two parts. One is haem i.e. iron-porphyrin part and other is globin i.e. polypeptide. Here the iron atom is Fe2+ (Ferrus) form and it converse in Fe3+ (ferric) form after combining with oxygen (O2). ­Each haem group is enfolded in one of two or four polypeptides called globin.

 So it can be said that, haemoglobin is the main functional unit of blood for exchanging or transporting O2. Hemoglobin acts in two basic ways- i) Giving off O2  when its tension is lowest in tissue and ii) Binding with O2 when its tension is higher in gill.

In as much as O2 taken up from the gill by blood and transported over the body is known as loading blood and when blood returns into gill with a lower density of O2 is known as unloading blood. It creates much tension/pressure to saturate the blood by O2 when the amount of CO2 decreases in blood.  On the contrary comparatively lower tension is needed to unloading the O2. This is known as Bohr Effect.

Role of blood as carrier of CO2: Generally CO2 diffuses rapidly in water than O2 as a result low amount of free carbon-di-oxide in natural water favors waste gas elimination at the gill by diffusion. CO2 in the venous blood of fishes is carried in solution in the plasma (about 10 poercent), as bicarbonates (about 70%) and as carbamino compound in R.B.C (about 20%). The bicarbonates turn into CO2 and H2O water by the catalyzation of carbonic anhydrase enzyme. This enzyme is found in acidophil cells of the gills and in other tissues.

CO2 from the tissues passes to the blood enters into the R.B.C. Then CO2 turns to carbonic acid and then bi-carbonate and hydrogen ion. The reaction between CO2 and water normally occurs very slowly but in the R.B.C, the reaction is catalyzed by the presence carbonic anhydrase. Associated chemical reactions are given in below-

C6H6O12+6H2O+6O= 6CO2+12H2O+673Kcal

The reactions of produced CO2 are here in below-

CO2 + H2O = H2CO3 (Carbonic acid)

This H2CO3 (Carbonic acid) again breaks down into CO2 by the reaction of carbonic anhydrase and this CO2 is released by the respiration of organism or fish.


Kumar S and Tembhre M (1996); Anatomy and Physiology of Fishes, Vikas Publishing House Pvt Ltd, India. pp. 77-98.

Lagler KF, Bardach JE, Miller RR, and Passino DR (1977); Ichthyology (2nd ed.), New York: John Wiley & Sons. pp. 228-260.

BdFISH Dictionary.


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MS Student, Department of Fisheries, University of Rajshahi, Rajshahi-6205, Bangladesh. Email:; More...

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