The Open Biology Journal, 2011, 4, 35-46

RESPIRATORY SYSTEM IN FISH 
The fish gill adapted a structure for extraction of oxygen 
from water that is formed by a large number of filaments 
spaced out along the gill arches on either side of the pharynx. 
Each filament has a series of plates projecting at right angles 
from its upper and lower surfaces, the secondary lamellae, 
which are extremely numerous, are the site of gaseous 
exchange and form a fine sieve which ensures that all the 
water comes into close contact with the blood [2, 18].
The gills are multifunctional organs that are responsible 
for the gas exchange (respiration) but also for the osmoregu-
lation, acid-base regulation, and excretion of nitrogenous 
waste [18, 19].
The epithelial surface of a gill arch is structurally and 
functionally zoned [2]. The filaments are covered by two 
distinct epithelial surfaces, the lamellar and filament epithe-
lia, also termed the secondary and primary epithelia respect-
ively [20]. Gas exchange occurs through the secondary 
lamellae, and the non-respiratory functions of the gills take 
place in the primary epithelium [2].
The primary epithelium contains the chloride cells, which 
vary in morphology and number according to the milieu 
where the fish lives. The presence of an accessory cell beside 
the chloride cell is characteristic of seawater or seawater-
adapted fish [20]. The secondary epithelium that covers the 
free part of the secondary lamellae has an exclusive 
relationship with the arterioarterial vasculature, i.e., the pillar 
cells. This epithelium consists of an outermost layer of pave-
ment cells that exhibits structural characteristics suggestive 
of cell coat secretion and an innermost layer of less diffe-
rentiated cells. In contrast to the primary epithelium, the 
secondary epithelium does not exhibit any obvious diffe-
rences between freshwater and seawater fish [20].
The lamellar structure helps to increase the surface area 
but depends on the following complex anatomy to maintain 
the flat space necessary for circulation: separation between 
epithelial sheets (by pillar cells) and connection between the 

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