Image: Fish Dissection - Internal organs

Fish Dissection - Internal organs

Some of the ventrally located internal organs.

  1. heart
  2. liver
  3. pyloric caecae
  4. adipose (fatty) tissue


The circulatory system in fishes is a single circuit, with blood flowing from the heart to the gills and then to the rest of the body. The heart is located a little behind and below the gills.

The typical fish heart has four chambers, however unlike mammals, blood moves through all four in sequence. Venous blood enters the sinus venosus (a thin walled sac) then flows into the atrium, followed by the ventricle (a thick walled pump). Blood then flows into the conus arteriosus (elasmobranchs) or bulbus arteriosus (teleosts) then to the gills and the rest of the body.

The heart of slow moving fishes is comparatively small, whereas active swimming species such as the Blue Mackerel have large hearts.


The liver has many digestive and storage functions. One of these is the production of bile, a solution which emulsifies fats and may assist in changing the acidic conditions of the stomach into the neutral pH of the intestine). The liver is also responsible in some species for the storage of fats, blood sugar, and vitamins A and D. Before it was possible to synthetically create vitamins A and D, sharks were caught for their livers which have high concentrations of these vitamins.

The liver acts as a food reserve and so changes with reproductive condition particularly in viviparous species. The livers of sharks in early pregnancy are large and lightly coloured. Those of individuals that have just given birth tend to be shrunken and darker coloured. Similarly, the livers of males vary depending upon mating activity (J. Stevens, CSIRO pers. comm.).

Pyloric caecae

Pyloric caecae (singular caecum) are fingerlike pouches connected with the alimentary canal (the gut). They are attached to the pylorus, the section of the intestinal tract immediately following the stomach. They range in number from three in a type of scorpionfish (Setarches) to thousands in the tunas (family Scombridae). The Blue Mackerel is a scombrid species with many long, thin pyloric caecae.

Pyloric caecae may have a digestive and/or absorptive function. The enzyme lactase has been found in the pyloric caecae of some fishes such as trout.

In some families, such as the Salmonidae (salmons and trouts), the number of pyloric caecae is an important character used to tell species apart.

Stuart Humphreys
© Australian Museum



  1. Helfman G.S., Collette, B.B. & D.E. Facey. 1997. The Diversity of Fishes. Blackwell Science. Pp. 528.
  2. Lagler, K.F., J.E. Bardach & Miller R.R. 1962. Ichthyology. John Wiley & sons. Pp. 545.

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