106 VERTEBRATE RESPIRATION 



plumbing in the branchial circulation (fig. 30b), have the result 

 that the blood volume is only one-third that of a typical selachian. 

 Furthermore, the pressures recorded in the ventral and dorsal 

 aorta are much greater, e.g., in the salmon, the ventral aorta 

 pressure is 75 mm. Hg. and 53 mm. Hg. in the dorsal aorta. 

 These modifications result in a faster circulation time than is 

 probable in the dogfish, but where they have been measured, as 

 in the eel, circulation is still slow (20-120 seconds) as compared 

 with man (30 seconds including the pulmonary circulation). 

 There are no valves in the veins of fishes but no doubt muscular 

 movements do play a part in maintaining the venous return. 



(b) Circulation in bony fishes with air-breathing organs 



Many fishes living in conditions unfavourable for branchial 

 respiration have evolved organs where oxygen may be absorbed 

 from air obtained by gulping from above the water surface. As 

 explained in Chapter 3, these organs are usually modified 

 regions of the alimentary canal and receive their blood supply 

 in a manner typical for their region of the gut. The blood supply 

 to some of these organs is given in Table 4, where a division is 

 made into those which return blood into the general body circu- 

 lation and those in which the return is direct to the heart. Before 

 considering the latter in some detail, it is of interest to notice 

 that some fishes have the circulation to the accessory breathing 

 organs in parallel with the gill circulation (e.g., the African and 

 Indian catfishes Clarias and Saccobranchus) so that the oxy- 

 genated blood which leaves the organ is distributed to all the 

 rest of the body. This contrasts with most other species where 

 the accessory circulation is in series with the gill circulation and 

 hence it only serves to boost the oxygen tension in the general 

 circulation. It is thus theoretically possible to have a single 

 circulation in which the gills are replaced by an air-breathing 

 organ, which might well have been a lung. In fact, of course, the 

 evolution of lung-breathing in tetrapods is associated intimately 

 with the evolution of a double circulation, which makes it 

 possible for the blood pressure to be raised before the oxy- 

 genated blood is circulated to the main organs of the body. It is 



