WATER, SALTS AND MINERALS 



51 



and their osmoregulatory mechanisms have been investigated extensively 

 (Fig. 2.16). The blood of marine elasmobranchs is nearly isosmotic with 

 sea water and has a higher osmotic pressure than that of marine teleosts 

 (Table 2.6). The salt concentration of the blood is somewhat higher in 

 elasmobranchs (about 240 mM CI) than in teleosts (about 180 mM CI). 

 The greater osmotic pressure of elasmobranch blood is mainly due to its 



-2-5 



20 



;-5 - 



o 



1-0 



0-5 



05 



10 1-5 20 



A Externa.! Medium 



-2-5 



Fig. 2.16. Variation in Blood Concentration of Three Fishes 

 in Waters of Different Concentrations 



A marine elasmobranch (dogfish), marine teleost (conger eel) and freshwater eel 

 (euryhaline teleost). (Data from Duval, 1925.) 



high urea content, reaching 1-5% (250 mM) and responsible for about a 

 third of the total osmotic pressure (127). 



Urea is retained by elasmobranchs as a useful metabolite, much of the 

 urea in the glomerular filtrate being absorbed in the kidney tubules. More- 

 over, the oral membrane, gills and integument are relatively impermeable 

 to urea, which is thereby conserved. As the result of the osmotic gradient 

 maintained by this high urea content water tends to flow into the blood 

 from the surrounding medium. The water content of the tissues is main- 

 tained at a steady level by the excretion of a hypotonic urine at a relatively 

 constant rate. Some data for urea levels in elasmobranchs are given in 



