RESPIRATION IN WATER 



39 



contact with the filaments. Figure 16 gives a diagram of the 

 blood supply to the gills and shows the leaf-like secondary 

 lamellae through which blood is flowing in a direction opposite 

 to that of the water flowing 

 between the leaves. This 

 counter-current principle is 

 of the greatest importance, 

 as emphasized by van Dam. 

 With blood and water flow- 

 ing in the same direction 



the maximum saturation of the blood would be deter- 

 mined by an equilibrium with the water leaving the gills. 

 On the counter-current principle the theoretical maximum 

 will be reached when blood leaving the lamellae is in equi- 

 librium with the water entering the gills, and even if this 



Fig. 14. 



Diagram of gill arches in the 

 perch, (van Dam.) 



Fig. 15. Upper part of two gill arches with two rows of gill filaments bearing 

 lamellae on each, (van Dam.) 



ideal is not practically attainable there is no doubt that the 

 oxygen tension in the blood leaving the gills is definitely higher 

 than that of the water leaving them. 1 The efficiency of the 



1 The counter-current principle is realized also in the placental circulation 

 of some mammals (Barcroft and others, 1934). From a functional point of 

 view the gas-exchange in the placenta is very similar to that in gills. 



