14 VERTEBRATE RESPIRATION 



blood circulates through spaces formed between the individual 

 pillar cells. It is a notable adaptation that the spaces between 

 the pillar cells are just large enough for the red blood corpuscles 

 to pass (fig. 2c). The distance for diffusion of O2 between the 

 water and corpuscles is small and can take place on both sides 

 of the secondary folds. Having been oxygenated the blood is 

 collected into a corresponding series of efferent vessels which 

 pass into the efferent branchial artery which joins the dorsal 

 aorta along which the blood passes back to the rest of the body. 



(ii) Counter-current principle 



The gills of fishes and many other aquatic animals show this 

 principle extremely well ; the essential feature being that the flow 

 of water is in a direction opposite to that of the blood (fig. 3a). 

 This pattern ensures that blood which has already become partly 

 loaded with oxygen meets water which has had little O2 removed 

 from it. Correspondingly, water which has already had much of 

 its oxygen removed is in contact with blood that is least saturated 

 with oxygen. Therefore, a fairly constant gradient of oxygen 

 tension between the blood and water is maintained throughout 

 their passage across the gill. In this way a greater degree of 

 saturation of the blood can be achieved and it is theoretically 

 possible for the blood to reach almost the same oxygen tension 

 as the inhalant water. In exchange systems with parallel flow, 

 the gradient is initially high but will progressively decrease (fig. 

 3c and d). The advantages of counter-flow in heat exchangers 

 are well known to engineers and, as we shall see (p. 88), they are 

 similarly employed in vertebrate animals where eflftcient heat 

 exchange is required. However, counter-flow is not always more 

 effective than parallel flow. For instance, if water saturated with 

 oxygen flows very rapidly past a respiratory surface in which a 

 smaller volume of blood is circulating in parallel and at a much 

 slower speed, the degree of oxygenation of the blood would be 

 very nearly as great as if counter-flow were present. In this case 

 the proportion of oxygen removed from the water would be 

 relatively low whereas in fact bony fishes can achieve 80% 



