RESPIRATION 



H5 



if there is a series of regulating taps can the pressure be adjusted safely 

 and smoothly. 



If the animals remain at a given depth, no further great fluctuations 

 in pressure will occur. However, in diving and surfacing there may still 

 be differences in pressure between the soft and hard parts of the lung, and 

 between the soft part of the lung and thorax, with adverse effects on the 

 delicate walls of the alveoli and the capillaries surrounding them. In this 

 case, too, the valves act as essential safeguards. 



We have dealt with dolphins and porpoises, and the reader may wonder 

 whether whales which dive so much deeper have a similar mechanism. 

 Oddly enough they have not, or at least not to a significant extent. How- 

 ever, they have a regulating mechanism of their own in the alveolar 

 ducts, i.e. the air pipes going to the alveoli themselves. These fairly wide 

 ducts, which are unsupported by cartilage, can be shut off by powerful 

 annular muscles, and so can each alveolus separately. Now, smooth muscle 

 fibres are also found in the alveolar ducts of all other mammals, in which, 

 however, they are not nearly as strongly developed. The overall effect in 

 whales is probably similar to that of the dolphins' valves, though it must 

 be remembered that deep divers take down far less air than animals which 

 stay closer to the surface. 



Until recently little was known about the fine structure of the lungs of 

 Bottlenose Whales. In 1957, however, microscopic investigations carried 

 out by Miss J. R. Goudappel, a student at Amsterdam University, 



Figure 84. Septum be- 

 tween alveoli in {a) man and 

 {b) a porpoise. Note that 

 the former contains one and 

 the latter two layers of 

 capillaries. At the top of the 

 septum: smooth muscle and 

 elastic fibres. 



K 



