84 RESPIRATORY MECHANISMS 



The sudden rise in blood lactic acid just after the dive 

 might indicate a suspension or at least a great reduction in 

 blood flow through the muscles during the dive, and the most 

 interesting and probably the most important adjustment tak- 

 ing place in diving is the extreme slowing of the heart rate 

 observed by several authors and amply confirmed by Scho- 

 lander. The electrocardiogram shown in Fig. 49 illustrates 



Fig. 49. Electrocardiographic record of seal before, during, and after dive. 



(Scholander.) 



the point. The rate drops from 120 per min. to about 10. 

 With such a very slow pulse blood pressure cannot possibly be 

 kept up unless the peripheral circulation is largely suspended, 

 and there is reason to believe (Irving, 1938) that at least in 

 muscles and perhaps also in many internal organs the arteries 

 are closed down. A corresponding bradycardia during diving 

 has been observed also in the beaver and in diving birds 

 (Irving, Scholander) but not so far in whales, where a mechan- 

 ism for shunting blood through the retia mirabilia is suggested 

 by studies made by Eriksson in Scholander's laboratory. 



We know from Barcroft's admirable book The Architecture of 

 Physiological Function that it is the central nervous system 

 which requires oxygen at a fairly high tension to work properly 

 while all other organs are very resistant, and we may reason- 

 ably assume therefore that during diving the blood circu- 

 lation and oxygen supply are reserved mainly for the brain 

 and medulla. 



The mechanisms for prolonged diving turn out to be closely 

 related to those allowing man to contract a large oxygen debt 



