260 



Comparative Animal Physiology 



mergence. The initial aerobic metabolism of the muscle during a dive is below 

 the normal resting rate, and the anaerobic rate in the later stages is greater 

 than at rest. ■^-•' A general lowering of the metabolic activity and the formation 

 of lactic acid probably are not limited to the muscles but occur elsewhere in the 

 diver as well. 



(2) Carbon dioxide insensitivity-this is characteristic of diving mammals 

 and reflects an adaptation to unusual conditions where lactic acid accumulates 

 in tissue and would otherwise stimulate the respiratory center to considerable 

 activity. Carbon dioxide insensitivity, due to both decreased circulation of 

 the blood and higher threshold of the respiratory center, has been demon- 

 strated in the beaver, seal, muskrat, and porpoise. *'''• ^^^- ^^■^' ^^' The entire 

 resting respiratory mechanism is adjusted to a high carbon dioxide tolerance 

 and relative apnea, with an extremely low breathing rate in these animals, 2-4 

 inspirations a minute in the porpoise and 2-3 in the seal. Furthermore, utiliza- 

 tion of the inspired oxygen is high; in Tursiops, the porpoise, for instance, an 



Fig. 59. Arterial blood changes in the seal, Cystophora, during experimental 

 diving conditions. After Scholandcr.'"'" 



animal weighing 170 kg. was found to operate with a resting breathing 

 frequency of once a minute, to inspire 10 liters with each breath, and to 

 consume about 1 liter of oxygen a minute— a "utilization" of approximately 10 

 per cent, two or three times that of man. 



(3) Cardiovascular changes— significant alterations in the heart and periph- 

 eral vessels tend to reduce the blood flow in most body regions with the excep- 

 tion of the brain. This bradycardia (heart slowing), first described by Paul 

 Bert in diving ducks, has been demonstrated in practically all diving mammals, 

 including man. In the seal the heart slows from a resting value of about 80 per 

 minute to 10 during a dive. This reduction in heart rate is a vagal reflex. 

 Another important reflex is the vasomotor change which effectively shuts the 

 circulation off from the main muscle masses of the body, as manifested by (a) 

 complete reduction of the muscle hemoglobin at a time when the blood is 

 still half saturated with oxygen, and (1?) accumulation of large amounts of 

 lactic acid in the muscles and its absence in the blood during the dive. 



