RESPIRATION 171 



In true diving animals contact of the respiratory openings with sea 

 water reflexly inhibits breathing. During submergence 2 is used up and 

 C0 2 accumulates in the blood. Compared with terrestrial species divers 

 are relatively insensitive to C0 2 . Diving birds (puffins and guillemots) 

 show unusual tolerance to C0 2 , up to a concentration of 15%. The low 

 sensitivity to C0 2 results from higher threshold of the respiratory centre 

 in birds and diving mammals. Divers carry a supply of oxygen, but this is 

 insufficient for aerobic respiration during a long dive. The oxygen store 

 of the bladder-nose seal Cystophora has been estimated to be sufficient for 

 about 5 min at rest, but the animal is of course highly active when diving 

 and its energy consumption will be much greater than at rest. A seal can 

 remain submerged for 1 5 min and is enabled to do so by contracting an 

 oxygen debt in addition to depleting its oxygen stores. The gaseous capac- 

 ity of the lungs of divers is only slightly if at all larger than that of terrest- 

 rial mammals of comparable size. Only a small proportion (about a third) 

 of the oxygen store is carried in the lungs. The remainder is carried largely 

 in the blood and to a lesser extent in tissue fluids and in cells. Some divers 

 (seals, ducks) have somewhat larger blood volumes and higher concentra- 

 tions of blood haemoglobin than terrestrial forms. In addition many 

 divers, especially whales and seals, have large stores of muscle haemoglobin, 

 which surrender their oxygen to the muscle cells during submergence. It 

 appears, then, that oxygen capacity and stores of divers may be somewhat 

 higher than in terrestrial mammals, but still insufficient to account for the 

 fact that they can remain submerged so much longer than the latter. On 

 surfacing after a dive lasting 15 min (suspended respiration) the seal dis- 

 plays deep and continuous respiration, lapsing to normal in 15 min. There 

 is a large increase in gaseous exchange, and around 80% of the oxygen 

 debt is paid off in the first 20 min. 



During a dive lasting 15 min the seal carries only enough 2 for at 

 most a third of its requirements. How is this allocated and utilized? In 

 all divers submergence causes reflex slowing of the heart. In the seal the 

 heart rate falls from a resting value of 80; min to 10 min when the animal 

 has submerged, and the oxygen consumption falls during the first minute 

 of the dive to about one-fifth of resting value (Fig. 4.15). At the same time 

 the peripheral circulation (in the muscle mass and probably the viscera) 

 is largely suspended, and the reduced circulation and oxygen supply are 

 reserved largely for the brain. In whales there seems to be a method for 

 shunting blood through retia mirabilia to the brain. While the peripheral 

 circulation is occluded during a dive, the muscle haemoglobin of the seal 

 provides a local store of oxygen for the muscles, enabling them to carry 

 on aerobically for 5-10 min without recourse to lactic acid formation. 

 Once these stores are depleted, lactic acid accumulates in the muscles 

 (anaerobic respiration). On surfacing, lactic acid appears with a surge in 

 the blood once the peripheral circulation is opened, and is gradually 

 removed over the course of the next half-hour (Fig. 4.15) (82, 83, 84, 85, 

 140, 143). 



