Respiration and Metabolism 259 



the unusual storage of oxygen or increase in lung volume, but rather the more 

 efficient utilization and availability of oxygen for the essential operating 

 processes, the dexelopment of anaerobic glycolysis, the high tolerance to lactic 

 acid, and the relatixe insensitivity to carbon dioxide of the animals while 

 submerged. 1 he rate of oxygen consumption of di\'ing mammals does not 

 differ greatly from that of land mammals, ranging from 546 cc. O^/kg./hr. in 

 the seal, down to 180 cc. in the manatee, comparable to a value of about 250 

 cc. for man. The xarious factors which play a [lart in the oxygen stores are 

 indicated as follows: 



(i) Lungs— the lung volume is only slightly larger in the dixing mammals 

 than in land animals, although the tidal air is considerably greater in most, as 

 high as 80 per cent of the lung \'olume in the porpoise. '""^ 



(2) Blood— in the seal the oxygen capacity of the hemoglobin is 1.78 cc. 

 Oo/gm. Hb, compared with 1.23 for man,'""' and the blood volume of the seal 

 is about 10 per cent of the body weight. '"^^ 1 he total ox\gen capacity ol the 

 blood is slightly higher than that in man in some divers (29.3 \ols. per cent 

 in the seal), but not in all (17.7 vols, per cent in the beaver). 



(3) Muscle hemoglobin— divers other than the manatee have a rich store 

 of muscle hemoglobin. Seal muscle vields 7715 mg. flb/lOO gm., compared 

 with 1084 mg. from beef.-"'i 



(4) Tissue Huids— they appear the same as in non-divers. 

 Scholander-'-'* summarizes the utilizable oxygen stores of a bladdernose seal 



weighing 29 kg. as follows, taking the highest estimates: 



Air in the lungs: 350 cc, 15% O^ 

 Blood: 4500 cc, 25% O, 

 Muscle: 6000 gm., 4.5% O2 

 Tissue fluid 



Total oxygen store 1 520 cc. 



The major part of the store, that of the blood, is used up at practically a 

 constant rate and can last 25 minutes, whereas the muscle oxygen forms an 

 isolated store which is used up during the first 5T0 minutes of the dive.^-'* 

 During a 25 minute period at rest this animal consumes about 6250 cc. of 

 oxygen, which is four or five times as much as the oxygen stores could possibly 

 supply for the dive. One may conclude therefore that the oxygen consumption 

 by the tissues during a dive is depressed to 20-25 per cent of the resting rate. 

 These findings are in accord with similar observations on penguins and whales, 

 manatees, and the burrowing three-toed sloth, Bradypxis.^^^- •'*-"• •*-- 



Ruling out oxygen stores as a critical factor in the diving mechanism, other 

 adaptations have been demonstrated to permit submergence for long periods.'-"^ 



(1) Anaerobic glycolysis— although lactic acid does not accumulate in the 

 blood to any great extent during the dive, large quantities literally flood the 

 blood stream immediately after surfacing^**" (Fig. 59). Apparently after the 

 oxvgen in the muscle is used up in the first few minutes of a dive, anaerobic 

 processes take over entirely and an ox\gen debt builds up, counteracted during 

 the dive only to the extent that some small quantities of oxygen may be brought 

 in bv the blood, itself pretty well blocked ofl^ from the muscle during sub- 



