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The Lancet • Saturday 19 September 1964

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The Lancet • Saturday 19 September 1964 The Lancet • Saturday 19 September 1964 COMPARATIVE PHYSIOLOGY OF OXYGEN fied if, according to the law of diniinishing metabolism TRANSPORT JON MAMMALS* (Zeuthen 1953, Lehmann 1956), the cells of a shrew Heinz Bartels require, on the average, a hundred times more oxygen (per M.D. Tubingen gramme of tissue) than the cells of an elephant ? A corres PROFESSOR OF APPLIED PHYSIOLOGY, UNIVERSITY OF TUBINGEN ponding increase in the size of heart and lungs is unthink Children can ask questions that place adults in quite able; reckoning 1 litre as 1 kg., the lung volume alone represents about 8% of the body-weight—which makes s predicament. Such as: " Why are there elephants ? ", a hundredfold increase impossible. The geometrically or " Why are mice so small ? " Examined more closely. similar structure of mammals reflects itself in a fairly these questions are not so stupid as we are at first inclined constant relation between lung volume and body-weight to suppose. Children have an unspoilt capacity for as graphically plotted (fig. 4) from the work of Tenney wonder—the ©avaaleiv of Epicurus—which adults lose and Remmers (1963). iS too soon as their minds become saturated with facts. Since the organs which transport oxygen through the Another question that might be asked, not by a child 3u: by an adult who is childish enough, concerns the body do not show any large adaptations of structure enabling them to cope with more intense metabolism, we weight at birth of various mammals. Fig. 1 shows that must look for adaptations of function. If the differences weight at birth tends to be related to the length of gestation. are functional, we should expect to find that lung ventila Nevertheless, there must be considerable differences in the tion and cellular blood-supply are proportional not to the rate of fcetal growth; for after the same gestation period i newborn hippopotamus weighs fifty times as much as body-weight but to the intensity of the metabolism. As i newborn chimpanzee. lung capacity cannot be expanded sufficiently to meet the metabolic demand for oxygen, we should expect to find Between seals and whales the difference is even greater. the rate of respiration increased. And in fact we do. As After almost the same gestation period, a newborn whale -s about two hundred times heavier than a newborn seal, fig. 5 shows, the respiration-rate increases in direct propor tion to the metabolic rate; from which it follows that tet the life of all of these animals begins with two cells, ventilation increases with increasing metabolism. which do not vary significantly in size from mammal to •aammal (Linzbach 1955). In fig. 2. constructed by Huggett and Widdas (1951), the comparative differences O fcetal growth-rate are made clear. A modern philosopher has said: " It is an important and necessary indication of intelligence to know just what questions one judiciously should ask." The reason why *"«- may think the question " Why are there elephants ? " BW particularly intelligent is that we cannot conceive an •-aswer for it. Certainly I am not going to provide one. Instead, I -^all modify the question and ask: " How is it possible *«*» elephants and mice both live in the same environ ment ? ". Both are mammals, and very similar in structure and function; yet the intensity of their cellular metabolism ^•cs to the second power and their body-weight to the 7*k (ng- 3) power. How can such very different metabolic aemands be satisfied with the same son of lungs and respiratory organs ? I shall attempt to find an explanation by examining the Process of oxygen transport in these animals from the air 10 the body cells. Most mammals live in an environment which offers theni oxygen at tensions from 120 to 150 mm. Hg. A Pressure gradient between the oxygen tension of the ac*nosphere and that of the body cells allows oxygen to °tPjss energy—so from the atmospherelong as enough to the oxygen cells enterswithout the expenditure lungs and *j*-ansported by the circulating blood to xhe capillaries. ^tow can the oxygen requirements of the cells be satis- Pfcial lecture given on invitation of the University of London to April, 1964, at St. Mary's Medical School, W.2. GESTATION PERIOD (days ) Fig. 1 —Birthweight in mammals as a function of gestation period. ■£.7360 SEPTEMBER 19, 1964 ORIGINAL ARTICLES ventilation of various animals yields some • ? Whale interesting information. The work of Agostoni m nac"m .i. .* . -■ *. -• , . ***'•«. ji 330 390 takes five breaths two hundred, mear fluctuation in alveolar as well "as in arterST" ter-" - ~"- ■--■■--■ tic the size of the animal, and the greater fluctuation. alveolar zas concenrrarinn i<* h**fFV>r-»r* k~ mcreased residual capacity. To achieve their greater respiratory ventilation i Hipp op ot cuni/s Rhinoceros small animals must perform more work in breathi** per gramme of body-weight (Crosfill and Widd? combe 1961). But, as the entire metabolic proce* is more intense in small animals, they do not spend (Red deer)- any greater proportion of their energy on respirator- needs. Thus we see that the varying oxygen requirement! of large and small animals, with similar lung capacitv are served by differences in ventilation. The qu^ 12 \ ! Uon'/PIg aw ^Rot-deer tion now arises whether the increase in ventilation • Puma /• Macacus -7*~ Chimpanzee of the lung is in fact related to increased oxvgenaucc ' "Mandrill of the blood. 1lg^- - Moschus The degree to which the lung is permeable to Uistiti oxygen is called the oxygen capacity of the lung. toat , It is expressed in millilitres of oxygen per minute! ou 120 180 2 40 300 360 420 480 540 Opossum and millimetres of mercury. This figure is obtained Fig. 2-P.ot of cube-root of bix-thweight'agaLt ge.t.tlon-tlme less the ^^^T "^ft Coefficient of Oxygen estimate of t„. (For further information see the original paper of "J*™51011 "** -Ung tlSSUe and lung Capillaries. Thus Huggett and widdas 1951.) ' ' a material constant, as well as the diffusion distance T„In addition,„ , ..„. a• rise. in. «hearr-rarp- , . , and h=>nH<* *-*"-<-• ro ->surface k'<--Tn<M- area r-™A\nr. of the „„„„lung, i . ..is „ available,. „ ° . .. for."* output. The smaller the animal, the higher the heart-rate; surface. so the heart-rate is also roughly proportional to the In man, the diffusion capacity is 20-30 ml. oxygen per intensity of the metabolism. mni.Hg per minute, and the exchange surface is assumed dot N •Maun Manatee ,/ B e a r % / • C o w pig */ Porpoise Goat.*/Man Oog I ..o Raccoon . -Slope = 102 Cat */ NSor-ef-p/j* N. N *»■*• ^w>->A--y«\ *Oog\ S C a t * \ \ R a b b i t • > v \ \ ^ V » P c t r p c / s e \ G o a t * \ N RatiA Gumeo-pig *wv a - o. r * \ \ • Man • »Cow Mmto • \. / % Mouse ^*Shrew %. *• ^ \Bat Dugana 0-0001 I 1 1 1 I I 0*01 0-1 10 10 100 1000 10.000 ; BODY-WEIGHT (g.) BODY-WEIGHT (kg.) g. 4—Logarithmic plot of lung volume a« a function' b ody-wc ight. weight (after Tenhey and Remmers 1963). -EMBER 19* 1964 ORIGINAL ARTICLES THE LANCET BREATHS per min. y' Whole /, 1000 Slope '■hOs^/ .•Cow Pig Bear/ 100 Coat \ * /* Porpoise Man */*. Manatee Dog / Ougotuj 10 r ~ o n - . - . ; - Rabbit • /,t/. R a c c o o n Armadillo •,/ car Woodchucks • Monkey 1*0 Rot /•Bumea-py 0-1 / /Mouse• • S h r e w Bat 0-01 1 1 1 ! 1 1 1000 10.000 Oj ( ml per kg per min.) Fig. 6—Logarithmic plot of alveolar surface area, a function of whole-body oxygen consumption (after Tenney and Remmers 1963). STROKES per mm. 5—Metabolic rate (ml. 0: per kg. per min.) as a function of tquencics of respiration and the heart respectively. .ncrease the amount of oxygen taken up by the blood, ess there is also an increase in the diffusion surface or decrease in the diffusion distance. During physical ivity. the diffusion capacity of the lung does indeed rease, because more blood flows through the lung itself. : an increase in perfusion and ventilation during physi- activity is unlikely to be the single way in which small mals meet their higher oxygen needs. For, even at rest, small animal—in comparison to the large animal—is a relative state of work. Probably- therefore, energy is conserved in small mals by improvement in the diffusion of oxygen in the ■gs. Since the material constants cannot be altered, this ^rovement can be achieved only by shortening the rusion distance or by a relative increase in the diffusion p02 (mm.Hg ) "face area. Macklin and Hartroft '1940) found that the Fig. 7—Oxygen dissociation curve of human blood, saturation per eolar diameter of the shrew is smaller to the second cent as c function of oxygen tension, at pK 7-2-7-6. wet than that ofthe manatee. This means an increase in eoli per unit of lung diffusion surface. glob in concentration of an animal's blood and the intensity II one roughly assumes the alveolar surface to be fully of its metabolism. The oxygen capacity of blood, an -"'jpied with capillaries, there appears to be a rather good expression of its haemoglobin concentration, ranges from "elation between the total alveolar surface and the 16 to 25 ml. oxygen per 100 ml. blood. But cats have -nsity of the particular animal's metabolism (fig. 6). the same oxygen capacity as elephants. *e smaller animals achieve their f- mer rate of metabolism not only by -ive increases in ventilation and -2 •»w* elation but also by relative enlarge- £ 35 -nt of the lung surface involved in ^" : - o u s e x c h a n g e .
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