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

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 . S , o '^hen oxygen is taken up by the $ ^ 30 "°d in the lung capillaries, it is trans- c *: r-ed to the tissue capillaries. The '~ -- • Go/ der* hdmster^. •Guntc-pig -"•sport medium, blood, is a physico- ^ - -mically specialised fluid that is far g 25 -'er suited for gas transport than j; y other fluid we could think of. <_ Qxygen is loosely bound to the c*> "otnoprotein haemoglobin. One nor- »•*• 21 % finds 12-19 g. of haemoglobin 10 *00 ml. blood. No correlation is p. g_Half.tatura BODY-WEIGHT ( g.) Fig. 8—Half-saturation oxygen tension (T„) as a function of body-weight. All animals' parent, however, between the haemo- bloodblood corrected corrected tot pH 7-4 and 37°C. 602 SEPTEMBER 19, 1964 ORIGINAL ARTICLES

tensions are plotted in relation to 1 < *"". ™ ON small animals have a definitely higher half-satur, So- t e n s i o n . " ^ * t * 80U*-30% j *-nis manner of representation was chc-w

''/Shrew ,32VC ; ]2\^7i/Shrew [ J l«gues and I have added the values for v'

elephant (Bartels et al. 1963a). It is evident Sf il/ : A. Krogh foretold, the larger animals wim reS! 80 100 0 2o 1 8 0 1 0 0 pU2lmmHg)p02 (mm.Hg.) meless there are many exceptions. For", ^TlSSofathdl"rUtl0nTV"*5»*?r^sa«aasr (Wlth """^ P« cent and -«—— volume. *? ^ ™* X^^s8'^^^^ « body-weight of 50 kg., has r" TT-,» ;«,«^-»— -i . ... _ . dehverv of the shrew. rnmn-ir»ri „r,ru .i , globin bond is that with a sufficient amount of oxveen in HrrhT .i-* !*&*** tCnSI°n m *■ dssues of » te the lung, haemoglobin can become saturated andwl"en 2*2? S^te Sri" "V"* desa^2 transport the oxveen ro -hP ri«i„. *«iui. ...-.._. agamst oo 0 for the shrew. In addition the mZ given_off to the cells. The oxygen dissociation lJt [tig. /) presents this relationship quite simply. With mak though whether the shrew acrua.

a mammal to oxygenate the blood in its lung capillaries arterv s pulmon-,- sufficiently at altitudes of about 12,000 ft. as well as at sea level. But, at lower oxygen tensions, between 50 and 1U mm. Hg, the oxygen-binding ability of hemoglobin decreases sharply. This means that when blood saturated with oxygen enters a tissue capiUary where the oxygen tension averages only 30 mm. Hg, almost half the chemic 11 / 31 ally bound oxygen can be given off to the tissue. The quotient AS2/'Ap02 rises as oxygen tension falls. Hence, for particular pressure differences, more and more oxygen can be given off. 6 Not only is the numerical value of this quotient a deciding factor in the delivery of oxygen from blood to WW/A tissue, but so also is the oxygen capacity of the blood and the relationship of the quotient to the dissociation curve At what oxygen tension does the oxygenated hemoglobin begin to undergo drastic deoxygenation ? This point is by no means the same in all ammmals. One can make comparisons by determining at what oxygen tension half the hemoglobin is saturated with oxygen. This is known as the half-saturation tension. If these half-saturation

Eleohant fcetus ( '2 months ) 30 Po-> (mm.Hg) / / -j Fi»-n—Oxygen dissociation curves of k

/ / - Another feature of the respiratory function of mac- 7 / M mdian biood concerns the intra-erythrocvte hemogtocr II ' eventration. Mammalianabout 33°„ hemoglobin, erythrocytes and it was generally thought thatconn= do figure varied very little. In shrews, white mice, and golde • ranging from 45 to 48% (Ulrich et al. 1963), the ware content being appreciably lower. These high hemoglofc /hamsters, however, we found hemoglobin concentrarice ou /u a io 20.30 40 so eo ?b Www «K>raarocm in conjunction with a high^ "Jaj* o2 pressure -,-:, - . ^1^* ™.?^Probably evolved as a result of hyV iiii im^mmmmn mmmmmmmm* - - 2c£SS3S-£*-

TEMBE.t ORIGINAL ARTICLES THE LANCET

Similarly high hemoglobin concentrations were found the red cell of the camel and the llama (Bartels et al. o Elephant

53a), which belong to the same family. Perhaps some- Horse . ; will be able to find a correlation with the extreme \ *k.oX« Cattle rdens placed upon the fluid balance of these animals. Dxygen-binding characteristics differ not only between . • English. Sette* •ious mammals but also during the life-span of the Haemoglobin iividual. The oxygen affinity of the blood is increased Blood x* < Solution "ore birth, and decreases after it. In all cases it falls tow the value for the adult. The half-saturation tension \ blood is 2-10 mm. Hg lower in the fcetus than in the "Goneo-Pic alt (Bartels et al. 1964). Only with a 5 months' elephant °Golder. Homster rjs in Uganda did we find a virtually identical dissocia- Shrew D curve in both mother and fcetus. In an elephant fcetus t White Mouse about 12 months the dissociation curve was shifted to : left, as in all other mammals investigated (fig. 10). -0-4 -0-5 -0-6 -0-7 -0-8 -0-9 -1*0 The shift in the dissociation curve is much greater in A l o g . P q , wborn goats than in the elephant or human. The pur & pH se of this mechanism seems (fig. 11) to be to increase Fig. 13—Logarithmic plot of body-weight as a function of the Bohr ygen uptake from the mother's blood in the placenta. effect in hemoglobin solutions and in blood. Compiled after After birth, the intake of oxygen is achieved more results of Riggs (1960), Hilpen etal. (1963), Baumann et al. (1963), .ciently through lung respiration. The blood no longer and Ulrich et al. (1963). 100 1 needs to have so high an oxygen affinity-, 32 and the resulting shift to the right improves 90 >" " the dissociation of oxygen from blood to 3C s.hdoy . «ssue. 80 as0;-27% Simultaneously with this shift there are 28 * *'0vo1-* _ changes in oxygen capacity: the two values $ 3« 70 1 seem to be closely related. Which is

,6 J? 60 20th doy primary and which secondary I shall not t, S0,-3B% *& — liTso^MX* 3-e'vo: % tT - extent t0 decide; clarified but by the the pictureobservation is to that some the 50 « 3-8vol. % 22 Oz- Capacity oxygen tension of venous blood is one 40 ■ of the factors regulating erythropoiesis 20 (Tribukait 1963). When blood has a rela- 30 ■• 7C 80 tively low oxygen affinity-, comparatively- AGE (doys) p02 ( mm.Hg ) high tensions occur in venous blood, slow- t 12—Oxygen capacity, half-saturation oxygen tension (T,6), and,i, arteriovenous ing down erythropoiesis. Thus it may be iiflerences in kids at the 5th and 20th day (after Bartels et al. 1963b).*3b). that a mechanism not yet fully understood first changes the oxygen affinity- of the blood and then increases or decreases the (Vot.X) rate of erythropoiesis. ■ Human placenta In man. the changes in oxygen affinity Shrew uman placenta . * r , . , . ., , White mouse and capacity after birth are such that a ter given difference in arterial and venous oxygen tensions during the first month of life coincides with a constant difference in the arterial-venous oxygen content (Bartels et al. 1960). This would mean that the oxygen supply is maintained constant Y a k ▶ .* despite the anzemia of the newborn result Dog -+O50 ing from reduced oxygen affinity. Cow t 00 fo understand how the growing organ Guineo pig Goat ism benefits from these changes, it is Camel Rabbit ) White mouse ■ necessary not only to investigate the G°lden hamster Newborn human • « — S h e e p } L a n respiratory function of the blood but also —Pig to obtain the actual in-vivo blood-gas Elephant. obblt values in arterial and venous blood. To be significant, these measurements should be made on non-an£esthetised animals. Elephant Guineaur*o pig pig Tq explain ^g meaning of these changes .. . . we have made every effort to obtain useful Goldenolaennamster hamster' information from , _ young , lambs , , and _„„,„ goats naJep (Bartels et al. 1963b). We placed indwelling catheters in the pulmonary artery and aorta. **• 14—Bohr effect and "effective Bohr effect" In mammals1 (after (aftcr HUpert mipert The animals were then observed for days w * 1M3, and Ulrich et al. 1963). M2 SEPTEMBER 19, 1964 ORIGINAL ARTICLES THE LA* ~- or weeks while they respired through a tracheal cannula or mask attached to a spirometer. In this manner we could —| follow the oxygen consumption, the blood gases, the car diac output, and the respiration from the first to about the thirtieth day after birth. From the first to the twentieth day, the half-saturation tension, as an expression of the shift to the right of the dissociation curve, increased from - 20 to 31 mm. Hg. Simultaneously, the oxygen. capacity decreased almost 30% (fig. 12). On the right side of the figure two in-vivo dissociation curves are shown. At an almost unchanged mixed venous oxygen tensions, an increase in dissociation resulting in better tissue oxida tion occurred. Despite a reduced oxygen capacity, 100 !000 CAPILLARIES (persq.mm.) the difference between arterial-venous oxygen content remained essentially the same. Fig. 15—Logarithmic plot of body-weight as a function of capilu. per sq. mm. in gastrocnemius muscle (data from Schm Also important for oxygen transport in blood is the Nielsen and Pennycuik 1961). Bohr effect. The oxygen-binding ability of haemoglobin is influenced by the amount of carbon dioxide bound to et al. 1946) and cytochrome-C concentration (Drabk the haemoglobin, as well as by the pH of the blood. We 1950). This further enables smaller animals with increas have enough experimental evidence to form ideas as to me metabolic intensity to meet the demands of thi possible purpose of this mechanism 'Manwell I960, metabolism. Riggs 1960, Hilpert et al. 1963). Conclusion Working with haemoglobin in solution, Riggs ''I960) As a starting-point for our discussion we took the law found a good correlation between the magnitude of the diminishing metabolism, and asked die question: How Bohr effect and body-weight 'fig. 13). Accordingly, die it possible, with geometrically similar structures, for ; elephant has the smallest, and the mouse the largest smallest animal to have a metabolic intensity a huncir Bohr effect. One can see from this that the lower oxygen times greater than that of the largest animal ? affinity is a further adaptation of the small animal to its We considered the part played by oxygen transport, a- intensive metabolism. saw that in smaller animals ventilation and perfusi. When, however, one examines our findings on the become relatively greater with increasing metabo extent of the Bohr effect as it actually occurs in the intensity. As metabolism grows more intense, the dema; blood (fig. 13), the preceding correlation is found to be for more gas transport can be satisfied by increased at somewhat diminished. To understand the working of the fusion only when the gaseous exchange surfaces oft Bohr effect in vivo, it must be interpreted in combination alveoli and tissues enlarge. We have also seen th with the oxygen capacity and affinity of the blood. We call improvement in the delivery of oxygen to the tissues this the " effective Bohr effect " (Hilpert et al. 1963)— achieved through lowered oxygen affinity of the blood. defined as the volume per cent given off by 100 ml. blood It will be very valuable if, through new comparati at 50% saturation, on acidification by 0-1 pH unit without experimentation, deviations from these concepts appes a change in oxygen tension. When we look at the results By more objective scrutiny we may eventually attain full of these measurements and calculations in fig. 14, our understanding of that part of Nature of which we ourselv original joy disappears; for we no longer see any correlation are a part. with the intensity of metabolism. As Goethe once said: " The reason I ultimately pre: To complete this discussion, a word must be said about to commune with Nature, is because she is always righ:- the nature of the blood-supply and extent of capillarisation the error can only be on my side." in the organs of various animals. Just as an increase in ventilation does not necessarily REFERENCES Agostoni, E., Thimm, F. F., Fenn, W. O. (1959) J. appl. Physiol. 14, 6' mean a ereater intake of oxvcen bv rh*» bInnH_ <*n an Bartels, H., Buss, I., Kleihauer, E., Luck, C, Metcalfe, J., Riegel, i increased blood-supply to the tissue does not necessarily — Wright,Hilpert, P.P., (1964) Barbey, (to K., be Betke, published). K., Riegel, K., Lang, E. M., Metcalfe mean a greater supply of oxygen to the tissue. In studies — (1963a)— Riegel, Amer. K. J.(1960) Physiol. Arch. 205, ges. 331. Physiol. 271, 169. of the skeletal muscle of the horse, dog, and guineapig — — — (1963b)ibid. 277,6. Baumann, P., Hilpert, P., Bands, H. (1963) ibid. p. 120. August Krogh (1919a and b) found that the smaller the Crosfill, M. L., Widdicombe, J. G. (1961) J. Physiol. 158, 1. animal the greater the number of capillaries per square Drabkin, D. L. 11950) J. biol. Chem. 182, 317. millimetre of muscle tissue. Other researchers have Hilpert, P., Fleischmann, R., Kempe, D., Bartels, H. (1963) Amer. J. Phy- further investigated these findings. Recently Schmidt- Huggett', A. St. G., Widdas, W. F. (1951) 7. Physiol. 114, 306. Krogh, A. (1919a) ibid. 52, 409. Nielsen and Pennycuik (1961) published measurements on — (1919b) ibid. p. 457. — (1940) Comparative Physiology of Respiratory Mechanisz the capillary density in animals ranging in body-weight Philadelphia. Kunkel, H. O., Campbell, I. E., Jr. (1952) J. biol. Chem. 198, 229. from 9 to 450,000 g. These findings agree only roughly — Spalding, I. F., de Franciscis, G., Futrell, M. F. (1956) Amer. with those of Krogh (1919a), for there is only a slight Physiol. 186, 203. Lehmann, G. (1956) Das Gesetz der Stotfwechselproduktion: Handbu difference in the capillary density in such widely dis der Zoologie, vol. vm, part 4, p. 1. similar animals as the rat and the pig (fig. 15). The concept Linzbach, A. J. (1955) Handbuch der allgemeinen Pathologic, vol. vi, pa p. 180. Berlin-Gotungen-Heideiberg. is partially supported by the astoundingly high density of Macklin, C. C, Hartroft, W. S. (1940) Extramural Report CP. 35, Caca Subcommittee on Physiological Aspects of Chemical Warfare. capillaries in the bat. Results are still lacking for such Manwell, C. (I960) Ann. Rev. Physiol. 22, 191. Riggs, A. J. (1960) J. gen. Physiol. 43, 737. large animals as the elephant. Schmidt-Nielsen, K, Larimer, J. L. (1958) Amer. J. Physiol. 195, 424. Finally, it has also been found that compared with — Pennycuik, P. (1961) ibid. 200, 746. Tenney, S. M., Remmers, J. E. (1963) Nature, Land. 197, 54. larger animals, smaller ones have higher tissue cytochrome Tribukait, B. (1963) Acta Physiol, scand. 57, 1. oxidase activity (Kunkel and Campbell 1952, Kunkel Ulrich, S., Hilpert, P., Bartels, H. (1963) Arch. tej. Physiol. 205, 331. Zeuthen, E. (1953) Quart. Rev. Biol. 28, 1.