the heart and pulmonary vasculature of the llama (lama glama)

Thorax (1974), 29, 463.

The heart and pulmonary vasculature ofthe llama (Lama glama)

D. HEATH, P. SMITH, D. WILLIAMS, P. HARRIS,J. ARIAS-STELLA, and H. KRUGER

The Department of Pathology, University of Liverpool, Cardiothoracic Institute, University ofLondon, and the High Altitude Research Unit, Cayetano Heredia Medical School, Lima, Peru

Heath, D., Smith, P., Wifliams, D., Harris, P., Arias-Stella, J, and Kriiger; H. (1974).Thorax, 29, 463-471. The heart and pulmonary vasculature of the llama (Lamaglama). A qualitative and quantitative histological study was made of the pulmonarytrunk and muscular pulmonary arteries of a male and a pregnant female llama born andliving at an altitude of 4,720 m above sea-level in the Peruvian Andes. A similar studywas made on the fetal llama. The individual cardiac chambers of the two adults wereweighed. Our results show that in the adult llama there is no hypertrophy of the rightventricle or of the media of the pulmonary trunk or small pulmonary arteries. Thisappears to be of evolutionary significance in respect of survival at high altitude andsuggests that the llama does not have a sustained significant pulmonary hypertension.The pulmonary arteries of the fetal llama are thick-walled and we associate this withthe physiological pulmonary hypertension of fetal life.

There is now much convincing evidence that inmost species the pulmonary arterial tree respondsto a hypoxic stimulus by constriction. This leadsto an increase in medial thickness of the terminalportions of the pulmonary arterial tree, an ele-vated pulmonary vascular resistance, and conse-quent right ventricular hypertrophy. ThusAbraham, Kay, Cole, and Pincock (1971)subjected rats to a simulated high altitude of5,500 m in a decompression chamber and foundthat during the course of 33 days they showed aprogressive rise in right ventricular mean pressureand weight and the development of muscularizedpulmonary arterioles and increased medial thick-ness of muscular pulmonary arteries. Arias-Stellaand Saldan'a (1963) showed that the same pheno-menon occurs naturally in men exposed to thechronic hypoxia associated with mountain dwell-ing. They found that in the Quechua Indiansliving permanently in Andean towns abovealtitudes of 3,440 m there is a greater amount ofmuscle in the more distal segments of the pul-monary arterial tree. In some species the effectof hypoxia on the pulmonary circulation is severe.Thus cattle living at high altitude in the region ofSalt Lake City may develop 'brisket disease' in

which an increased pulmonary vascular resistanceis produced by the hypoxia and leads on to rightventricular hypertrophy and congestive cardiacfailure (Hecht et al., 1959); the increased pul-monary vascular resistance is associated withmedial hypertrophy of the small pulmonaryarteries (Alexander, 1962). In a previous study wefound that the pulmonary arteries of cats and dogsliving at high altitude are thick compared torepresentatives of the same species living at sealevel (Heath, Castillo, Arias-Stella, and Harris,1969). In the same study we found that the pul-monary arteries of a llama were much thinner andsuggested that this might be of evolutionary signifi-cance in respect of survival at high altitude. Thisview was subsequently challenged by Banchero,Grover, and Will (1971) who illustrated a thick-walled muscular pulmonary artery and amuscularized pulmonary arteriole in a llama takento high altitude in the Colorado Rockies. On arecent expedition to the Peruvian Andes in 1973we were again impressed by the remarkabletolerance of llamas to life at high altitude anddecided to examine the pulmonary vasculature offurther members of this species to check ourprevious findings.

463

D. Heath, P. Smith, D. Williams, P. Harris, J. Arias-Stella, and H. Kruger

METHOD AND MATERIALS

We studied a male and a female llama. Both were2 years old. They had been born and lived all theirlives on an estancia at Rancas which is situated at analtitude of 4,720 m above sea level (Fig. 1). Thefemale llama was pregnant, and this offered anopportunity for the study of the lungs and pulmonaryvasculature of the fetal llama.The animals were killed and the heart and lungs

were removed and taken to the High Altitude Re-search Station at Cerro de Pasco, a few kilometresdistant. There the right lungs from all three llamaswere distended with 10% formol saline until thepleural surfaces were smooth. The lungs were cutinto slices 2 cm thick after fixation and these weretransported to Liverpool for histological study. Blocksof lungs were resected and paraffin sections 5 ,um thickwere cut and stained with haematoxylin and eosin,and by the Miller method to demonstrate elastic tissuewith a Van Gieson counterstain to demonstrate muscleand collagen. A histological investigation of the vari-ous classes of pulmonary blood vessel was carriedout. A quantitative study of the medial thickness ofthe 'muscular pulmonary arteries' was made. Measure-ments were made of the diameter and average thick-ness of the media of arteries up to 250 Am in diameterin the adult llamas and up to 125 Am in diameter inthe fetal llama. Only vessels that were virtually cir-cular in transverse diameter were measured; thismethod of selection decreased the number of arteriesincluded in each study but avoided error in themeasurement of the vessel diameter. The diameterwas taken as the mean of two measurements, at right-angles to each other, of the distance between dia-metrically opposite points on the external elasticlamina. The medial thickness was estimated as themean of four measurements taken at points approxi-mately equally spaced around the vessel wall. From

these data the thickness of the media was expressedas a percentage of the external diameter.

In both cases the heart was dissected free of fat.It was opened and the circumferences of the cardiacvalves were measured. The individual cardiac cham-bers were then separated and weighed according tothe method of Fulton, Hutchinson, and Jones (1952).The weight of the right ventricle was expressed as aninverse ratio of left ventricular weight.

Circumferential rings of pulmonary trunk andaorta were resected 1 cm distal to the respectivevalves. The tissue was embedded, and paraffin sections5 pm in thickness were stained by the Miller methodfor elastic tissue to delineate the media. Ten measure-ments of the medial thickness of both arteries weremade and a mean medial thickness was then calcu-lated for both. A direct ratio of the medial thicknessof the pulmonary trunk to that of the aorta (PT/A)was then calculated for both animals.

RESULTS

The circumferences of the cardiac valves and theweights of the chambers of the heart in the adultllamas are shown in Table I. The medial thicknessof the pulmonary trunk and aorta and the PT/Aratio in these two animals are shown in Table II.The percentage medial thicknesses of muscularpulmonary arteries in the three llamas are shownin Table III.

STRUCTURE OF THE DIFFERENT CLASSES OFPULMONARY BLOOD VESSEL

Pulmonary trunk The pulmonary trunk of thellama is very muscular compared to that of man.In the outer media and extending into the

FIG. 1. Llamas at Rancas inof 4,720 m above sea level.

464

The heart and pulmonary vasculature of the llama (Lama glama)

TABLE IWEIGHTS (g) OF HEART AND INDIVIDUAL CARDIACCHAMBERS AND CIRCUMFERENCES (mm) OF VALVES

IN ADULT LLAMAS

Chamber Female Male Valve Female Male

Right ventricle 605 52 5 Pulmonary 50 52

Left ventricle 206-9 1 560 Tricuspid 98 100and inter-ventricularseptum

Right atrium 15-4 13-3 Aortic 55 55

Leftatrium 15 4 13 4 Mitral 84 85

LV+S 3.5 3-0

RV

Total weight 298-2 235-2

TABLE IIMEAN MEDIAL THICKNESS (Cm) OF AORTA ANDPULMONARY TRUNK AND PT/A RATIO IN TWO

ADULT LLAMAS

Female Male

Aorta 2,510 2,230

Pulmonary trunk 1,620 1,330

PT/A 0 64 057

TABLE IIIPERCENTAGE MEDIAL THICKNESS OF MUSCULAR

PULMONARY ARTERIES IN THREE LLAMAS

Llama No. of Range of Percentage MedialVessels Diameters Thickness

Examined (gLm)Range Mean"

Female 67 35-198 0x57-10-26 3x8

Male 101 28-233 1*82-10-58 4 9

Fetus 94 26-111 8-91-28-57 20-0

'Expressed to first place of decimals.

adventitia are thick bands and nodules of smoothmuscle (Fig. 2). The media also contains muchelastic tissue. In the outer half of the media theelastic fibrils are thick, long, and wavy and ap-pear to be connected by much thinner fibrils.Between the elastic fibrils are bands of smoothmuscle arranged circumferentially in the main andprogressing in size to the muscular nodules at thejunction of media and adventitia described above.In the inner half of the media smooth muscle isless prominent, although individual fibres are seen(Fig. 3). Collagen is more in evidence here. Inthis inner part of the wall of the pulmonary trunkthe elastic fibrils are arranged both circum-ferentially and longitudinally, and as a result even

FIG. 2. Part of transverse section of pulmonarytrunk from an adult llama. The outer third of themedia is shown, the adventitia lying to the left. Thickbands and nodules of smooth muscle (m) are seen tothe left. The elastic fibrils of the outer media, seen tothe right, are thick, long, and crenated. (Elastic vanGieson, X 105.)

at low magnification this inner zone can bedelineated easily. There is a thick internal elasticlamina and immediately beneath this is a verynarrow zone of tightly packed elastic fibrilsarranged longitudinally (Fig. 4).Elastic pulmonary arteries The branches of themain extrapulmonary arteries have much elastictissue within their media and are consequentlytermed 'elastic pulmonary arteries'. Even thesmallest representatives of this class of vessel seenway out in the periphery of the lung have up toeight thick, well-defined elastic fibrils spacedthroughout the media and running circum-ferentially. Finer wispy elastic fibrils are seen be-tween these large fibrils. The elastic tissue tends tobe thicker in the inner media. There is muchcircularly arranged smooth muscle between theelastic fibrils. Smaller amounts of fibrous tissueare present. There is no intimal fibrosis. Theadventitia of collagen contains elastic fibrils.Muscular pulmonary arteries (Figs 5 and 6) Theelastic pulmonary arteries give rise to muscularpulmonary arteries. As the elastic arteries get

F

465


-'asi§:0Small pulmonary veins (Fig. 9) The pulmonaryhb;;\NJSF -4 x WFvenules drain into small pulmonary veins whichhave a thick, crenated elastic lamina external to

fXtt-9tX}.;X a thin muscular media which is ill-defined and4! £sSBff^ U <devoid of a distinct internal elastic lamina. The

elastic fibrils within the media are thin and shortand orientated circumferentially. The small veins

xKv ;*Jet r W M do not show intimal fibrosis. There is a fibrous%

J1g a ~~~~~~~~adventitia.Large pulmonary veins There is a well-defined

g.i X k} media in this class of vessel. The elastic fibrils ofthe media are thinner than in the elastic pul-monary artery. They are well-defined and run

A;j$44,4C,tfE,4Sta ,pC,S circumferentially. Individual smooth muscle fibresICX4V.v "NW.f * ^4& js0g fi btare present between the elastic fibrils. There is

more collagen in the media than in that of theelastic pulmonary arteries, and the tissue com-ponents of the media are less compactly arranged.FThere is no intimal fibrosis. The fibrous adventitia"44.5t\t',.:>............... ;i 4 ^} g $Ft'contains wispy elastic fibrils.

*'^iFETAL LLAMA

Elastic pulmonary arteries (Fig. 10) These haveFIG. 3. Part of the same transverse section. In the a thick media in which are situated about 10-15middle third of the media shown in the figure themuscle is in the form of individual fibres between 4the elastic laminae. (Elastic van Gieson, X 105.)

smaller the number of elastic fibrils within themedia becomes fewer, and finally branches arisewhich have the structure of a 'muscular pul- Kmonary artery'. In spite of this designation, taken v-from the classification of Brenner (1935) for 34 jcomparable vessels in the human lung, the mostcharacteristic feature of the 'muscular pulmonaryartery' in the llama is that it is very thin-walled. VThe media consists of a thin layer of circularly kjtJ .S Wdorientated smooth muscle sandwiched between }minternal and external elastic laminae. Small wisps jof elastic tissue are also seen within the media.There is no intimal fibrosis. The adventitia ofcollagen contains an admixture of thin elasticfibrils.Pulmonary arterioles (Fig. 7) These arise as sidebranches and terminations of the thin-walledmuscular pulmonary arteries, and their walls + i Vconsist of a single elastic lamina. The lung ofthe adult llama is not characterized by large num-bers of thick-walled, muscular arterioles like those v " 7found in the systemic circulation of mammals. FIG. 4. Part of the same transverse section. The

inner third of the media is shown, the intima lyingPulmonary venules (Fig. 8) The venules resemble to the right. There is a thick internal elastic laminathe arterioles in structure with walls consisting and beneath this is a narrow zone of tightly packedof a single elastic lamina. They arise by confluence elastic fibrils, many of which are orientated longi-of pulmonary capillaries. tudinally. (Elastic van Gieson, X105.)

466


*W.- ..., ..

*,0..: .48

\V1-

FIG. 6. Part of the preceding figure at higher magni-fication to show the thin media and the elasticlaminae. (Elastic van Gieson, X1,270.)

FIG. 5. Transverse section of a muscular pulmonaryartery from an adult llama showing the thin mediaof circularly orientated smooth muscle bounded byinternal and external elastic laminae. Contrast withthe thick-walled pulmonary artery of the fetal llamashown in Fig. 11. (Elastic van Gieson, X345.)

concentric elastic laminae which are thick andwell defined. Wispy elastic fibrils are found be-tween the main laminae. There is no intimalfibrosis. A thick fibrous adventitia contains a fewthin elastic fibrils.

Muscular pulmonary arteries (Fig. 11) In strik-ing contrast to the state of affairs in the adultllama, the muscular pulmonary arteries of thefetus are very thick-walled and composed ofcircularly orientated smooth muscle bounded byinternal and external elastic laminae. Theendothelial cells are prominent but there is nointimal fibrosis.

Small pulmonary veins These are composed of athick elastic lamina lined on its inner aspect by afew smooth muscle fibres and a surroundingfibrous adventitia.

Large pulmonary veins In this class of vessel themedia is thinner than in the elastic pulmonaryarteries. The elastic laminae are thin and indi-vidual muscle fibres are seen between themtogether with considerable amounts of collagen.There is no intimal fibrosis.

FIG. 7. Part of transverse section of a muscular pul-monary artery (m) from an adult llama showing theorigin of an arteriole (a). The media of the artery isvery thin and consists of muscle sandwiched betweeninternal and external elastic laminae. The arterioledoes not have a muscular media. (Elastic van Gieson,X345.)

467

T.! I'iliv... .'

% ipk. .4 i

..m..:::-'.&:

f. 14,


..~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.

0kK~ ~ ~......'.'....,.:.,:X

FIG. 8. Transverse section of a pulmonary venule from the fetal llama. Itconsists of a single elastic lamina. (Elastic van Gieson, X600.)

P.L ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.,.},~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.......

FIG. 9. Transverse section of a small pulmonary vein from an adult llama. Themedia is muscular and the outer elastic lamina is thick. (Elastic van Gieson,X375.)

468

The heart and pulmonary -vasculature of the llama (Lama glama)

I.

3- .

Af

FIG. 10. Part of transverse section of an elastic pul-monary artery from a fetal llama. The thick internalelastic lamina is seen to the right and the fibrousadventitia to the left. Within the thick media arewell-defined thick elastic laminae. (Elastic vanGieson, X230.)

t, ' tnL '; v-Z_~~~~~~Ax

FIG. 11. Transverse sectionof a muscular pulmonaryartery from the fetal llama.The media is very thick andmuscular and is bounded byinternal and external elasticlaminae. This is in strikingcontrast to the very thinmedia of the 'muscular pul-monary artery' of the adultllama. (Elastic van Gieson,X552.)

469

qX. 01, - t. *S


DISCUSSION

Comparison of quantitative data on ventricularweights in the two adult llamas with values ac-

cepted as normal in man suggests that thepulmonary circulation in the llama is not subjectedto a sustained rise in pulmonary arterial pressure.Thus the ratio of left to right ventricular weightin these two animals was 3-5 and 3 0, while therange given as normal in man is 2-3 to 3-3(Fulton et al., 1952). Similarly, the PT/A ratiosin the two llamas were 0 64 and 0 57, values whichlie in the normal range of 0-4 to 0-7 for thehuman lung after the age of 2 years (Heath,DuShane, Wood, and Edwards, 1959). The per-

centage medial thickness of the small pulmonaryarteries in the two adult llamas was respectively3-8 and 4*9 and in our previous investigation of athird adult llama it was 3-6 (Heath et al., 1969).In the human lung a percentage medial thicknessof less than 5 may be regarded as normal (Heathand Whitaker, 1955). Thus, although the llama isborn into and exposed throughout life to thechronic hypoxia of high altitude, it does not showeither muscularization of the terminal portion ofits pulmonary arterial tree or right ventricularhypertrophy when compared with observations inman and other mammals. This suggests to us thatthe llama does not have a sustained significantpulmonary hypertension.The llama has lived in the Andes for many cen-

turies and it seems likely that the lack ofstructural change in its heart and pulmonaryvasculature in the face of chronic exposure to thehypoxia of high altitude is an expression ofevolutionary adaptation to these conditions. Thedata we have collected on the present two animalssupport our findings and conclusions from aprevious study on a single llama (Heath et al.,1969). Such adaptation is generally regarded as a

genetic, heritable, anatomical or biochemicalmodification of an organism which has enabled itto explore a particular environment to the bestadvantage.Banchero et al. (1971) doubt this view partly

on the grounds that in three young male sea-levelllamas, aged 5, 7, and 14 months respectively,who were born and bred at St. Louis, Missouri theaverage resting mean pulmonary artery pressurerose from 14 mmHg to 23 mmHg after spending10 weeks at an altitude of 3,420 m at Climax,Colorado. There was an increase of resistance toblood flow in these animals. We interpret thesefindings as indicating that in sea-level llamas atleast, acute exposure to hypoxia may produce a

mild pulmonary hypertension. However, we be-lieve that our anatomical and histological findingsshow that in llamas born and living at highaltitude chronic hypoxia does not induce signifi-cant chronic pulmonary hypertension with associ-ated structural changes in the right ventricle,pulmonary trunk, and terminal portions of thepulmonary arterial tree such as occur in theQuechua Indian (Arias-Stella and Saldania, 1963;Saldafia and Arias-Stella, 1963). Banchero and hisassociates (1971) also base their doubts on theirobservation of the pulmonary vasculature in oneof their llamas who died from pulmonary throm-boembolism. These authors do not providequantitative data on the pulmonary arteries astheir paper is physiological rather than anatomicalin nature. Instead they illustrate a single pul-monary arteriole showing muscularization of itsmedia and a single muscular pulmonary arteryshowing a very thick media. We find it difficultto reconcile these histological features with ourown findings on one llama previously (Heathet al., 1969) or in the two adult llamas describedhere. The thinness of the media of the muscularpulmonary artery of the adult llama can beappreciated if it is compared with the thick smallpulmonary arteries of the lung of the fetal llama(Figs 5 and 11). In the fetus the mean medialthickness is in the region of 20% (Table III) andthis is explained by the physiological pulmonaryhypertension which characterizes the fetus.

Adaptation to life at high altitude exhibited bythe llama may be contrasted with acclimatizationshown by the Quechua Indian. Acclimatization isa reversible, non-inherited change in the anatomyor physiology of an organism which enables it tosurvive in an alien environment. Changes in highaltitude man which appear to aid acclimatizationinclude hyperventilation, a decreased A/a gradientsaid to be associated with dilated alveoli and capil-laries facilitating the area for gas exchange,polycythaemia, an increased myoglobin content,and so on.However, not all the changes which are found

in the Quechua Indian can be accepted withoutreservation as being due to acclimatization orindeed as desirable. Thus, as noted above, heresponds to the chronic hypoxia of mountaindwelling by chronic pulmonary hypertension withassociated right ventricular hypertrophy,muscularization of the pulmonary arterioles, andhypertrophy of the pulmonary trunk. It has beensuggested that the elevated pulmonary arterialpressure may aid perfusion of the pulmonary

470


capillary bed, facilitating gas exchange. How-ever, all these changes can be induced in rats ina hypobaric chamber in the laboratory and willeventually terminate in death from congestivecardiac failure. They do not occur in the rightventricle or pulmonary circulation of the llamaand it seems likely that this has favoured the sur-vival of this species at high altitude.

It is of interest that, although the small pul-monary arteries of the llama are thin-walled, itspulmonary trunk is very muscular compared tothat of man. Nevertheless its histology, exten-sibility, and chemical composition do not differsignificantly from those of the pulmonary trunkof other high-altitude animals that we haveexamined previously (Heath, Harris, Castillo, andArias-Stella, 1968). Chemically the content ofelastin in the pulmonary trunk of the llama issimilar to that found in man, although theamount of collagen is lower (Castillo et al., 1967).The ratio of elastin to collagen tends to increasewith age in man so that the figures we found fora single llama are higher than one would expectfrom man at sea level but comparable to thosefound in persons living at high altitude who havean aortic pattern of elastic tissue in the media ofthe pulmonary trunk. In spite of its musculoelasticstructure the extensibility of the tissue of thepulmonary trunk of the llama is greater than theaverage found in the first two decades of life inman (Heath et al., 1968). This is probably becauseextensibility is simply a direct function of timerather than a proportional function of the averagelongevity of the species.

Professors Harris and Heath acknowledge the receiptof a research grant from the Nuffield Foundation toundertake this study.

REFERENCES

Abraham, A. S., Kay, J. M., Cole, R. B., andPincock, A. C. (1971). Haemodynamic andpathological study of the effect of chronic hypoxiaand subsequent recovery of the heart and pul-monary vasculature of the rat. CardiovascularResearch, 5, 95.

Alexander, A. F. (1962). The bovine lung: normalvascular histology and vascular lesions in high

mountain disease. Medicina Thoracalis (Base!),19, 528.

Arias-Stella, J. and Saldania, M. (1963). The terminalportion of the pulmonary arterial tree in peoplenative to high altitudes. Circulation, 28, 915.

Banchero, N., Grover, R. F., and Will, J. A. (1971).High altitude-induced pulmonary arterial hyper-tension in the llama (Lama glama). AmericanJournal of Physiology, 220, 422.

Brenner, 0. (1935). Pathology of the vessels of thepulmonary circulation. Archives of InternalMedicine, 56, 211.

Castillo, Y., Kruger, H., Arias-Stella, J., Hurtado, A.,Harris, P., and Heath, D. (1967). Histology, ex-tensibility, and chemical composition of pul-monary trunk in persons living at sea-level and athigh altitude in Peru. British Heart Journal, 29,120.

Fulton, R. M., Hutchinson, E. C., and Jones, A. M.(1952). Ventricular weight in cardiac hypertrophy.British Heart Journal, 14, 413.

Heath, D., Castillo, Y., Arias-Stella, J., and Harris,P. (1969). The small pulmonary arteries of thellama and other domestic animals native to highaltitudes. Cardiovascular Research, 3, 75.DuShane, J. W., Wood, E. H., and Edwards,

J. E. (1959). The structure of the pulmonarytrunk at different ages and in cases of pulmonaryhypertension and pulmonary stenosis. Journal ofPathology and Bacteriology, 77, 443.

Harris, P., Castillo, Y., and Arias-Stella, J.(1968). Histology extensibility and chemical com-position of the pulmonary trunk of dogs, sheep,cattle and llamas living at high altitude. Journalof Pathology and Bacteriology, 96, 161.

and Whitaker, W. (1955). The pulmonary vesselsin mitral stenosis. Journal of Pathology andBacteriology, 70, 291.

Hecht, H. H., Lange, R. L., Carnes, W. H., Kuida,H., and Blake, J. T. (1959). Brisket disease. I.General aspects of pulmonary hypertensive heartdisease in cattle. Transactions of the Associationof American Physicians, 72, 157.

Saldafia, M. and Arias-Stella, J. (1963). Studies on thestructure of the pulmonary trunk. III. Thethickness of the media of the pulmonary trunkand ascending aorta in high altitude natives.Circulation, 27, 1101.

Requests for reprints to: Professor D. Heath,Department of Pathology, University of Liverpool,Liverpool L69 3BX.

471

the heart and pulmonary vasculature of the llama (lama glama)

Documents