Organism and Environment as Illustrated by the Physiology of Breathing

What is the significance of the fall in alveolar CO2 pressure at low barometric pressures? It might be thought that the teleological significance at any rate is clear enough, since lowering of alveolar CO2 pres- sure means raising of the oxygen pressure, thus com- pensating to some extent for any want of oxygen caused by the lowered oxygen pressure. But there may be no evident signs of want of oxygen, and lower- ing of alveolar CO2 pressure is in itself a very dis- turbing influence, as has already been shown. When we first observed the persistent lowering of alveolar CO2 pressure in connection with shorter experiments in a steel chamber we thought that lactic acid must have been formed in consequence of oxygen want, and that the persistence of the lowered alveolar CO2 pressure after the experiment was due to lactic acid remaining in the body. But further observations by Boycott and Ryffel failed to confirm this theory ; and the persistence observed after longer observations in the chamber, and stays in the Alps, was far too great to justify the lactic acid theory. As already men- tioned the excess of lactic acid produced by muscular work disappears from the blood within about an hour. Barcroft meanwhile found on the Peak of Teneriffe that the dissociation curve of the oxyhaemoglobin in human blood was displaced to the right if the deter- mination is made in presence of 40 mm. pressure of CO2 (that of the alveolar air at sea level), but was normal if made in presence of the existing lowered alveolar CO2 pressure. From this it could be con- cluded that there is no appreciable change in the reaction of the arterial blood within the body at the higher altitude. The lowered alveolar CO2 pressure

just compensated sensibly for diminished alkalinity of the blood. This we confirmed on Pike's Peak at a higher altitude. As a result of the whole of the Pike's Peak and previous experiments we came to the conclusion that the point of alkalinity to which the kidneys, etc., regu- late the blood is altered in the direction of slightly diminished alkalinity, so that, assuming the reaction of the respiratory centre to alkalinity to be steady, the alveolar CO2 pressure has to be kept lower in order to preserve the balance. The very slight diminution of alkalinity required to account for the increased breathing is so small as to be at present beyond the range of measurement, as already explained. Hassel- balch and Lindhard have more recently published the results of electrometric measurements of the arterial blood alkalinity which show a sensibly unaltered reaction after acclimatisation to lowered barometric pressure in a steel chamber, with the alveolar CO2 pressure much reduced.

It thus appears that the regulation of the alkalinity of the blood by the kidneys and liver is dependent on the oxygen pressure of the air. The change in envi- ronment has altered the setting of the regulator. This is a very striking example of the intimate connection between internal physiological regulation and external environment; but we have now to consider other instances. It has long been known that the percentage of haemoglobin and relative number of red corpuscles increases at high altitudes. Figure 3 represents the

results of Miss Fitz Gerald's observations on the haemoglobin percentages in persons permanently living at different altitudes. These observations were all made by the colorimetric method of determination which I introduced a few years ago, and with a care- fully standardised instrument. It will be seen that just as the alveolar COg rises with fall in the baro- metric pressure, so the haemoglobin percentage rises. It appears also that in an atmosphere with a higher oxygen pressure than air at sea level a decrease in the haemoglobin percentage below what is termed "nor- mal" would occur. Here also, then, the setting of the regulation of haemoglobin percentage is altered by change in environment.

Using the carbon monoxide method of Lorrain Smith and myself, we found that on going to a high altitude not only the percentage amount, but also the total amount of haemoglobin in the blood is increased. The total volume of the blood seems to diminish at first, thus raising the concentration of haemo- globin; but after a few days the volume of the blood increases above normal. The regulation of total haemoglobin, concentration of haemoglobin, and blood volume are thus all dependent on the oxygen pressure of the air breathed.

I now come to what was the most striking result of the expedition. In the lungs the blood is separated from the alveolar air by an extremely thin membrane consisting of the "protoplasm" of flattened epithelial cells. Do these cells play any active part in the gaseous exchange between the air and the blood? Or does the gas simply pass through them by ordinary diffu- sion? This question has been debated ever since a suggestion that they may play some active part was made by Ludwig forty or fifty years ago.

By means of an apparatus known as the aeroto- nometer, Pfliiger and his pupils compared the pressure of CO2 in the blood with that in alveolar air, and found it to be about the same. The aerotonom- eter was then improved by Bohr of Copenhagen, an old pupil of Ludwig. His results seemed to show that sometimes there is a lower pressure of CO2, and a higher pressure of oxygen in the arterial blood than in the lung air, in which case an active secretion of oxygen inwards, and of CO2 outwards, must be assumed. Fredericq then got results in favour of the simple diffusion theory. Last of all Krogh of Copen- hagen improved the aerotonometer still further, and obtained results which again favoured the diffusion theory.

Meanwhile Lorrain Smith and I attacked the prob- lem by a new method, which was suggested to me by the study of CO poisoning, and which eliminated cer- tain sources of very serious error in the aerotonometer method of measuring the arterial oxygen pressure. When blood is saturated with a mixture containing both oxygen and CO the haemoglobin combines partly with CO and partly with oxygen in perfectly definite proportions depending on the relative pressures of the two gases, although in consequence of the far greater affinity of CO for haemoglobin the pressure of oxygen must be about 300 times greater than the

pressure of CO if the haemoglobin of human blood at body temperature is to be divided equally between the two gases. Clearly, therefore, if the pressure of CO is known, and also the percentage saturation of the blood after equilibrium has occurred, the pressure of oxygen can be calculated very exactly. We there- fore breathed an exactly known small percentage of CO until the blood ceased to take up any more CO. We then determined the percentage saturation of the haemoglobin with CO, and the pressure of CO in the alveolar air. From these data we calculated the pres- sure of oxygen in the blood leaving the Im^s. CO, as already mentioned, is, apart from its action on haemo- globin, a physiologically indifferent gas like nitrogen or hydrogen. It is not oxidised in the body, and it appears to pass freely by simple diffusion, like nitro- gen or hydrogen. We could therefore assume that it diffuses freely into the blood and finally reaches a pressure which is the same in the blood of the lungs as in the alveolar air.

In the human experiments we reached the appar- ently unmistakable result that the oxygen pressure in the blood leaving the lungs is considerably higher than in the alveolar air, and that there is therefore active secretion inwards. Experiments with animals showed, further, that when the percentage of CO was in- creased so as to produce symptoms of oxygen want the evidence of active secretion became much more striking. On repeating the human experiments at a later date we could not get the same results. Douglas and I then

improved the method further, and found that both in animals and in ourselves we got results wholly con- sistent with the diffusion theory, provided that the percentage of CO was kept very low. If sufficient CO was given to produce symptoms of oxygen want we got active secretion. We also got active secretion if oxygen want was produced in a group of muscles by fatiguing work. Nevertheless the human experi- ments gave on the whole a much less striking result than the former ones, and we could not at the time see any reason for this.

The apparent acclimatisation to oxygen want in mountaineers or persons living at high altitudes then attracted our attention, and in conjunction with Yan- dell Henderson the Pike's Peak Expedition (in which he, Douglas, Schneider and I participated, while Miss Fitz Gerald made observations at neighboring mining camps and towns) was planned. When we reached the summit of Pike's Peak (14,100 feet) we were all more or less blue in the lips, as were other newcomers. We then suffered in various degrees for two or three days from mountain sickness, after which the blueness entirely disappeared, although our alveolar oxygen pressures remained nearly the same as while the blue- ness was present, and our haemoglobin percentages had not as yet risen appreciably. After this we made a number of determinations of the arterial oxygen pres- sure, and each one without exception showed a consid- erably higher pressure of oxygen in the arterial blood than in the alveolar air. On the other hand, when we breathed during the experiment air rich in oxygen, so

as to bring the alveolar oxygen to about the normal at sea level, the difference between arterial and alveolar oxygen pressure almost disappeared. We then deter- mined the arterial oxygen pressure in a newcomer who was still blue, but did not become mountain-sick till some hours later. It was very little above the alveolar oxygen pressure; but three days later when he was acclimatised and well, his arterial oxygen pressure was as high as our own. The mean result was that on Pike's Peak, after acclimatisation, the arterial oxygen pressure was during rest only about 13 mm. lower than at sea level, but was 35 mm. higher than the alveolar oxygen pressure. The complete absence of any blue- ness after acclimatisation was thus easily intelligible. The lungs were actively secreting oxygen into the blood, even during rest. Nevertheless the blueness reappeared temporarily during prolonged muscular exertions, as in a long climb. The lung epithelium could thus apparently be fatigued by the extra work thrown upon it.

As already seen, the lung epithelium is at all times capable of actively secreting oxygen inwards if the requisite stimulus arising from oxygen want in the tissues is present. But at high altitudes this capacity is greatly increased, and secretion goes on continuously after acclimatisation. The stimulus, moreover, is essentially the same stimulus as produces the changes in the regulation of blood alkalinity and in the haemo- globin of the blood. The stimulus is want of oxygen in some form; but how does the want of oxygen act? The haemoglobin of the arterial blood must, after

acclimatisation, be practically as fully saturated as usual ; and considering the increase in the haemoglobin percentage the amount of oxygen in the arterial blood must be greater than normal. The oxygen consump- tion during rest was the same on Pike's Peak as at sea level, and the circulation rate, so far as our tests could determine it, was about the same.^ Hence the oxygen pressure in the capillaries of the body would be somewhat higher than usual, and our unusually rosy color seemed to confirm this.

The most probable explanation as to how oxygen want produces these effects is that there is some sub- stance which normally undergoes almost complete oxi- dation in the lungs at each round of the circulation. At high altitudes it escapes past the lungs in abnormal quantity in consequence of the lowered oxygen pres- sure, and probably also of the longer time required by the blood in the lungs to reach its full oxygen pres- sure. There are many facts pointing to the assump- tion that such a substance exists and that its presence in the blood is the source of various phenomena accom- panying oxygen want.

The increase in the capacity of the lung epithelium to secrete oxygen is comparable to the increased efficiency produced in almost any organ by increased use. This increased capacity suggests the probable explanation of why in the original human experiments of Lorrain Smith and myself we obtained much more 1 By more accurate tests Krogh and Lindhard have re- cently shown definitely that there is no alteration in the circulation rate after acclimatisation in a steel chamber.

striking results than in the later experiments of Doug- las and myself. The earlier experiments were very long ones, and we were frequently exposing ourselves to oxygen want for many hours at a time. We had probably thus both become more or less acclimatised, so that our lung epithelium reacted very promptly to the slight oxygen want produced by the CO. In no other way can I explain the fact that we were able to breathe with complete impunity percentages of carbon monoxide which in subsequent isolated experiments were found to produce severe symptoms. The same criticism applies to my own early experiments as to the effects of definite percentages of CO. I was breathing CO every day often for hours, and doubt- less had become highly acclimatised to want of oxy- gen, so that I underestimated the effects of CO on ordinary unacclimatised persons.

The part played by the lung epithelium in acclimati- sation to want of oxygen makes it possible to under- stand how mountaineers have succeeded in reaching such great heights as they have. In his recent explora- tions in the Himalayas the Duke of the Abruzzi reached the height of 24,600 feet, the barometric pres- sure being only 312 mm. An unacclimatised person at this pressure is rapidly disabled completely ; but the Duke's party did not suffer at all from mountain sick- ness or other serious physiological inconvenience. Dr. Filippi, a member of the expedition, in his account of it expresses the opinion that there is no such thing as mountain sickness due to rarefaction of the air. He was entirely deceived by the influence of acclimatisa-

tion, just as I was in the case of CO poisoning. On rereading Glaisher's account of his balloon expe- riences I was much interested to see that though he did not clearly understand the cause of mountain sickness he was quite convinced that repeated ascents produced acclimatisation. I have recently found that the effects of acclimatisation can easily be observed at ordinary atmospheric pressure in a closed chamber in which the oxygen percentage has been greatly reduced. An acclimatised person remains of a normal colour, and has no unpleasant symptoms, while an unacclimatised person soon becomes blue in the face, and may faint.

In acclimatisation to high altitudes there are evi- dently three factors — the increased activity of the lung epithelium in absorbing oxygen, the increased breathing, and the increased percentage of haemo- globin. Of these the last raises the oxygen pressure in the capillaries of the body, the second diminishes the fall in alveolar oxygen pressure, and the first raises the arterial oxygen pressure much above the alveolar oxygen pressure, whereas at sea level the arterial oxygen pressure is no higher, as a rule, than the alveolar oxygen pressure. The teleological sig- nificance of these changes seems clear, and a vitalist would naturally point to this as evidence of the inter- ference of the vital principle. But we must analyse the facts further.

We must now attempt to analyse the meaning of the fact that the pressure of oxygen may be, and at high altitudes always is, higher in the arterial blood than in the alveolar air. The layer separating the blood from the alveolar air in the lungs appears under the microscope as an extremely thin layer of moist albuminous material made up of flattened cells. In such a layer gases are soluble, just as they are in water ; and it seems natural that the membrane should take up a gas in contact with it till it is saturated, and give it off on the other side if the gas pressure is lower there. During rest at sea level this is in fact what happens with oxygen, as well as with every other gas which has been tested. The gas passes so readily that complete equilibrium between the gas pressure in the alveolar air and that in the blood has occurred before the blood leaves the lungs ; and the gas pressure in the arterial blood is thus equal to that in the alveolar air. For CO2 and nitrogen this has been shown by the aerotonometer and other methods : for oxygen it has been shown by the carbon monoxide method, the aero- tonometer method being unreliable for oxygen.

suddenly reminds us that it is alive : for it begins to do something which at once recalls living things when it delivers oxygen at a higher pressure than that at which it receives it. The passage of oxygen molecules is accelerated in the inward direction, and this accel- eration applies to them alone, and not to other mole- cules, so it is selective. It does not occur in a non- living membrane, and its presence is evidently depend- ent, firstly upon the peculiarities of the living mem- brane, and secondly upon the presence of a special stimulus acting on the membrane. We know, also, that the specific peculiarities of living tissues depend upon the maintenance of their external environment. Hence we can say that the acceleration depends, not only upon the factors just mentioned, but upon the integrity of the general environment of the mem- brane— in more familiar words, upon its nutrition, temperature, etc., and upon the regulated removal of so-called waste products.

Active secretion of oxygen is not a new phenomenon in physiology. It is now over a century since the famous physicist Biot made the discovery that the gas in the swim bladder of deep sea fishes is nearly pure oxygen. The pressure of oxygen in sea water is only about a fifth of an atmosphere, and is doubtless less than a tenth of an atmosphere in the blood circulating outside the walls of the swim bladder. Yet inside the swim bladder the oxygen pressure in the case of deep sea fishes may be 100 atmospheres or more. It was shown in 1877 by Moreau that fishes secrete just sufficient oxygen into their swim bladders to bring

their specific gravity equal to that of the water at whatever depth they may be, or even to counterbal- ance the effects of a float or weight attached to them. I have in my library Ludwig's copies of Moreau's papers. They are an interesting clue to what was passing through his mind in suggestions he made as to the possibility of oxygen secretion in the lungs. It was discovered by Bohr that the oxygen secretion in the swim bladder is, like salivary secretion, under nervous control ; and Dreser found that oxygen secre- tion can be excited by pilocarpin, a drug which also excites secretion in other glands.

The cells in the wall of the swim bladder which secrete the oxygen are columnar, and arranged like the cells of many other secreting glands, whereas the lung epithelium is extremely thin. Nevertheless the elementary structure of the lung is glandular, just as in the case of the swim bladder ; and both lung and swim bladder are developed as outgrowths of about the same part of the alimentary canal. Before the lungs expand at birth the lung epithelial cells are cubi- cal, and similar to those of other secreting glands. That the secreting cells should be thicker in the swim bladder is natural considering the enormously greater pressure against which the cells have to secrete.

The pressure difference against which oxygen can be secreted in the lungs is evidently quite limited. This is shown by measurements of the oxygen pressures in the blood in CO poisoning, when the stimulus to secre- tion is pushed up to what is presumably a maximum. If there were no limit the secretion of oxygen would afford complete protection, similar to that produced, as already described, when the oxygen pressure of the arterial blood is greatly raised by placing the ani- mal in compressed oxygen. The pressure difference against which oxygen can be secreted in the lungs is also dependent on the pressure of oxygen in the alveo- lar air. When this becomes very low the pressure dif- ference is diminished ; and the flow of oxygen may be actually reversed if the alveolar oxygen pressure is low enough. A similar reversal seems to occur in the case of the swim bladder ; and sometimes the air in the swim bladder seems to be utilized as a store of oxygen, drawn upon when the blood is insufficiently oxygenated by the gills. Possibly the active secretion current is reversed in direction.

Let us now compare the secretion of oxygen with that of other substances by other secreting glands. In the case of the kidney, various salts and crystalloid substances, particularly urea, are actively secreted by the gland cells, so that their concentration in the urine is far greater than in the blood. For instance there is usually about ten or fifteen times as much urea in a given volume of urine as in the same vol- ume of blood, and when the kidneys secrete sugar there may be twenty or thirty times as much sugar in the urine as in the blood. Here then we have other cases of the flow of one kind of molecules being accel- erated in one direction. In the kidney secretion we also see that the acceleration may be in either direc- tion, and that it depends upon the molecular concen- trations in the liquids on the two sides of the secreting

cells. We cannot by any means force up indefinitely the concentration of a substance in the urine; and if the concentration in the blood of a constituent of urine falls below a certain point, the secretion of that con- stituent ceases. If, for instance, the concentration of sodium chloride in the blood falls below normal, sodium chloride disappears at once from the urine, though it is still abundant in the blood. Sugar is not secreted at all by the kidneys unless its concentration in the blood exceeds the normal. In both these cases the acceleration is in the opposite direction to secretion, so that the passage of these substances is actively prevented.

The secretory action of the kidneys is strikingly dependent in other ways on the environment of the secreting cells. Their activity is easily abolished by want of oxygen, for instance, or by minute doses of various poisons, and may be increased by the admin- istration of various drugs. When we look at other cases of secretion we find that often enough some one or other of the sub- stances secreted is not present as such in the blood, but is formed in the secreting cells. Instances of this are the formation and secretion of hippuric acid in the kidney, of urea, bile acids and pigments in the liver, or of casein and milk-sugar in the milk glands. The constituents or precursors of these substances are taken up from the blood, and their combination or decomposition takes place in the secreting cell. The resulting substances are then accelerated outwards from the secreting cell to the duct, while their precur-