Paper - The terminals of the human bronchiole (1922)
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Willson HG. The terminals of the human bronchiole. (1922) Amer. J Anat. 30: 267-296.
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The Terminals of the Human Bronchiole
Herbert G. Willson
University of Toronto
Nine Figures
In the hope of throwing some light on certain questions about which there has been controversy, the construction of a wax model of a respiratory bronchiole was begun at the University of Toronto in October, 1919. The work was carried out in collaboration with Prof. J. Playfair McMurrich, by whom the problem had been suggested and to whom the writer is very greatly indebted for advice and assistance.
The extreme complexity of the terminal branches of the bronchial tree is not generally appreciated. The maze of channels which occur even in a minute piece of lung tissue cannot be visualized accurately from a mere comparison of serial sections. The larger passages of the lung may be injected with wax or metal and a cast obtained by corroding away the lung tissue, but one cannot be certain of obtaining in this way a complete cast of the smaller tubes. The method of wax reconstruction of serial sections is the only plan by w^hich one may hope to get clear ideas regarding the finer tubes, and even this method is especially difficult to apply to the bronchioles. So complicated are the branchings and so carefully has nature economized space in the lung, that if all the air passages in a piece of lung tissue are reconstructed in wax on a magnified scale, the result is practically a solid, and the model has to be dissected in order to show the relationships of tubes and air-cells.
Malpighi in 1661 demonstrated the vesicular nature of lung tissue and showed how the trachea terminates in bronchial filaments, but after his time there was no important contribution to the knowledge of the histology of the lung until the early part of the nineteenth century, when Soemmering, Rossignol, Reisseisen, and others pubUshed the results of their researches, and Henle, by his discoveries in general histology, laid the foundation for many special investigations. Controversy now arose in regard to such questions as the exact shape of the terminal bronchioles, their method of branching, and as to whether or not there were direct communications between alveoli.
Rossignol, writing in 1847, refers to the most distal divisions of the bronchial tree as 'infundibula,' and these he describes as being thickly beset with alveoli. He notes that the alveoli of the infundibulum are of an unusually great depth, and that while the alveoli are scattered and few in the proximal part of the respiratory bronchiole, they are soon arranged close together, covering the whole surface of the last bronchial divisions. As to the method of branching, he concludes that there are both dichotomous and trichotomous divisions. In his investigations Rossignol inflated and dried the lung, after having injected the blood vessels.
In 1860 Waters described monopodial, dichotomous and trichotomous branching. His conclusions were based on the study of single sections. In man he found no alveoli in the terminal bronchiole, but only in the infundibulum. He states that at a certain place the terminal bronchiole widens into a cavity into which open six, eight, or ten canals, beset with alveoli. These canals he terms air-sacs, these being again identical with Rossignol's infundibula.
F. E. Schulze in 1871 used the term 'Alveolengang' to denote all the parts of the tubular system on which there are alveoli, excepting, however, the terminal sacs, for which he employed the term infundibula.
In 1892 W. S. Miller announced the discovery of a new element in the series of pulmonary air-spaces, terming it the 'atrium' and locating it between the air-sacs (infundibula) and the terminal bronchiole (alveolengang). This space seems to be identical with the enlargement of the terminal bronchiole described by Waters as giving origin to the air-sacs, but Miller describes it as something more than a mere enlargement, having a more or less spherical form with numerous alveoli on its walls and giving origin to from two to five air-sacs. The lung of the dog was used in Miller's first investigation; but in 1900 and again in 1913 he published accounts of further researches, and maintained that his description held good for lung of cat, ox, child, and adult man.
In his article of 1900 he gives the following table of nomenclature for the air-spaces of the lung:
B. X. A.
8CHAFER
SCHULZE
KOLLIKER
Bronchus
Bronchiolus respiratorius
Bronchial tube
Alveolengang
Alveolengang
Terminal
Ductuli
Lobular
bronchiole
alveolares
bronchus
Atrium
Air-sac
Air-sac
Infundibulum
Infundibulum
Air-cell
Alveolus pulmonis
Air-cell
Alveolus
Alveolus
But at the same time he revised his own earlier terminology, substituting the B. N. A. terms for 'Bronchus' and 'Terminal bronchiole.' In his investigations Miller used the method of wax reconstruction. His results have found wide acceptance by the authors of text-books, in spite of several dissenting voices.
In 1900 Justesen gave an account of his investigations of the structure of the lung in oxen. He used corrosion preparations and also serial sections, drawings of which were made on transparent paper, so that by superposing the drawings, successive sections might be compared. He finds that each 'bronchiolus simplex' forms dichotomously two respiratory bronchioles, each of which again divides dichotomously, each of the branches so formed ending in a large cavity which he identifies with the atrium of Miller. His atria are variable in size, sometimes quite distinct, and sometimes only slight enlargements of the bronchioles, and while Miller finds two to five air-sacs on each atrium, and Waters six to ten, Justesen believes that there are normally four. Two first bud out and these then divide, so that each of the four occupies a position corresponding to one of the angles at the base of a four-sided pyramid, the apical angle of which is occupied by the atrium. In other words, the air-sacs do not arise as accidental growths from the atria, but are formed by two successive dichotomies in planes at right angles. In the adult ox he found occasionally but three air-sacs on an atrium — a condition which he explains by supposing that in the case of the primary air-sacs the secondary dichotomy had failed owing to space exigency.
Justesen holds that the bronchial branchings occur in a definite mathematical plan and are fundamentally dichotomies, but that in the majority of the branches the dichotomy becomes modified into a sympodial arrangement, the terminal branches still retaining the dichotomous plan. If this be so, and the mathematical regularity of the dichotomies persist, the lateral branches of each sympodial stem might be expected to show a decreasing number of air-sacs as they were traced peripherally, one arising from an earlier dichotomy having twice as many airsacs as that which arose from the succeeding dichotomy. Justesen believed that he obtained evidence in favor of this arrangement in his observation on the pig where the eparterial bronchus gave rise to as many lateral branches as did the stem branches for the rest of the lung.
F. E. Schulze, writing in 1906, takes the view that Miller's atria are not new spaces, but only those parts of the ductuli alveolares into which the sacculi open. He states:
So wenig, wie man an einem sich unregelmassig verzweigenden Baumast diejenigen Stellen, wo sich ein Ast in zwei oder auch mehrere Endaste teilt, als besondere typische Stellen charakterisieren und mit einem eigenen Namen, sondern einfach als Teilungsstellen zu bezeichnen pflegt, so wenig scheint mir in dem respiratorischen Gangsystem der Lunge die Auszeichnung dieser Stellen durch eine besondere Benennung ('Atrium') erforderlieh oder auch nur zweckmassig zu sein.
Schulze claims that normally in man and in many mammals there are direct communications between alveoli — 'alveolar pores' — and in this view he is supported by Hansemann, Hassall, Zimmerman, Nicolas, and Merkel, but is opposed by Piersol, W. S. Miller, Laguesse, and Oppel.
In 1907 J. Miiller investigated the lungs of most of the domestic animals, using metal corrosions as well as sections. His conclusions regarding the occurrence of atria he states as follows :
Hinsichtlich des neuen Luftraumes, des Atriums, war es mir nun weder an den Korrosionspraparaten noch an den Schnitten bei irgend einem unserer Haussaugetiere moglich, ihn als einen Luf traum sui generis bestatigen, Wenn auch da und dort einmal ein Alveolengang vor seiner Auflosung in die Infundibula eine buchtige Erweiterung zeigte, welche etwa dem 'Atrium' Justesens entsprechen konnte, so habe ich doch niemals zwischen jedem Infundibulum und dem Alveolargang, noch auch zwischen mehreren Infundibeln und einem solchen einen, oder mehrere kugelige Hohlraume eingeschaltet gesehen, welche fiir das konstante Vorkommen der Millerschen Atrien sprechen konnten.
Muller found alveolar pores in various animals, but not in young animals. He thinks these are pathological.
Just as my first model was completed, I received the number of The American Journal of Anatomy that contained two artir cles by the Japanese investigator Ogawa, who by an interesting coincidence had been working in the University of Kyoto at exactly the same problem as myself and by similar methods, but had evidently begun the construction of his model some months before I started with mine. Ogawa worked with human material, and constructed both a negative and a positive model of the terminal branchings of the lung, the former being at a magnification of 100 diameters and measuring 8 x 12 x 8 cm.^ while the latter was enlarged 80 diameters and measured ll.S X 24 X 20 cm. Like Schulze and Miiller, he reaches the conclusion that Miller's atrium is an unnecessary term, at least for the human lung." In his second paper he states that alveolar pores are normally found in many mammals, and only seldom cannot be seen." Further reference to Ogawa's work will be made when his results are compared with my own.
Material
The material used was exclusively human and consisted of portions of the lungs of two individuals obtained at autopsies performed soon after death. One of the individuals was a woman of thirty years who had died of heart disease (mitral stenosis), while the other was a child whose age could not be definitely ascertained, but was certainly less than thirteen years. In the case of the adult, portions of suitable size were taken from the lungs and immersed in Bouin's fluid, but in the case of the child's lung the entire organ was first injected through the bronchus under gentle pressure with Bouin's fluid, and then immersed in the same fluid, portions suitable for sectioning being taken only after the tissue had been fixed in this manner. The portions selected were carried through the various grades of alcohol and imbedded in paraffin, and to secure satisfactory penetration of the paraffin they were, while in 70 per cent alcohol, placed under the bell-jar of an air-pump and the air exhausted till bubbles ceased to rise from the cut surface of the tissue.
By this method a perfect infiltration of the paraffi.n was obtained, and the tissue was cut into serial sections, 20 ju thick in the case of the adult lung and 30 /x in that of the child. Both series were stained with Weigert's elastic tissue stain, this being chosen with the intention of studying later the distribution of the elastic fibers in the human lung. Wax reconstructions of the air spaces, i.e., negative reconstructions of portions of each lung were made at a magnification of 100. To ensure accuracy in the superposition of the wax plates in the model of the adult, numerous bridges were left in cutting out the air-spaces, but in the second model the necessary accuracy was obtained by the use of a duplicate series of drawings of the sections made upon transparent paper. A duplicate drawing of a section about the middle of the series was covered by a sheet of glass, and on this the pieces of wax representing the corresponding air-spaces were placed one after the other, as they were cut from the wax plate. The next succeeding drawing was then carefully oriented upon that first chosen, so that the position of the air-spaces shown in the one could be accurately determined with reference to those of the other, and from the information thus obtained the pieces of wax representing the air-spaces of the second section could be accurately adjusted on those cut from the first plate. Dealing in this way with successive drawings and wax plates, half the model was built up. This completed portion was then detached from the sheet of glass, turned upside down, and the other portion of the model was then built up in the same way. This method of orientation was found to be much more economical of time than was the use of bridges and entailed no sacrifice of accuracy.
To follow a respiratory bronchiole from its beginning to its terminals, it was necessary to^make drawings from 128 sections of the adult lung. As each section was 20 ^ thick, it is evident that the piece of lung containing all these branches had a thickness of 2560 M, i.e., 2.56 mm. or a little over 1/10 of an inch. It will be evident also that the height of the completed model would be 256 mm., or a little over 10 inches. In the case of the child's lung, drawings of sixty-three sections were required, and as each section was 30 ^ thick, the thickness of the piece of lung reconstructed was 1890 /x, or 1.89 mm., and the height of the completed model approximately 7.5 inches.
Results
The first impression received from inspection of the completed models is that the branchings of a respiratory bronchiole are far more complicated than is revealed in the text-books, and one feels also that there is difficulty in 'labeling' the various parts according to the terms commonly used. In some places a number of alveoli are represented in the reconstruction as opening into a cavity which seems too small to deserve the name of an air-sac, while in another place one finds an alveolus which is several times as large as the ordinary alveolus. The models indicate that the minute passages in the lung are not formed in strict accordance with the usual descriptions. The two models when placed side by side suggest at once that the child's lung is a miniature of the adult lung, just as the child's hand is a miniature of the adult hand, there being no apparent difference in complexity of structure. More air-sacs occur in the volume of child's lung represented than in the greater volume of adult lung represented in the first model.
A photograph of the model from the adult lung is shown in figure 1. It starts with a non-respiratory bronchiole which is marked 3a, and is so designated because in tracing it back through the serial sections it was found to represent the third dichotomy from a bronchus which contained cartilage in its wall. The 3a dichotomizes into branchings marked ^a and 4^, of which 4b has not been followed any further, but ^a again divides dichotomously into 5a and 5b, whose walls show alveolar outbranchings, so that they are to be regarded as respiratory bronchioles. The 5b is followed only a short distance, but 5a again divides into two stems, one of which was followed for some distance, but its reconstruction is omitted in the photograph for the sake of simplicity. The other stem, which may be designated 6a, is completely reconstructed, and gives rise to all that portion of the model which is colored. It can be followed into a further dichotomy, one branch of which gives rise to the portions colored orange and green, while from the other all the remaining portions originate. The orange and green portions have been separated from the rest of the model in order that its parts might be more completely shown.
A photograph of the model on this scale, though useful for a general orientation of its parts, does not sufficiently reveal the details. These are more clearly shown in figure 2, which represents a part of the portion colored orange in figure 1 at a greater magnification. It shows a number of infundibula or air-sacs with their alveolar outbranchings, and it shows also how difficult it is to determine exactly what shall be termed an air-sac and what an air-cell. Thus the lower of the two portions colored yellow might equally well be regarded as a single air-sac with a number of complicated air-cells, or as at least two air-sacs with a common basal portion. Similarly, the upper yellow portion might be regarded as a single large air-sac or as three, according to the point of view of the observer. The terms infundibulum or air-sac (ductulus alveolaris B. N. A.) and alveolus or air-cell are all useful in conveying an idea as to the arrangement of the terminal air-spaces of the lung, but it must be remembered that in the human lung, at least, transitions exist between them; particular cases may be found where it is difficult to say whether one is dealing with an air-sac or an air-cell.
To obtain a clearer picture of the terminal branchings, those represented in both models were projected upon a single plane, the projections being based partly on tracings of various sections used in the construction of the models, and partly on sketches of the smaller parts. The result obtained in the case of the model of the child's lung is shown in figure 3. The stem marked i is a non-respiratory bronchiole which was traced through seventy-four sections (2.22 mm.) to reach its origin from a bronchiole with cartilage in its wall. Four dichotomies occurred in this distance. In the model of the adult lung three dichotomies occurred between the first respiratory bronchioles and the bronchiole with cartilage in its wall. The stem (1) divides into two branches, only one of which (2) was followed; this was a respiratory bronchiole, alveoli occurring on its wall. It in turn undergoes a dichotomy, only one limb of which (5) was followed, and then two additional dichotomies succeed in rapid succession, only one branch of each being followed. That followed from the last of these dichotomies (6) again divides into 7 and 8, these again into 9 and 10 and 11 and 12, respectively, but beyond this the branchings become irregular, and while it would be possible to interpret some of these divisions as dichotomies, there are others where the branching could be more accurately termed a trichotomy. In fact, the branching in some parts is so irregular that almost any 'method' might be read into it. The truth seems to be that, as the terminals are approached, no one system of branching is followed, but one edict is obeyed, i.e., that there must be no waste of space.
Embryological investigation has shown that in the early development the branching is dichotomous, and apparently this is continued, with some modification in certain of the branches, until there comes a time when the small bronchioles are competing with one another for space, and then they branch or send out processes in any possible direction. This competition for space of the infundibula and alveoli, seen in the complicated interdigitation of these elements from different respiratory bronchioles and in their varying form and size, is the most striking impression that one receives from a study of the models. An idea of the manner in which the infundibula fit in with one another may be obtained from figure 4, which is a photograph of the pleural surface of the model of child's lung. The numbers on the infundibula correspond with those on the branchings shown in figure 3.
From the diagram and the accompanying photographs it will be evident that the models reveal no definite space which corresponds to Miller's atrium. Careful study of the models shows, it is true, enlargements of the respiratory bronchioles where several infundibula communicate with them, but these enlargements exhibit no definite delimitation from the remaining portions of the bronchioles, and they never assume a spherical form. Schulze was probably correct in his contention that a special name is not needed for that part of a branch from which a number of subordinate branches arise.
It is interesting to note that Justesen's description of the branchings of a respiratory bronchiole applies very closely to the branchings revealed by the models, except in regard to the atrium. Both Waters and Justesen held very decided views as to the planes in which successive branchings occur. Waters believed the plane of two diverging branches to be always at right angles to the plane of the two branches preceding. Justesen claims that there is a strong tendency of the dichotomous divisions to lie in alternating planes cutting one another at right angles," but this was not universal. Examination of the models indicates that Waters' rule is by no means constantly true. Four angles which, according to Waters' rule, would be 90°, were found to be approximately 85°, 90°, 10°, and 45°.
The number of branchings that intervene between a nonrespiratory bronchiole and an air-sac was determined in seven cases, and in three the air-sacs were reached at the fifth division, in three at the sixth, and in one case at the seventh. Ogawa found from two to nine ramifications, with an average from fourteen cases of 5.57. Laguesse found six or seven branchings.
The alveoli or air-cells on seven difi"erent air-sacs were counted, the numbers being as follows: 22, 14, 16, 18, 12, 16, and 20, giving an average of 16.8. Ogawa's average is 11. However, many of the air-sacs, as Justesen says, are bifurcated or deeply indented, and a good deal depends on whether the subdivisions are considered as separate air-sacs or not.
Calculating from the lengths of the tubes in the model the following are the actual lengths of these tubes in the adult lung.
mm.
No. 1 1.6
No. 2a : 0.8
No. 3a 0.5
No. 4a 0.5
No. 5a 0.5
No. 6a 0.4
No. 6b 0.2
The greatest and least diameters of the non-respiratory bronchioles represented in the same model were estimated as follows :
mm.
No. 3a 0.3 X 0.40
No. 4a 0.3 X 0.25
No. 4b 0.4 xO.22
Similar estimates in the case of the respiratory bronchioles were :
mm.
No. 5a 0.4 x 0.30
No. 5b 0.3 xO.35
No. 6a 0.3 X 0.30
No. 6b 0.4 X 0.30
Measurements of the greatest and least diameters of those tubes into which the air-sacs open gave the following results in three cases, — the figures indicating the actual dimensions in the lung :
0.3 mm. X 0.2 mm.
0.5 mm. X 0.3 mm. <
0.4 mm. X 0.3 mm.
This gives an average of 0.4 mm. X 0.27 mm.
Ogawa found the average diameter of an alveolar duct to be 0.24 mm., and he quotes Kolliker's estimate as 0.27 mm. and that of Schulze as from 0.4 to 0.2 mm.
It will be seen from these measurements and from the illustrations that the bronchial tree in its finer ramifications by no means shows a decrease in the diameter of successive branches towards the periphery. There is, indeed, sometimes an increase even before the air-sacs are reached.
The air-sacs themselves show a great diversity of shape and size and frequently they are recurrent. Figure 2 shows plainly the tendency of the air-sacs to widen out to a greater diameter than the bronchiole from which they arise. It was, no doubt, this widening-out tendency which caused the early investigators to use the term 'infundibulum,' though the air-sacs are not funnel-shaped. Calculations of the actual size of eight air-sacs gave the following results, the measurements being taken in three dimensions:
0.4 X 0.8 xO.4 mm. 0.3 X 0.6 X 0.3 mm. 0.7 xO.4 X 0.3 mm. 0.6 xO.3 X 0.4 mm. 1.0 X 0.4 X 0.5 mm. 0.3 X 0.4 X 0.3 mm. 0.5 X 0.3 X 0.2 mm. 0.4 X 0.6 X 0.2 mm.
The air-sacs or alveoli, as shown in the model, vary greatly in size and shape. The following estimates were made of the actual diameters in three directions of the alveoli of the lung: (All the measurements given above have reference to the adult lung.)
0.05 X 0.06 X 0.07 mm.
0.08 X 0.08x0.12 mm.
0.08 X 10x0.13 mm.
0.06 X 0.08x0.10 mm.
0.12 x0.15 x0.20 mm.
0.08 X 0.10x0.13 mm.
0.08 X 0.05x0.15 mm.
0.08 X 0.10x0.10 mm.
Average: 0.075 x 0.09 x 0.125 mm. Extremes: 0.05 and 0.20 mm.
Ogawa in the case of a man of thirty-one years found an average of 0.1 mm. for depth and breadth of an alveolus, and in the case of a man of fifty-six years, his estimates are 0.15 mm. depth and 0.19 mm. breadth. Ogawa's extreme estimates, counting both his cases, are 0.04 and 0.21. It will be seen that many of the alveoli represented in the model are elongated to a greater extent than is usually described, though, as has been mentioned, Rossignol noted that certain alveoli had unusual depth. The models confirm the observations of Rossignol regarding alveoli. In the construction of the models and in the examination of the sections, careful search was made for evidence of interalveolar communications, but no evidence of their existence was found. The air-sacs interlock with wonderful closeness, so that there is absolutely no waste of space, and because of this close interlocking it sometimes requires great care to satisfy oneself of the absence of interalveolar communications, but in no case could such communications be demonstrated.
The Area of Pulmonary Air-Spaces
During the course of this work the interesting question of the total area of the respiratory air-spaces naturally suggested itself, and an attempt was made to answer it by estimating the total area of the respiratory epithelium in a cubic millimeter of lung tissue and then multiplying this by the total volume of the lung, expressed in cubic millimeters. It is evident that such a method can give only approximately the actual respiratory surface in the lung, since it takes no account of the variations that may occur in the size and number of the air-spaces in various cubic millimeters of the lung, and it fails to make allowance for the larger non-respiratory bronchioles and bronchi. Yet the calculation seemed worth carrying out, as it promised, at least, a maximum figure beyond which the total respiratory surface could not possibly extend.
In order to estimate the area represented in one cubic millimeter of lung tissue, a square of 100 mm. side was marked out on each of fifty successive drawings in the series of adult lung, the squares being oriented so that the series of squares represented successive sections of tissue. Since the sections were 20 m thick, the fifty squares together represented sections totaling 1 cu. mm. in volume. The total perimeter of the various air-passages in each of these squares was measured. This was done by transferring the drawings within each square to millimeter paper, and counting the number of millimeters in the perimeter of each airspace in that square, and totalling the amount. The grand total for the fifty squares amounted to 69346 mm. Since the magnification was 100 diameters, thecorrespondingperimeter in the actual
lung would be -^^ mm. and if this be multiplied by the thick B 100 F J'
20
ness of the sections the result will be nearly 14 sq. mm.
(1000) ^ ^
of respiratory surface in 1 cu. mm. of lung tissue.
The lung tissue used in this estimation was obtained after the
lungs had collapsed — the pleura having been opened. Vier ordt estimates the volume of the lungs in this condition to be
from 3005 to 3975 ccm. Taking the volume as the average of
these, 3400 ccm. or 3400000 cmm., the area of the walls of the
air-passages (respiratory and non-respiratory) is approximately
69346 20 o.^nnnn U x . ^7
X X 3400000 sq. mm., or about 47 square meters.
100 1000
Now% the volume varies as the cube of like dimensions, while
the area varies as the square of like dimensions, so that the area
would not be doubled if the volume of the lung were doubled by
expansion of air-passages. According to Arnold, the volume
of the lung when fully inflated is 6805 ccm., and Vierordt states
that the volume is 9521 ccm. 'bei starkster Fiillung.' For the
areas corresponding to these estimates the extreme limits might fairly be placed at 70 and 90 square meters, respectively. Vier ordt's estimate possibly refers to artificial inflation of the lungs
after death. For the volume of the lungs on deep inspiration,
Arnold's estimate seems a. reasonable one, since 5500 ccm., in the case of the adult lung, is the approximate total volume of complemental, tidal, supplemental, and residual air. We are thus led to the conclusion that on ordinary deep inspiration the total area of the respiratory and non-respiratory epithelium is approximately 70 square meters, and the respiratory area alone must be considerably less than this. In order to estimate how much less, one would have to know the proportionate amount of respiratory to non-respiratory epithelium in the air-passages, and this is not known.
It might be pointed out that the method of multiplying the perimeter by the thickness of the section is exact only in the case of a tube of uniform diameter. To illustrate, it is plain that the cylinder formed by a pile of coppers has an area on its curved surface equal to the sum of the areas of the edges of the coppers, while the area of the curved surface of a cone is really greater than the total area of the edges of a number of discs of gradually diminishing diameter piled up to represent a cone. Here, then, is a source of error which tends towards making the result too low, while errors of omission in the counting or tracing would tend in the same direction. On the other hand, the cubic millimeter of lung tissue on which our calculation is based contained only the finer branchings of air-passages, so that the result would be accurate only if the whole lung were made up of such fine branchings. This source of error, tending to make a too high result, can hardly be canceled by the factors referred to above. The figures given probably represent the extreme upper limits of area corresponding to the respective degrees of expansion.
In Hermann's Handbuch der Physiologie there is given a calculation by Zuntz of the area of the respiratory surface. Zuntz assumes the average diameter of an alveolus to be 0.2 mm. w^hen the lung is moderately inflated, and the total air-space of the lung to be 3400 to 3700 ccm. He considers that at least 3000 ccm. of this space is occupied by alveoli and infundibula. He calculates the volume of an alveolus as though it were a sphere, and arrives at the following result for volume and area of a single alveolus:
Volume, 0.00414 ccm. Area, 0.125+ sq.mm.
Reducing the 3000 ccm. to cubic millimeters, he divides this volume by the volume of a single alveolus, and reaches the conclusion that the number of alveoli in the lung is 725 million. On the basis of this result and the estimated area of a single alveolus, he concludes that the area of the respiratory surface is 90 square meters, w^hen the lung is inflated to a moderate extent.
Aeby used the same figures as Zuntz for volume and area of a single alveolus, assuming the average alveolus to be of spherical form and to have a diameter of 0.2 mm. Since the vohime of such a sphere is 0.004+ cmm., Aeby conchides that in a cubic miUimeter of lung tissue there would be 250 such alveoli, each with an area of 0.125+ sq. mm., so that in a cubic millimeter of lung tissue the total area would be 250 X .125 or 31.25 sq. mm. Nicolas gives estimates of the different areas of respiratory epithelium corresponding to the various degrees of expansion. He considers the maximum volume of air which the lungs will hold to be 4970 ccm. in the average man. His statements are based on calculations by Aeby:
Le nombre total des alveoles est immense. Hiischke I'avait evalue a 1700 ou 1800 millions. Selon Aeby ce chiffre est ])eaiicoiip trop eleve. D'apres ses calcvils chaque millimetre cube de poumon comprcndrait 250 alveoles representant ime surface de 31.2 millimetres carres. En estimant le volume du pomnon a 1617 centimetres cul^es chez I'homme et a 1290 chez la femme, on obtiendrait chez le premier une sonmie totale de 404 millions d'alveoles et chez la seconde de 322 millions, (en chiffres ronds). Cette quantite correspondrait a une surface de 50 a 40 metres carres pendant ['expiration forcee, de 79 (homme) a 63 metres carres (femme) pendant I'etat mo.yen de repos, et enfin de 129 (homme) a 103 metres carres (femme) lors d'une dilation complete.
The result here given of 129 square meters is greatly in excess of my maximum result, in spite of the fact that in the calculations of Aeby and Nicolas the figure representing the vohmie of the lungs is smaller. The difference arises from the difference in the estimate of the number of square millimeters of area per cubic millimeter of lung tissue. Nicolas uses Aeby's estimate of 31.2 sq. mm., while my estimate is 14 sq. mm. To obtain a result of 31.2 sq. mm., according to my method of calculation, the air-spaces cut in an area of 1 sq.mm. would have to be much more numerous than those of any of my sections of adult lung.
F. E. Schulze also refers to Aeby's calculations. After explaining that Aeby assumes the average diameter of an alveolus to be 200 n, he states that he cannot agree with Aeby's estimate of the number of alveoli and corresponding total respiratory surface. Schulze used his own estimate of the volume, 1500 ccm,, for the lungs of the average man. To get the volume of the respiratory parenchyma he deducts 20 per cent, leaving 1200 ccm. He takes the volume of an alveolus to be 200^ cubic microns. Reducing the 1200 ccm. to cubic microns, he divides the volume of a single alveolus into this total volume and arrives at the conclusion that there are 150 million alveoli in the lung, thus:
— ^^^-^ = 150,000,000 2^ X 10«
He then calculates the area of an alveolus as 5 X 200- square microns, and estimates the total respiratory surface as 5 X 200- X 150,000,000 square microns, or 30 square meters.
The estimates of Schulze and Zuntz are much higher than mine in proportion to the figures which they use for the volume of the lung.
In our calculation it was found that in the adult, a cubic millimeter of lung tissue represented the following area of lining of air passages:
69346 20 -.oo^n 1 -..
X = 13.859 sq. mm., or nearly 14 sq.mm.
100 1000 ' J 1
Similar calculations were made of the corresponding area in the child's lung, and estimates were made also from sections of emphysematous human lung and from the lung of an opossum. The results are given below.
ADULT NORMAL
CHILD
MAX OF 61, EMPHYSEM.4.TOUS
OPOSSCM
14 sq.mm. (nearly)
19 sa.mm.
6 sq.mm. to 8.713 sq.mm.
27 sq.mm.
Some of the tracings on which these calculations are based are reproduced in figures 5 to 9.
In the child's lung only a few typical sections were counted, and only one reading w^as taken of the opossum lung.
In the emphysematous lung, the total perimeters of twentyfive consecutive sections were counted, the sections being of tissue near the pleura, though there were other parts, also near the pleura, where the emphysema was much more marked. The readings of the twenty-five squares gave a total of 21784 mm., an average of 871.3 mm., which corresponds in the actual lung tissue to an average perimeter of 8.713 mm. Using a more direct method of calculation than previously, this average perimeter of 8.713 mm. multiplied by 1 (millimeter) gives 8.713 sq. mm., the approximate area of lining epithelium in each cubic millimeter of emphysematous lung. The readings of eight other sections of emphysematous lung, taken from near the pleura, gave a total of 4728 mm., or an average of 591 mm., corresponding in the actual lung to 5.91 mm., or nearly 6 mm. This average perimeter in 1 sq.mm. corresponds to an area of 6 sq.mm. per cubic millimeter of lung tissue.
Figs. 5 to 7 Each figure represents a square millimeter of a section of lung tissue, traced with a projection apparatus. The figures were traced at the same magnification (X 100) and reduced in reproduction to X 60. The double lines represent blood vessels.
Fig. 5, section of a child's lung.
Fig. 6, section of an opossum lung.
Fig. 7, section of an adult human lung.
Figs. 8 and 9 Each figure represents a square millimeter of a section of emphysematous (human) lung tissue, traced with projection apparatus. Both figures were traced at a magnification of 100 and reduced in reproduction to X 00.
It will be seen that the average for all the readings of the emphysematous lung indicates that a man with emphysema might possibly have only half the normal amount of respiratory epithelium per unit of lung volume.
Conclusions Regarding The Human Lung
- In the branching of the respiratory bronchioles there is far greater complexity, irregularity, and a greater degree of interlocking than is usually described.
- There is no spherical space, or 'atrium,' such as has been described by Miller.
- The method of branching of the bronchioles is dichotomous until the terminals are approached, and then the branching becomes irregular.
- Counting as the first branch, a respiratory bronchiole arising from a non-respiratory one, the air-sac is usually reached at the fifth to seventh branch.
- There are normally no direct communications between adjacent alveoli.
- The bronchioles do not decrease in diameter as the periphery is approached, but remain of fairly uniform size until the air-sacs are reached, and the air-sacs are, as a rule, of greater diameter than the tubes from which they arise.
- Waters' rule, that the planes of successive dichotomies cut one another at right angles, is only exceptionally confirmed.
- The lung of the child is just as complex in structure as that of the adult.
- It is calculated that during ordinary deep inspiration the total area of respiratory and non-respiratory epithelium in the adult lung is not greater than 70 square meters.
Bibliography
Aeby, Chr. 1880 Der Bronchialbaum der Stiugetiere unci des Menschen.
Henle 1873 Handbuch der Anatoniie des Menschen, Bd. 2.
JusTESEN, P. Th. 1900 Zur Entwiekelung iind Verzweigung des Bronchial baumes der Saugethierlunge. Arch. f. mikr. Anatomic, Bd. 56.
Miller, W. S. 1892 The lobule of the hing and its blood vessels. Anat. Anz., Bd. 7.
1893 The structure of the lung. Jour. Morph., vol. 8.
1900 Das Lungenlappchen, seine Blut und Lymphgefjisse. Archiv. f. Anat. u. Physiol., Anat. Abt.
1902 Article 'Anatomy of the lungs.' Reference Handbook of the Medical Sciences, New York.
1907 A criticism of some of the recent literature on the structure of the lung. Anat. Rec.
1913 The air spaces in the lung of the cat. Jour. Morph., vol. 24.
MuLLER, J. 1907 Zur vergleichenden Histologic der Lungen unserer Haus siiugetiere. Archiv. f. mikr. Anatomic u. Entw., Bd. 69.
Nicolas, A. 1903 Appareil respiratoirc in Poirier's 'Traite d'Anatomie humaine.'
Paris, T. 4, Fasc. 2. McLeod, J. J. R. 1920 Physiology and biochemistry in modern medicine, 3rd edition.
Ogawa, C. 1920 The finer ramifications of the human lung. Am. Jour. Anat., vol. 27, no. 3.
1920 Contributions to the histology of the respiratory spaces of the vertebrate lungs. Am. Jour. Anat., vol. 27, no. 3.
Schulze, F. E. 1906. Beitriige zur Anatomic der Saugethierlungen. Sitzungs bericht kgl. preuss. Akademie d. Wiss.
ViERORDT, H. 1906 Daten und Tabellen fiir Mediziner. Jena. Zuntz, N. 1882 Hermann's Handbuch der Physiologic, IV Band Theil. S. 90.
Plates
\
Plate 1
Explanation Of Figure
1 Peconstruction from an adult lung. This model was made at a magnification of 100 diameters. The distance in the model from the top of the part colored orange to the bottom of the part colored green is 7 inches.
Plate 2
Explanation Of Figure
2 Part of a reconstruction of an adult lung, more highly magnified. This part is the right half of the portion colored orange in plate 1, seen from above. The illustration shows the part at its actual size in the model, which was constructed at magnification of 100 diameters.
Cite this page: Hill, M.A. (2024, April 19) Embryology Paper - The terminals of the human bronchiole (1922). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_terminals_of_the_human_bronchiole_(1922)
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