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Respiratory System
1911 Encyclopedia Britannica
or Anatomy - The respiratory tract consists of the nasal cavities, the pharynx, the larynx, the trachea, the bronchi and the lungs, but of these the two first parts have been treated in separate articles (see Olfactory System and Pharynx).
The larynx is the upper part of the air tube which is specially modified for the production of notes of varying pitch, though it is not responsible for the whole of the voice. Its framework is made up of several cartilages which are moved on one another by muscles, and it is lined internally by mucous membrane which is continuous above with that of the pharynx and below with that of the trachea or windpipe. The larynx is situated in the front of the neck and corresponds to the fourth, fifth and sixth cervical vertebrae. For its superficial anatomy see Anatomy, Superficial and Artistic. The thyroid cartilage (see fig. 1) is the largest, and consists of two plates or alae which are joined in the mid-ventral line. At the upper part of their junction is the thyroid notch and just below that is a forward projection, the pomum Adami, best marked in adult males. From the upper part of the posterior border of each ala the superior cornu rises up to be joined to the tip of the great cornu of the hyoid bone by the lateral thyro-hyoid ligament, while from the lower part of the same border the inferior cornu passes down to be fastened to the cricoid cartilage by the crico-thyroid capsule. From the upper border of each ala the thyro-hyoid membrane runs up to the hyoid bone, while near the back of the outer surface of each the oblique line of the thyroid cartilage runs downward and forward.
The cricoid cartilage (see figs. 1 and 2) is something like a signet ring with the seal behind; its lower border, however, is horizontal. To the mid-ventral part of its upper border is attached the mesial part of the crico - thyroid membrane, which attaches it to the lower border of the thyroid cartilage though the lateral parts of this membrane pass up in ternally to the thyroid cartilage and their upper free edges form the true vocal cords. On the summit of the the two arytenoid After D. J. Cunningham, from Cunningham's FIG. 2. - Cartilages and Ligaments of Larynx, as seen from behind.
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forms a pyramid with its apex upward and with an anterior posterior and internal or mesial surface. The base articulates with the cricoid by a concave facet, surrounded by the crico-arytenoid capsule, and the two arytenoids are able to glide toward or away from one another, in addition to which each can rotate round a vertical axis. From the front of the base a delicate process projects which, as it is attached to the true vocal cord, is called the vocal process, while from the outer part of the base another stouter process Hyoid bone Cartilago triticea Thyro
epiglottidean ligament Superior cornu of thyroid cartilage Cartilage of Santorini Arytenoid cartilage Muscular process of arytenoid cartilage Inferior cornu of thyroid cartilage Inferior tubercle Inferior cornu of i thyroid cartilage Crco-thyroid membrane Cricoid cartilage VlIDNIrrria z .After D. J. Cunningham, from Cunningham's FIG. i. - Profile View of the Cartilages and Ligaments of the Larynx.
signet part of the cricoid are placed cartilages (see fig. 2), each of which.
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Epiglottis Superior tubercle on theala of thyroid cartilage blique tine Hyoid bone Cartilago triticea Thyro-hyoid membrane Superior cornu of thyroid cartilage attaches the two crico-arytenoid muscles and so is known as the muscular process. Above each arytenoid are two smaller cartilages known as the cornicula laryngis or cartilages of Santorini and the cuneiform cartilages, but they are not of any practical importance.
The epiglottis (see fig. 3), on the other hand, is a very important structure, since it forms a lid to the larynx in swallowing: only the box moves up to the lid instead of the Hyo-epiglottidean ligament lid moving down to the box. It is leafshaped, the stalk (thyro-epiglottid e a n ligament) being attached to the junction of the thyroid cartilages inside the larynx, while the anterior surface of the leaf is closely attached to the root of the tongue and body of the hyoid bone. The posterior or laryngeal surface is pitted for glands, and near the point where the stalk joins the leaf is a convexity which is known as the cushion of the epiglottis. All the cartilages of the larynx are of the hyaline variety except the epiglottis, the cornicula laryngis and the cuneiform cartilages, which are yellow elastic. The result is that all except these three tend to ossify as middle age is approached.
The muscles of the larynx are: (I) the crico-thyroids, which are attached to the lower border of the thyroid and the anterior part of the cricoid, by pulling up which they make the upper part of the signet, with the arytenoids attached to it, move back and so tighten the vocal cords. (2) The thyro-arytenoids (see fig. 4), which run back from the junction of the thyroid alae to the front of the arytenoids and side of the epiglottis; they pull the arytenoids toward the thyroid and so relax the cords.
(3) The single arytenoideus muscle, which runs from the back of one arytenoid to the other and approximates these cartilages.
(4) The lateral crico-arytenoids (see fig. 4) which draw the muscular processes of the arytenoids forward toward the ring of the -cricoid and, by so doing, twist the vocal processes, with the cords attached, inward toward one another; and (5) the posterior crico-arytenoids (see fig. 4) which run from the back of the signet part of the cricoid to the back of the muscular processes of the arytenoid and, by pulling these backward, twist the vocal processes outward and so separate the vocal cords. All these muscles are supplied by the recurrent laryngeal nerve, except the crico-thyroid which is innervated by the external branch of the superior laryngeal (see Nerves, Cranial). The mucous membrane of the larynx is continuous with that of the pharynx at the aryteno-epiglottidean folds which run from the sides of the epiglottis to the top of the arytenoid cartilages (see (fig. 3). To the outer side of each fold is the sinus pyriformis (see Pharynx). From the middle of the junction of the alae of the thyroid cartilage to the vocal processes of the arytenoids the mucous membrane is reflected over, and closely bound to, the true vocal cords which contain elastic tissue and, as has been mentioned, are the upper free edges of the lateral parts of the crico-thyroid membrane. The chink between the two After D. J. Cunningham, from Cunningham's Text-Book of Anatomy. FIG. 4. - Dissection of the Muscles in the Lateral Wall of the Larynx. The right ala of the thyroid cartilage has been removed.
true vocal cords is the glottis or rim y glottidis. Just above the true vocal cords is the opening into a recess on each side which runs upward and backward and is known as the laryngeal saccule; its opening is the laryngeal sinus. The upper lip of this slit-like opening is called the false vocal cord. The mucous membrane is closely bound down to the epiglottis and to the true vocal cords, elsewhere there is plenty of submucous tissue in which the products of inflammation may collect and cause " oedema laryngis," a condition which is mechanically prevented from passing the true vocal cords. In the upper part of the front and sides of the larynx and over the true vocal cords the mucous membrane is lined by squamous epithelium, but elsewhere the epithelium is of the columnar ciliated variety: it is supplied by the superior laryngeal branch of the vagus nerve and above the glottis is peculiarly sensitive.
The Trachea or windpipe (see fig. 5) is the tube which carries the air between the larynx and the bronchi; it is from four to four and a half inches long and lies partly in the neck and partly in the thorax. It begins where the larynx ends at the lower border of the sixth cervical, and divides into its two bronchi opposite the fifth thoracic vertebra. The tube is kept always open by rings of cartilage, which, however, are wanting behind, and, as it passes down, it comes to lie farther and farther from the ventral surface of the body, following the concavity of the thoracic region of the spinal column. In the whole of its downward course it has the oesophagus close behind it, while in front are the isthmus of the thyroid, the left innominate vein, the innominate artery and the arch of the aorta. On each side of it and touching it is the vagus nerve.
The cervical part of the tube is not much more than an inch in length, but it can be lengthened by throwing back the head. This, of course, is the region in which tracheotomy is performed, and it should be remembered that in children, and sometimes in adults, the great left innominate vein lies above the level of the top of the sternum.
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In transverse section the trachea is rather wider from side to side than from before backward. In life the former measurement is said to be about 12.5 mm. and the latter II mm. It is made up of an external fibro-elastic membrane in which the cartilaginous rings lie, while behind, where these rings are wanting, is a layer of unstriped muscle which, when it contracts, Arytetto-epiglottidean muscle Hyoid Cuneiform cartilage Thyro-epiglottidean muscle Thyro-hyoid membrane Saccule of larynx Muscular process of arytenoid cartilage Thyro-arytenoid muscle Thyroid cartilage Crico-arytepoideus lateralis Crico-arytenoideus posticus Crico-thyroid membrane Cricoid cartilage E pig lot tis Thyroid cartilage Elevation produced by cuneiform cartilage ,False vocal cord 41t? Phiftrum ventriculi Elevation produced by arytenoid cartilage Laryngeal sinus True vocal card Arytenoid muscle Cartilage of epiglottis Fatty pad Thyro-hyoid membrane Processus vocalis Cricoid cartilage Cricoid cartilage After D. J. Cunningham, from Cunningham's Text-Book of Anatomy. FIG. 3. - Mesial Section through Larynx to show the outer wall of the right half.
draws the hind ends of the rings together and so diminishes the calibre of the tube. Inside these is plentiful submucous tissue Pulmonary artery After D. J. Cunningham, from Cunningham's Text-Book of Anatomy. FIG. 5. - The Trachea and Bronchi. The thyroid body is indicated by a dotted line.
containing mucous glands and quantities of lymphoid tissue, while the whole is lined internally by columnar ciliated epithelium. The Bronchi (see fig. 5) are the two tubes into which the trachea divides, but, since the branches, which these tubes give off later, are also called bronchi, it may be clearer to speak of primary, secondary and tertiary bronchi. Each primary bronchus runs downward and outward, but the right one is more in a line with the direction of the trachea than the left. The right primary bronchus has also a greater calibre than the left because the right lung is the larger, and for these two reasons when a foreign body enters the trachea it usually enters the right bronchus.
The first secondary bronchus comes off about an inch from the bifurcation of the trachea on the right side and, as it lies above the level of the pulmonary artery, it is known as the eparterial bronchus. On the left side the first branch is about two inches from the bifurcation and, like all the remaining secondary bronchi, is hyparterial: the left primary bronchus is therefore twice as long as the right. After the eparterial secondary bronchus is given off the direction of the right primary bronchus is carried on by the hyparterial secondary bronchus, and this, just before reaching the hilum of the lung, divides into upper and lower tertiary bronchi, while the left lower secondary hyparterial bronchus does not divide before reaching the hilum of its lung. Into the hilum or root of the right lung, therefore, three bronchial tubes enter, while on the left side there are only two. The firmly rooted habit of associating the term bronchi with those parts of the main tubes which lie between the bifurcation of the trachea and the point where the first. branch comes off makes it very difficult to suggest a nomenclature which calls up any picture of the actual state of things to the mind. Certainly the classification into primary, secondary and tertiary bronchi only goes a very little way toward this,. and it should be realized that, call them what we may, there are two long tapering tubes which run from the bifurcation of the trachea to the lower and back part of each lung, and give off a series of large ventral and small dorsal branches. The upper part of each of these long tubes or stem bronchi is outside the lung and in the middle mediastinum of the thorax, the lower part embedded in the substance of the lung. The structure of the bronchi is practically identical with that of the trachea. ( See G. S. Huntington's " Eparterial Bronchial System of the Mammalia," Am. Journ. Med. Sci. (Phila. 1898). See also Quain's Anatomy, London, last edition.) The Lungs are two pyramidal, spongy, slate-coloured, very vascular organs in which the blood is oxygenated. Each lies in its own side of the thorax and is surrounded by its own pleural cavity (see Coelom and Serous Membranes), and has an apex which projects into the side of the root of the neck, a base which is hollowed for the convexity of the diaphragm, an outer surface which is convex and lies against the ribs, an inner surface concave for the heart, pericardium and great vessels, a sharp anterior border which overlaps the pericardium and a broad, rounded posterior border which lies at the side of the spinal column. Each lung is nearly divided into two by a primary fissure which runs obliquely downward and forward, while the right lung has a secondary fissure which runs horizontally forward from near the middle of the primary fissure. The left lung has therefore an upper and lower or basal lobe, while the right has upper, middle and lower lobes. On the inner surface of each lung is the root or hilum at which alone its vessels, nerves and ducts (bronchi) can enter and leave it. The structures contained in the root of each lung are the branches and tributaries of (1) the pulmonary artery, (2) the pulmonary veins, (3) the bronchi, (4) the bronchial arteries, (5) the bronchial veins, (6) the bronchial lymphatic vessels and glands, (77 ), the pulmonary plexuses of nerves. Of these the first three are the largest and, in dividing the root from in front, the veins are first cut, then the arteries and last the bronchi. As has been pointed out already, the eparterial bronchus on the right side is above the level of the artery, but all the others (hyparterial) are on a lower level.
The bronchial arteries supply the substance of the lung;. there are usually two on each side, and they lie behind the bronchi. The blood which they carry is chiefly returned by the pulmonary veins bringing oxidized blood back to the heart,. so that here there is a normal and harmless mixture of arterial and venous blood. If there are any bronchial veins (their presence is doubted by some, and the writer has himself carefully but unsuccessfully searched for them several times), they open into the azygos veins of their own side. The bronchial lymphatic vessels lie behind the pulmonary vessels and open into several large glands which are black from straining off the carbon left in the lungs from the atmosphere.
There is an anterior and posterior pulmonary plexus of nerves on each side, the fibres of which are derived from the vagus and the upper thoracic ganglia of the sympathetic.
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Structure of the Lungs
As the bronchi become smaller and smaller by repeated division, the cartilage completely surrounds them and tends to form irregular plates instead of rings - they are therefore cylindrical, but when the terminal branches (lobular bronchi ) are reached, the cartilage disappears and hemispherical bulgings called alveoli occur (fig. 6 A). At the very end of --Thyroid cartilage Crico-thyroid membrane Cricoid cartilage ! Part of trachea covered by isthmus of thyroid body Common carotid artery Subclavian artery Aorta Left bronchus Pulmonary artery Hyparterial bronchi (Esophagus Eparterial bronchus Hypar ter ial bronchus each lobular bronchus is an irregular chamber, the atrium (fig. 6 At), and from this a number of thin-walled sacs, about 1 mm. in diameter, open out. These are called the infundibula (fig. 6 I), ? and their walls are
pouched by hemispherical air-cells or alveoli like those in the lobular bronchi. Each lobular bronchus with its atrium and infundibula forms what is known as a lobule of the lung, and these lobules are separated by connective tissue, and their outlines are evident on the surface of the lung.
The muscular tissue, which in the larger tubes was confined to the dorsal part, forms a complete layer in the smaller; but when the lobular bronchi are reached, it stops and the mucous membrane is surrounded by the elastic layer. In the lobular bronchi, too, the lining epithelium gradually changes from the ciliated to the stratified or pavement variety, and this is the only kind which is found in the infundibula and alveoli. Surrounding each alveolus is a plexus of capillary vessels so rich that the spaces between the capillaries are no wider than the capillaries themselves, and it is here that the exchange of gases takes place between the air and the blood.
Embryology
The respiratory system is developed from the ventral surface of the foregut as a long gutter-like pouch which reaches from just behind the rudiment of the tongue to the stomach. Limiting the anterior or cephalic end of this is a (1-shaped elevation in the ventral wall of the pharynx which separates the ventral ends of the third and fourth visceral bars and is known as the furcula; it is from this that the epiglottis, aryteno-epiglottidean folds and arytenoid cartilages are developed. Later on the respiratory tube is separated from the digestive by two ridges, one on each side, which, uniting, form a transverse partition. In the region of the furcula, however, the partition stops and here the two tubes communicate. The caudal end of the respiratory tube buds out into the two primary bronchi, and the right one of these, later on, bears three buds, while the left has only two; these are the secondary bronchi, which keep on dividing into two, one branch keeping the line of the parent stem to form the stem bronchus, while the other goes off at an angle. By the repeated divisions of these tubes the complex " bronchial tree " is formed and from the terminal shoots the infundibula bud out. The alveoli only develop in the last three months of foetal life. The thyroid cartilage is probably formed from the fourth and fifth branchial bars, while the cricoid seems to be the enlarged first ring of the trachea. Before birth the lungs are solid and much less vascular than after breathing is established. Their slaty colour is gradually gained from the deposit of carbon from the atmosphere. (For further details see Quain's Anatomy, vol. i., Lond. 1908.) Comparative Anatomy. - It has been shown (see Pharynx) that in the lower vertebrates respiration is brought about by the blood vessels surrounding the gill clefts. In the higher fishes (Ganoids and Teleosteans) the " swim bladder " appears as a diverticulum from the dorsal wall of the alimentary canal, and its duct ( d. pneumaticus ) sometimes remains open and at others becomes a solid cord. In the former case it is probable that the blood is to some extent oxidized in the vascular wall of this bladder. In the Dipnoi (mud-fish) the opening of the swim bladder shifts to the ventral side of the pharynx and the bladder walls become sacculated and very vascular, so that, when the rivers are dried up, the fish can breathe altogether by means of it. In the S. American and African species of mud-fish the bladder or lung, as it may now be called, is divided by a longitudinal septum in its posterior (caudal) part into right and left halves. In this sub-class of Dipnoi, therefore, a general agreement is seen with the embryology or ontogeny of Man's lungs. In the Amphibia the two lungs are quite separate though they are mere sacculated bags without bronchi. A trachea, however, appears in some species (e.g. Siren) and a definite larynx with arytenoid cartilages, vocal cords and complicated muscles is established in the Anura (frogs and toads). In most of the Reptilia the bag-like lungs are elaborated into spongy organs with arborizing bronchi in their interior. From the crocodiles upward a main or stem bronchus passes to the caudal end of the lung, and from this the branches or lateral bronchi come off. The larynx shows little advance on that of the Anura.
The respiratory organs of birds are highly specialized. The larynx is rudimentary, and sound is produced by the syrinx, a secondary larynx at the bifurcation of the trachea; this may be tracheal, bronchial or, most often, tracheo-branchial. The lungs are small and closely connected with the ribs, while from them numerous large air sacs extend among the viscera, muscles and into many of the bones, which, by being filled with hot air, help to maintain the high temperature and lessen the specific gravity of the body. This pneumaticity of the bones is to a certain extent reproduced by the air sinuses of the skull in crocodiles and mammals, and it must be pointed out that the amount of air to the bones does not necessarily correspond with the power of flight, for the Ratitae (ostriches and emeus) have very pneumatic bones, while in the sea-gulls they are hardly pneumatic at all.
In mammals the thyroid cartilage becomes an important element in the larynx, and in the Echidna the upper and lower parts of it, derived respectively from the fourth and fifth branchial bars, are separate (R. H. Burne, Journ. Anat. and Phys. xxxviii. p. xxvii.). The whole larynx is much nearer the head than in Man, and in young animals the epiglottis is intra-narial, i.e. projects up behind the soft palate. This prevents the milk trickling into the larynx during suckling, and is especially well seen in the Marsupials and Cetacea, though evidences of it are present in the human embryo. In the lower mammals an inter-arytenoid cartilage is very frequent (see J. Symington, " The Marsupial Larynx," Anat. and Phys. xxxiii. 31, also " The Monotreme Larynx," ib. xxxiv. 90) .
The lungs show a good deal of variation in their lobulation; among the porcupines as many as forty lobes have been counted in the right lung, while in other mammals no lobulation at all could be made out. The azygous lobe of the right lung is a fairly constant structure and is situated between the post-caval vein and the oesophagus. It is supplied by the terminal branch of the right stem bronchus and, although it is usually absent in Man, the bronchus which should have supplied it is always to be found. (F. G. P.) (2) Physiology So far as is known, the intake - of oxygen, either free or combined, and the output of carbon dioxide, are an essential part of the life of all organisms. The two processes are so closely associated with one another that they are always included together under the designation of respiration, which may thus be defined as the physiological process which is concerned in the intake of oxygen and output of carbon dioxide. According to the evidence at present available, it is only within living cells that the respiratory oxygen is consumed and the carbon dioxide formed. The mere conveying of oxygen from the surrounding air or water to these cells, and of carbon dioxide from them to the air or water, is, however, in itself a complex process in the higher animals; and accordingly an account of animal respiration naturally falls into two divisions, the first of which (I.) is concerned with the manner in which oxygen and carbon dioxide are conveyed to and from the living tissues, and the second (II.) with the consumption of oxygen and formation of carbon dioxide by the living tissues themselves.
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I. In all the more highly organized animals there are special respiratory organs: the lungs in the higher vertebrates; the gills in fishes; the tracheae in insects; and various rudimentary forms of lungs or gills in other higher invertebrates. In the Lob.
FIG. 6. - Diagram of Two Lobules of the Lung. B. Bronchus. A. Alveolus. I.
Infundibulum. L.B. Lobular bronchus. At. Atrium. Lob. Lobule.
present article attention will be specially confined to the case of the higher vertebrates, and in particular to man.
Air is brought into the lungs by the movements of breathing (see above, Movements of Respiration). Oxygen from this air passes through the delicate lining membrane of the air-cells of the lungs into the blood, where it enters into loose chemical combination with the haemoglobin of the red corpuscles (see Blood). In this form it is conveyed onwards to the heart, and thence through the arteries to the capillaries, where it again parts from the haemoglobin, and passes through the capillary walls to the tissues, where it is consumed. Carbon dioxide passes out from the tissues into the blood in a corresponding manner, enters into loose combination as bicarbonate, and possibly in other ways, in the blood, and is conveyed by the veins to the lungs, whence it passes out in the expired air. Pure atmospheric air contains 20.93% of oxygen,
03% of carbon dioxide and 79.0 4% of nitrogen (with which is mixed about 0.9% of argon). The dried expired air in man contains about 3.5% of carbon dioxide and 17% of oxygen, so that roughly speaking the carbon dioxide is increased by about 3-5% and the oxygen diminished by 4%. Expired air as it leaves the body contains about 6% of moisture, compared with usually about z % in the inspired air. The added moisture and higher temperature of expired air make it decidedly lighter than pure air.
Owing to the unpleasant effects often produced in badly ventilated rooms it was for long supposed that some poisonous volatile " organic matter " is also given off in the breath. Careful investigation has shown that this is not the case. The unpleasant effects are partly due to heat and moisture, and partly to odours which are usually not of respiratory origin. The carbon dioxide present in the air of even very badly ventilated rooms is present in far too small proportions to have any sensible effect.
The average volume of air inspired per minute by healthy adult men during rest is about 7 litres or
25 cub. ft. In different individuals the frequency of breathing varies considerably - from about 7 to 25 per minute, the depth of each breath varying about inversely as the frequency. During muscular work the volume of air breathed may be six or eight times as much as during rest. The volume of carbon dioxide given off varies from about half a cubic foot per hour during complete rest to 5 cub. ft. during severe exertion, but averages about 0.9 cub. ft. per hour, and will reach or exceed r cub. ft. per hour during even very light exertion. The volume of oxygen consumed is about a seventh greater than that of the carbon dioxide given off.
The breathing is regulated from a nervous centre situated in the medulla oblongata, which is the lowest part of the brain. If this centre is destroyed or injured the breathing stops and death rapidly results. From the respiratory centre rhythmic efferent impulses proceed down the motor nerves supplying the diaphragm, intercostals and other respiratory muscles. Afferent impulses through various nerves may temporarily affect the rhythm of the respiratory centre. Of these afferent impulses by far the most important are those which proceed up the vagus nerve from the lungs themselves. On distention of the lungs with air the inspiratory impulses from the respiratory centre are suddenly arrested or " inhibited "; on the other hand, collapse of the lung strongly excites to inspiratory effort. On section of the vagus nerve these effects disappear, and the breathing becomes less frequent and much more laboured. The vagus nerve is thus the carrier of both inhibitory and exciting stimuli.
As the physiological function of breathing is to bring oxygen to and remove carbon dioxide from the blood, it would naturally be expected that breathing would be regulated in accordance with the amount of oxygen required and of carbon dioxide formed; but until quite recently the actual mode of regulation was by no means clear. It was commonly supposed that afferent nervous impulses in some way regulated the otherwise automatic action of the centre, want of oxygen or excess of CO 2 in the blood being only an occasional and relatively unimportant factor in the regulations. The phenomenon of "apnoea" or complete cessation of natural breathing which occurs after forced breathing, was attributed mainly to the already mentioned distension effect through the vagus nerves. To go further back still, it was even supposed that the rate and depth of breathing, and the percentage of oxygen in the inspired air, determine the consumption of oxygen and formation of carbon dioxide in the body, just as the air-supply to a fire determines the rate of its combustion. This old belief is still often met with - for instance, in the reasons given for recommending " breathing exercises " as a part of physical training.
It is evident that if the breathing did not increase correspondingly with the greatly increased consumption of oxygen and formation of CO 2 which occurs, for instance during muscular work, the percentage of oxygen in the air contained in the lung cells or alveoli (alveolar air) would rapidly fall, and the percentage of carbon dioxide increase. The inevitable result would be a very imperfect aeration of the blood. Investigation of the alevolar air has furnished the key to the actual regulation of breathing. Samples of this air can be obtained by making a sudden and deep expiration through a piece of long tube, and at once collecting some of the air contained in the part of this tube nearest the mouth. By this means it has been found that during normal breathing at ordinary atmospheric pressure the percentage of carbon dioxide (about 5 . 6% on an average for men) is constant for each individual, though different persons vary slightly as regards their normal percentage. The breathing is thus so regulated as to keep the percentage of carbon dioxide constant; and under normal conditions this regulation is surprisingly exact. The ordinary expired air is a mixture of alveolar air and air from the " dead space " in the air passages. The deeper the breathing happens to be, the more alveolar air there will be in the expired air, and the higher, therefore, the percentage of carbon dioxide in it, so that the expired air is not constant in composition, though the alveolar air is. If air containing 2 or 3% of carbon dioxide is breathed, the breathing at once becomes deeper, in such a way as to prevent anything but a very slight rise in the alveolar carbon dioxide percentage. The difference is scarcely appreciable subjectively, except during muscular exertion. The effect of 1% of carbon dioxide in the inspired air is so slight as to be negligible, and there is no foundation for the popular belief that even very small percentages of carbon dioxide are injurious. With 4 or 5% or more of carbon dioxide, however, much panting is produced, and the alveolar carbon dioxide percentage begins to rise appreciably, since compensation is no longer possible. As a consequence, headache and other symptoms are produced. If, on the other hand, the percentage of carbon dioxide in the alveolar air is abnormally reduced by forced breathing, the condition of apnoea is produced and lasts until the percentage again rises to normal, but no longer. Forced breathing with air containing more than about 4% of carbon dioxide causes no apnoea, as the alveolar carbon dioxide does not fall.
If oxygen is breathed instead of air there is no appreciable change in the percentage of carbon dioxide in the alveolar air, and no tendency towards apnoea. Want of oxygen is thus not a factor in the regulation of normal breathing. During muscular work the depth and frequency of breathing increase in such a way as 'CO prevent the alveolar carbon dioxide from rising more than very slightly. It is still the carbon dioxide stimulus that regulates the breathing, although with excessive muscular work other accessory factors may come in to some extent.
Under increased barometric pressure the percentage of carbon dioxide in the alveolar air no longer remains constant; it diminishes in proportion to the increase of pressure. For instance, at a pressure of 2 atmospheres it is reduced to half, and at 6 atmospheres to a sixth; while at less than normal atmospheric pressure it rises correspondingly unless symptoms of want of oxygen begin to interfere with this rise. These results show that it is not the mere percentage, but the pressure (or " partial pressure ") of carbon dioxide in the alveolar air that regulates breathing. The pressure exercised by the carbon dioxide in the alveolar air is of course proportional to its percentage, multiplied by the total atmospheric pressure. It follows from this law that at a pressure of 6 atmospheres i % of carbon dioxide in the inspired air would have the same violent effect as 6% at the normal pressure of z atmosphere. To take a concrete practical application, if a diver whose head was just below water were supplied with sufficient air to keep the carbon dioxide percentage in the air of his helmet down to 3% at most, he would be quite comfortable. But if, with the same air supply as measured at surface, he went down to a depth of 170 ft., where the pressure is 6 atmospheres, he would at once experience great distress culminating in loss of consciousness, owing, not to the pressure of the water, which has trifling effects, but to the pressure of carbon dioxide in the air he was breathing. The air supply must be increased in proportion to the increase of pressure if these effects are to be avoided, and ignorance of this has led to the common failure of diving work at considerable depths.
The foregoing facts enable us to understand the regulation of breathing under normal conditions. The pressure of carbon dioxide in the alveolar air evidently determines that of the carbon dioxide in the arterial blood, and the latter in its turn determines the carbon dioxide pressure in the respiratory centre, which is very richly supplied with blood. The centre itself is extremely sensitive to the slightest increase or diminution in carbon dioxide pressure; and thus it is that the alveolar carbon dioxide pressure is so important. That the stimulus of carbon dioxide is from the blood and not through nerves is proved by many experiments. The function of the vagus nerves in regulating the breathing is apparently to, as it were, guide the centre in the expenditure of each separate inspiratory or expiratory effort; for as soon as inspiration or expiration is completed the inspiratory or expiratory effort is cut short by impulse proceeding up the vagus nerve, and much waste of muscular work and risk of injury to the lungs is thereby prevented.
Under ordinary conditions the regulation of carbon dioxide pressure in the alveolar air ensures at the same time a normal pressure of oxygen, since absorption of oxygen and giving off of carbon dioxide normally run parallel to one another. If, however, air containing abnormally little oxygen is breathed, the normal relation between oxygen and carbon dioxide in the alveolar air is disturbed. A similar state of affairs is brought about by any considerable diminution of atmospheric pressure. Not only does the partial pressure of oxygen in the inspired air fall, but this fall is proportionally much greater in the alveolar air; and the effects of want of oxygen depend on its partial pressure in the alveolar air. It has been known for long that any great deficiency in the proportion of oxygen in the air breathed increases the depth and frequency of the breathing; but this effect is not apparent until the percentage of oxygen or the barometric pressure is reduced by more than a third, which corresponds to a reduction of more than half in the alveolar oxygen pressure. In contrast with this an increase of a fiftieth in the alveolar carbon dioxide pressure has a marked effect on the breathing. Along with the increased breathing caused by deficiency of oxygen there is more or less blueness of the skin and abnormal effects of various kinds, such as partial loss of sensibility, memory and power of thinking. Long exposure often causes headache, nausea, sleeplessness, &c. - a train of symptoms known to mountaineers as " mountain sickness." That the primary cause of " mountain sickness " is lack of oxygen owing to the low atmospheric pressure there is not the slightest doubt. Lack of oxygen is thus not only an important, but also an abnormal form of stimulus to the respiratory centre, since it is accompanied by quite abnormal symptoms. A further analysis of the special effect of lack of oxygen on the respiratory centre has shown that this effect still depends on the partial pressure of carbon dioxide in the alveolar air. The lack of oxygen appears, in fact, to have simply increased the sensitiveness of the centre to carbon dioxide, so that a lower partial pressure of carbon dioxide excites the centre, and the breathing is correspondingly increased. By prolonged forced breathing so much carbon dioxide is washed out of the body that the subsequent apnoea lasts until the oxygen in the alveolar air is nearly exhausted. The subject of the experiment becomes very blue in the face and is partially stupefied by want of oxygen before he has any desire to breathe. The probable explanation of these facts is that want of oxygen does not itself excite the centre, but that some substance - very probably lactic acid, which is known to be formed abundantly - is produced abnormally in the body during exposure to want of oxygen and aids the carbon dioxide in exciting the centre. It is known that the blood becomes less alkaline at high altitudes,, and that acids in general excite the centre. A person on a high mountain thus gets out of breath much more easily than at sea-level. The extra stimulus to the centre during work still comes from the extra carbon dioxide formed, but has a greater effect than usual on the breathing. If the extra stimulus came directly from want of oxygen the person on the mountain would probably turn blue and lose consciousness on the slightest exertion. By analysing the alveolar air it can be shown that after a time even a height of 5000 to 6000 ft., or a diminution of only a sixth in the barometric pressure, distinctly increases the sensitiveness of the respiratory centre to carbon dioxide, so that there seems to be a slow accumulation of acid in the blood. The effect also passes off very slowly on returning to normal pressure,. although the lack of oxygen is at once removed.
| [PHYSIOLOGY The blueness of the skin (" cyanosis ") produced by lack of oxygen is due to the fact that the haemoglobin of the red corpuscles is imperfectly saturated with oxygen. Haemoglobin which is fully saturated with oxygen has a bright red colour, contrasting with the blue colour which it assumes when deprived of oxygen. According to the existing evidence the saturation of the haemoglobin is practically complete under normal conditions in the lungs, or when thoroughly shaken at the body temperature and normal atmospheric pressure with air of the same composition as normal alveolar air. As the partial pressure of the oxygen in this air falls,. however, the saturation of the haemoglobin becomes less and less complete, and the arterial blood assumes a more and more blue tinge, which imparts a blue or leaden colour to the skin, accompanied by the symptoms, already referred to, of lack of oxygen. Normal arterial blood in man yields. about 19 volumes of physiologically available oxygen for each ioo volumes of blood. Of these 19 volumes about 182 are loosely combined with the haemoglobin of the red corpuscles,. the small remainder being in simple solution in the blood.. Venous blood, on the other hand, yields only about 12 volumes. The combination of haemoglobin with oxygen is only stable in the presence of free oxygen at a pressure of about that in normal alveolar air. As this pressure falls the compound is progressively dissociated. From this it can be readily understood why the blood loses its oxygen in passing through the tissues, which are constantly absorbing free oxygen, and regains it in the lungs. The marked effects produced by abnormal deficiency in the pressure of oxygen in the alveolar air are also readily intelligible; for even although the arterial blood still contains sufficient oxygen to cover the normal difference between the oxygen content of arterial and that of venous blood, yet this oxygen is given off to the tissues less readily - i.e. at a lower pressure, and thus fails to supply their demands completely. It is evident also that in pure air at normal pressure increased ventilation of the lungs does not appreciably increase the supply of oxygen to the blood, whereas in air largely deprived of its oxygen, or at low pressure, the increased alveolar oxygen pressure produced by deep breathing helps greatly in saturating the blood with oxygen, and may thus relieve the symptoms of want of oxygen. Hence it is that the increased sensitiveness of the respiratory centre to carbon dioxide, and consequent increased depth of breathing, at high altitudes compensates to a large extent for deficiency in the oxygen pressure. Addition of carbon dioxide to the inspired air produces exactly the same result. Indeed Professor Angelo Mosso was led by observation of the beneficial effects of carbon dioxide at low atmospheric pressure to attribute mountain sickness to lack of carbon dioxide, a condition which he designated by the word " acapnia. " When impure air is vitiated, not only by deficiency of oxygen, but also by carbon dioxide, the carbon dioxide causes panting, which not only gives warning of any danger, but prevents the alveolar oxygen percentage from falling in the way it would do if the carbon dioxide were absent. In this way the carbon dioxide greatly lessens the danger. To give instances, air progressively and very highly vitiated by respiration is much less likely to cause danger if the carbon dioxide is not artificially absorbed, and not nearly so dangerous as the great diminution of atmospheric pressure (and consequently of oxygen pressure) which occurs in a very high balloon ascent. Indeed the dangers of a very high balloon ascent are notorious, and a number of deaths or very narrow escapes are on record. Just as oxygen forms a dissociable compound with the haemoglobin of the blood, so does carbon dioxide form dissociable compounds. One of' these compounds appears to be with haemoglobin itself, and another is sodium bicarbonate, which is far more easily dissociated in the blood than in a simple watery solution, owing to the presence of proteid and possibly other substances which act as weak acids and thus help the dissociation process. The whole of the carbon dioxide can therefore be removed from the blood by a vacuum pump, just as the whole of the oxygen can. Venous blood contains roughly speaking about 40 volumes of carbon dioxide per roo of blood, and arterial blood about 34 volumes. Of this carbon dioxide only about 3 volumes can be in free solution, the rest being loosely combined. The conveyance of carbon dioxide from the blood to the lungs is thus readily intelligible, as well as the fact that any increase or diminution of the pressure of carbon dioxide in the alveolar air will naturally lead to a damming back or increased liberation of carbon dioxide from the blood, and that by forced breathing carbon dioxide can be washed out of the blood to such an extent that a prolonged cessation of natural breathing (apnoea ) follows, since even in the venous blood the partial pressure of carbon dioxide has become too low to excite the respiratory centre. It will be evident from the foregoing that in order to supply efficiently the respiratory requirements of the tissues not only must the breathing, but also the circulation, be suitably regulated. In hard muscular work the consumption of oxygen and output of carbon dioxide may be increased eight or ten times beyond those of rest. Unless, therefore, the blood supply to the active tissues were correspondingly increased, deficiency of oxygen would at once arise, since the amount of oxygen carried by a given volume of the arterial blood is very limited, as already explained. It is known that the supply of blood to each organ is always increased during its activity. This increase can, for instance, readily be seen and measured in the case of contracting muscles or secreting glands; and the volume and frequency of the pulse are greatly increased during muscular work. But while it is evident enough that the flow of blood through the body is determined in accordance with the metabolic activities of each tissue, our knowledge is as yet very scanty as to the means by which this determination is brought about. Probably, however, carbon dioxide may be nearly as important a factor in the regulation of the circulation as in that of breathing. Just as the rate of breathing was formerly supposed to determine, and not to be determined by, the fundamental metabolic processes of the body, so the circulation was supposed to be another independent determining factor; and under the influence of these mechanistic conceptions the direction of investigation into the phenomena of respiration and circulation has been largely diverted to side issues. Since the circulation, no less than the breathing, is concerned in the supply of oxygen to and removal of carbon dioxide from the tissues, it can readily be understood that defective circulation, such as occurs, for instance, in uncom pensated valvular affections of the heart, may affect the breathing and hinder the normal respiratory exchange. Conversely, also, defects in the aeration or oxygen-carrying power of the blood may be compensated for by increase in the circulation. For instance, in the very common condition known as anaemia, where the percentage of haemoglobin, and consequently the oxygen-carrying power of the blood, is often reduced to a third or less, the respiratory disturbances may be so slight that the patient is going about his or her ordinary work. A miner suffering from the now well-known " wormdisease," or ankylostomiasis, may be working underground, or a housemaid suffering from chlorosis may be doing her work, with only a third of the normal oxygen-carrying power of the blood. There seems to be no doubt that in such cases an increased rate of blood circulation compensates for the diminished oxygen-carrying power of the blood. It is well known that at high altitudes a gradual process of adaptation to the low pressure occurs, and the shortness of breath and other symptoms experienced for the first few days gradually become less and less. This adaptation is partly, at least, due to a marked increase in the percentage of haemoglobin in the blood, though probably circulatory and perhaps other compensatory changes are also involved. In connexion with respiration the action of certain poisons is of great interest. One of these, carbon monoxide, is of very common occurrence, and causes numerous cases of poisoning. Like oxygen, it has the property of combining with the haemoglobin of the blood, but its affinity for haemoglobin is far more strong than that of oxygen. In presence of air containing as little as. 05% of carbon monoxide, the haemoglobin will become about equally shared between oxygen and carbon monoxide, so that, since air contains 20.0% of oxygen, the affinity of carbon monoxide for haemoglobin may be regarded as about 400 times greater than that of oxygen. The blood of a person breathing even a small, percentage of carbon monoxide may thus become gradually saturated to a dangerous extent, since the haemoglobin engaged by the carbon monoxide is for the time useless as an oxygen-carrier. Air containing more than about o.r % of carbon monoxide is thus more or less dangerous if breathed for long; but the blood completely recovers in the course of a few hours if pure air is again breathed. The poisonous action of carbon monoxide can be abolished by placing the animal exposed to it in oxygen at an excess pressure of about an atmosphere. The reason for this is that, in consequence of the increased partial pressure of the oxygen, the amount of this gas in free solution in the blood is greatly increased in accordance with Dalton's law, and becomes sufficient to supply the tissues with oxygen quite independently of the haemoglobin. Even at ordinary atmospheric pressure the extra oxygen dissolved in the blood when pure oxygen is breathed is of considerable importance. Carbon-monoxide poisoning is the chief cause of death in colliery explosions and fires, and the sole cause in poisoning by lighting gas and fuel gas of various kinds. Its presence in dangerous proportions may be readily detected with the help of a small bird, mouse or other small wain-blooded animal. In such animals the respiratory exchange is so rapid that symptoms of carbon-monoxide poisoning are shown far more quickly than in man. The small animal can thus be employed in mines, &c., to indicate danger from carbon monoxide. A lamp is useless for this purpose. There are various other poisons, such as nitrites, chlorates, dinitrobenzol, &c., which act by disabling the haemoglobin, and so cutting off the oxygen supply to the tissues.
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