RESPIRATION
Life means change. We cannot imagine its continuance without
liberation of energy. Arrest of molecular activity is death. There
is no possibility of its revival. A watch that has stopped may be
started by shaking. On the cessation of molecular activity an animate
being becomes inanimate. Dead, it is liable to further chemical
changes. Bacteria invade it. They shake down its complex unstable
compounds into simple, stable, so-called “inorganic groups”; but the
ordered combination with oxygen, which constitutes living, can never
recommence. Putrefaction may be prevented by the exclusion of germs.
The inanimate mass of organic material may remain unchanged. Its return
to life would be a miracle. From time to time a frog is found enclosed
in old red sandstone, or some other rock which for countless ages has
lain beneath the surface. The cleft through which the frog entered
a few hours or days before it was discovered is overlooked. It is
supposed to have lived “in a state of suspended animation” for millions
of years. The fact that no frogs are to be found among the fossils of
the old red sandstone is an objection too casuistical to be seriously
entertained. The physiologist’s demand to know what has become of the
mountains of solid carbonic acid, water, and urea which the frog must
have produced during its unimaginable term of incarceration is regarded
as the natural expression of his prejudice—that life cannot continue
without molecular change. And he is bound to admit his inability to
prove that it cannot. Nevertheless, his experience that, whenever and
however he may, by experimental methods, arrest change, he loses the
power of causing it to recommence justifies him in his conviction that
life is change. Even a living seed is to his mind an organism whose
complex constituents are slowly—however slowly—setting free energy by
settling down the steps which lead to stability and ultimate, inanimate
rest; and the only source of this energy is combination with oxygen. In
the case of a seed the oxygen need not come from without. Seeds retain
their power of germination after long occlusion in nitrogen or other
neutral gases. But all the time some change is occurring, some internal
oxidation which resolves their less stable into more stable compounds.
Otherwise they would not be alive. A physiologist is willing to believe
that this may continue for ten years, fifteen years—for any period
that the botanist tells him that he has, under verifiable conditions,
observed that it does occur; but when he is told that peas taken from
the hand of an Egyptian mummy, or seeds set free by the spades of
navvies after a far longer burial, have been found to retain their
vitality, his credulity is stretched beyond breaking-point. He cannot
imagine a change so slow as to be spread over a geological period,
still without exhaustion of all changeable compounds.
The term “respiration” has been extended until it is synonymous with
“oxidation.” At one time it was supposed that the combination of oxygen
with oxidizable substances occurred in the lungs. The lungs were the
hearth of the body, to which the blood brought fuel which burned
in the air drawn into them. When it was understood that the actual
combination of combustible material with oxygen occurs, not in the
lungs, but in the tissues, a somewhat illogical distinction was made
between “external respiration”—the combination of oxygen and blood
in the lungs—and “internal respiration”—the combination of oxygen
and tissue-substances. The terms are not comparable. The taking up of
oxygen by the hæmoglobin of blood is a different process to the union
of oxygen, after the hæmoglobin has parted with it, with the carbon,
hydrogen, and nitrogen of the tissue-substances.
The blood-stream carries both fuel and oxygen to the tissues, but the
fuel while in the blood is not in an oxidizable condition. The foods
are taken up by the tissues. They enter into combination with their
protoplasm. Oxygen also combines with tissue-substances. In proportion
as the tissues are active oxidized compounds are split off. They fall
into the lymph, whence they are absorbed by the blood. If they are
nitrogenous compounds, they are carried to the liver, formed into urea,
and passed to the kidneys for elimination. If carbonic acid, it is
carried to the lungs for exhalation. The water formed by combination of
hydrogen and oxygen may escape from the lungs, the kidneys, or the skin.
Two or three pounds of mixed foods are consumed every day. By the blood
they are carried to the tissues, whence an equivalent quantity of
waste—that is to say, oxidized—material is removed. About 1½ pounds
of oxygen is required to burn the day’s fuel.
The problems of respiration are twofold. In the first place we have to
consider the physics and chemistry of the combination of hæmoglobin
with oxygen, and of the elimination of carbonic acid from the blood in
the lungs; secondly we have to explain the transference of oxygen from
hæmoglobin to the tissues, and the reception in the blood of carbonic
acid produced by the tissues.
The apparatus by which air is brought into relation with the blood
consists of lungs and windpipe. At its upper end, where it joins the
portion of the alimentary tract common to deglutition and respiration,
the special respiratory tube is protected by the larynx. The nasal
chambers belong to the respiratory tract; the gullet, or pharynx, is
common to the two functions.
The mucous membrane which lines the nose and windpipe is kept moist
in order that it may catch particles of dust drawn in with the air.
At the same time the nasal chambers serve to warm the air, and to add
moisture to it if it be too dry; for the lining epithelium of the lungs
would suffer if dry air came in contact with it. The wall-surface
of the nasal chambers is increased by the projection of folded and
chambered “turbinate bones.” The importance of warming the air before
it is admitted to the lungs is remarkably illustrated in the case of
certain sea-birds. The nasal chambers of the frigate-bird, and of some
other birds which resemble it, are exceptionally complicated. Since the
animal is devoid of any sense of smell, and the air which it breathes
must be nearly saturated with moisture, the only function which can be
assigned to these convoluted passages is that of warming inspired air.
The larynx will be more minutely described when it is considered
as the organ of voice. In connection with respiration, it must be
regarded as primarily a valve which closes the entrance to the windpipe
during swallowing. It is overhung by a leaf-like appendage—the
epiglottis—formed of exceedingly elastic tissue. It was thought until
lately that the epiglottis drops over the aperture of the larynx when
food is passing down the gullet, and springs up again as soon as the
act of deglutition is over; but recent observations have shown that
during deglutition the epiglottis is pressed against the back of the
tongue, and that the closure of the larynx is effected by its own
sphincter muscles. The mucous membrane of the larynx is extremely
sensitive to stimulation by anything which would be prejudicial to the
tissue of the lungs. When its sensory nerve—the superior laryngeal—is
stimulated, the larynx closes. It is the agent in carrying out many
reflex actions, in which not the larynx only, but also the muscles of
the chest and diaphragm, take part. For example, it immediately stops
inspiration if an irritating vapour is present in the air. It stops
respiration if any foreign body, such as a crumb of bread or a drop
of water, touches the mucous membrane. When the trunk of the nerve is
stimulated by an electric current, respiration is inhibited. Further,
under suitable stimulation the nerve brings about respiratory movements
in which inspiration is gentle and expiration sudden, violent,
convulsive. Rib-muscles and diaphragm combine to produce a cough, which
ejects the noxious body. Again, its stimulation in a different way
probably helps to produce constriction of the smaller bronchi which
regulate the amount of air supplied to the air-cells of the lungs;
although this constriction may be largely due to a reflex which starts
in the air-cells. The epithelium of the air-cells has an immensely rich
supply of sensory nerves. In some persons this protective mechanism
is very prone to overact its part. A little dust or foul gas in the
air leads to such marked contraction of the bronchi that respiration
becomes very difficult. Such an exaggerated tendency to reflex action
constitutes the neurosis, asthma. In this malady the mechanism is
unduly sensitive. Very slight stimulation leads to a maximum discharge
of impulses to the muscular tissue of the bronchi.
The trachea has a length of about 4 inches. It extends from the lower
edge of the cricoid cartilage, which is easily felt in the neck beneath
the thyroid cartilage (Adam’s apple), to the under side of the arch
of the aorta, where it divides into the right and left bronchi. The
epithelium which lines the trachea and bronchi is ciliated. The cilia
propel the secretion which accumulates on its surface upwards towards
the larynx. The wall of the windpipe is kept open by rings of cartilage
which are incomplete behind, where the trachea and œsophagus are in
contact. Rings and plates of cartilage also support the bronchi. The
bronchi divide and subdivide until their diameter is reduced to about
0·2 millimetre. Each bronchiole then breaks up into a bunch of very
thin-walled, elongated infundibula, club-shaped, and with a diameter
about five times that of the bronchiole with which they are connected.
They may be three or four times as long as they are broad. The wall
of an infundibulum is pitted like a piece of honeycomb into shallow
chambers—the air-cells or alveoli.
The walls of the air-chambers, or alveoli, are formed of a membrane
upon which is spread a network of capillary bloodvessels. The
air-chambers are so closely packed together that a common wall
separates one chamber from the next adjoining. Minute bloodvessels
pierce the partitions which separate the chambers, appearing now on
one side of the wall, now on the other. The air-chambers are lined by
thin epithelial scales or tiles. The blood in the capillary vessels
is separated from the air in the air-chambers by the wall of the
capillary; by a lymph-space, probably rather potential than actual; and
by the epithelial tiles. This covering suffices to prevent the escape
both of red corpuscles and of plasma, yet offers very little resistance
to the passage of gases from the blood into the air, and from the air
into the blood.
Leucocytes make their way between the tiles, and creep over their
internal surfaces, searching for cell débris or foreign matter.
Anything that they find they carry to the clumps of lymphoid tissue
which occur in the outer wall of the bronchi. In a town-dweller,
leucocytes are found in these lymph-thickets, charged with particles of
soot. They show droplets of fat and other evidences of degeneration. At
other spots are to be seen little collections of soot which have been
left behind after the dissolution of the leucocytes which brought them
there.
It is not possible to make anything like an accurate estimate of the
number of alveoli in the lungs; 725,000,000 is a figure arrived at by
measuring the average cubic capacity of an alveolus, and comparing it
with the total cubic capacity of the lungs. Each alveolus supports some
forty or fifty capillary vessels. The superficial area of vascular
membrane exposed is placed at 90 square metres, or about 100 times the
area of the skin. Figures such as these convey very little meaning, but
they help one to realize the magnitude of the provision made for the
aeration of the blood.
Pneumonia is a condition in which the lining of the air-chambers is
inflamed, usually, possibly always, owing to the entrance of bacteria.
Lymph exudes through the walls of the alveoli. Epithelial scales flake
off. Pus cells (dead leucocytes) accumulate in the air-chambers.
Respiration is curtailed, and dyspnœa results. After a time, if the
case progresses favourably, “resolution,” as it is technically termed,
begins to occur. The exuded substances are either expectorated or
absorbed, and the lung-tissue returns to a normal condition.
Here a few words may be devoted to respiratory sounds. _Spirare_
means to sigh. Breathing received the name by which it is known in
physiology from the sound which accompanies the exit of air from
the nostrils. Since the introduction of auscultation as a means of
ascertaining the condition of the lungs, other sounds, not heard until
the ear or a stethoscope is placed against the chest, have acquired
great importance. These sounds, termed “murmurs,” may be divided
into two classes. (_a_) When the ear is placed against the windpipe,
or in the middle of the back between the shoulder-blades, a murmur
is audible, due to the movement of air through the larynx. If the
larynx, the trachea, or the bronchi contain mucus, it is a harsh,
rough, bubbling, or crackling sound. It accompanies both inspiration
and expiration. (_b_) A softer, more delicate murmur is heard when
the ear is placed against the front or the side of the chest. This is
the vesicular or pulmonary murmur. It is heard during inspiration,
and is due to the passage of air out of the smallest bronchi into
the more spacious infundibula in which they end. These two kinds of
murmur must be rigidly distinguished—the laryngeal murmur, heard in
situations in which no lung-tissue intervenes between the ear and the
great tracheal or bronchial tubes; and the pulmonary murmur, heard
over all regions where the bronchi are buried in lung. Healthy lung is
as bad a conductor of sound as a sponge or a wad of cotton-wool. The
laryngeal murmur is inaudible in regions in which lung lies beneath the
chest-wall. It would be far beyond the scope of this book to attempt to
describe the very varied alterations in the chest-sounds which may be
produced by disease. The student would do well to familiarize himself
with the nature of the sounds which are heard in health, and the
situations in which they are heard, in order that he may be able, in
abnormal conditions, to recognize that something is wrong.
The chief departures from the normal may be grouped under the following
heads: (1) The pulmonary murmur may lose its soft, smooth, sighing
character owing to inflammation of the alveoli and infundibula. It may
be as loud in expiration as in inspiration. Only a practised ear can
estimate the significance of these changes. (2) The laryngeal murmur
may be reinforced by “râles”—a convenient term for supplementary
sounds. The source of such râles may be a cold in the chest,
laryngitis, or bronchitis of various degrees. (3) The laryngeal murmur
may be heard in situations in which lung intervenes between the ear
and the larger bronchial tubes. This can be due only to the lung being
in an abnormal condition as a conductor of sound. Instead of being
as spongy as well-made Vienna bread, its air-spaces are filled with
solid or fluid deposit. It is as firm as dough. To such a condition
it attains at the height of pneumonia—a stage termed “hepatization”
because in section it looks like liver rather than lung.
Breathing is the enlargement and diminution of the chest, which causes
air to be drawn into and expressed from the lungs. The windpipe being
open, the air inside the lungs is, of course, at the same pressure as
the atmosphere. Expansion of the chest results in the equal expansion
of the lungs. Since there is no air-space between the outer surface
of the lungs and the inner surface of the chest-wall, the lungs
cannot separate from the chest-wall when it expands. But the lungs
contain elastic tissue always slightly on the stretch. If the chest be
punctured, and air admitted between the chest-wall and the lungs, the
lungs collapse. The expiratory movement, the contraction of the chest,
is due to the elasticity of the lungs. This tendency on the part of the
lungs to contract is sufficient in quiet respiration to restore the
chest to its usual size after inspiration, and thus to expel air. The
lungs are held open owing to the negative pressure in the space which
separates them from the chest-wall. This negative pressure has a most
important relation to their permeability by air. Imagine the condition
reversed. Picture a lung into which air is forced by a muscular pump.
After each stroke of the pump the lung would collapse. Its finest
tubes and their dilated terminations could be maintained as open
spaces, between the strokes of the pump, only by giving a considerable
thickness and firmness to their walls. Such a substantial structure
would be unfavourable to an interchange of gases between the blood and
the air. The reverse of this condition is found in Nature. The lung is
stretched from without. Its tissue, delicate as crêpe, cannot collapse
even at the end of the deepest expiration.
The ribs are united by intercostal muscles, disposed in two sheets.
The fibres of the external intercostals are directed downwards and
forwards, those of the internal intercostals downwards and backwards.
In tranquil respiration the chest is enlarged by the external
intercostal muscles, which raise the ribs, and the diaphragmatic
muscle, which renders peripheral portions of the diaphragm flat.
The rôle of the internal intercostal muscles is a subject still
under discussion. For the most part, physiologists regard them as
accessory to expiration, but some hold that they combine with the
external intercostals in raising the ribs and twisting them outwards
during inspiration. The diaphragm is a partition which separates the
thoracic from the abdominal cavity. It is in the form of a vault. The
central portion of the dome is membranous, its margins muscular. Its
membranous centre is in contact with the pericardium, which encloses
the heart. The level of this part is therefore fixed, except in
forced inspiration, when it descends slightly. It constitutes a fixed
plane for the muscles of the diaphragm, which are attached below
to the vertebral column and the ribs. When the muscles contract in
inspiration, the curvature of the marginal portions of the diaphragm
is diminished, and the chest-cavity consequently enlarged. During
expiration the space between the muscle of the diaphragm and the
chest-wall closes up, and the lower border of the lung slips out of it.
There is a marked difference in the relative extent of the costal and
diaphragmatic movements in men and women. In women respiration is
chiefly costal; in men it is chiefly diaphragmatic. In men the abdomen
moves forwards, as the diaphragm descends in tranquil breathing; in
women the chest rises. Men who wish, for the purposes of athletics, or
singing, or public speaking, to retain the power of making the most of
their chest-capacity are wise in not allowing themselves to fall into
the habit of lazy, abdominal breathing.
[Illustration:
FIG. 8.—THE DIAPHRAGM AND ORGANS IN CONTACT WITH IT—A,
IN EXPIRATION; B, AT THE END OF A DEEP INSPIRATION.
TRANSVERSE VERTICAL SECTIONS IN THE LINE OF THE ARMPIT.
A, At the end of an ordinary expiration the lung does
not extend below the upper border of the eighth
rib. From this level to the middle or lower border
of the tenth rib the two layers of the pleura
covering respectively the inner wall of the chest
and the upper surface of the diaphragm are in
contact. B, When the lung is distended with air it
occupies the whole of the pleural cavity.]
When additional pressure is required, when respiration is forced,
various external muscles attached to the spinal column, the
shoulder-blades, and the clavicles, as well as the muscles of the
abdomen, come into play.
The chest is lined and the lungs covered by a serous membrane—the
pleura. Normally there is only just sufficient lymph in the space
between the visceral layer of the pleura which invests the lungs and
the parietal layer which lines the chest-wall to prevent friction
during respiration. When the pleura is inflamed, one layer of the
membrane rubs against the other. In the early or “dry” stage of
pleurisy, the physician recognizes this condition by the friction-sound
which he hears on placing his stethoscope against the chest. In a
later stage lymph (pleuritic fluid) is poured out. It accumulates in
the lower part of the chest, and is recognized by the absence of the
resonant note which, under normal conditions, is given out by the chest
when percussed.
The lungs are not compressed during expiration; they are not squeezed,
as a pair of bellows or a sponge may be squeezed, emptying it of its
contents. At the end of tranquil expiration the lungs still contain
about 3½ litres of air. At the top of tranquil inspiration the volume
of their contents does not exceed 4 litres. It is evident, therefore,
that air is not drawn into and driven out from the air-chambers by
the movements of respiration. The tide of air does not extend far
beyond the ends of the bronchi. The gases in the air-chambers are
exchanged with the fresh air drawn into the infundibula by diffusion.
The composition of the air which is in contact with the bloodvessels
is constant. It is about 4 per cent. poorer in oxygen and 3 per cent.
richer in carbonic acid than the outside air.
Of the air drawn into the windpipe during an inspiration, about
one-third returns to the open with the following expiration; two-thirds
remains in the lungs. If, therefore, the air taken in at each tide
equals one-seventh of the quantity already in the lungs, and if of this
one-seventh two-thirds remains, each alveolus renews about one-tenth of
its air. Its contents are completely changed in ten respirations.
Fresh air is composed of 21 per cent. oxygen, 79 per cent. nitrogen,
and a trace (0·04 per cent.) of carbonic acid. Forced by a syringe
through lime-water, fresh air does not produce any appreciable
milkiness, whereas air breathed through a tube into lime-water renders
it turbid owing to the formation of carbonate of lime. Carbonic acid
(CO₂) occupies the same volume as its oxygen (O₂) would occupy if free.
The oxygen which breathed air has lost slightly exceeds in amount the
carbonic acid which it has gained in exchange. The difference is due
to the retention of some of the oxygen for the purpose of uniting with
hydrogen to form water, and of forming urea. The proportion between
carbonic acid gained and oxygen lost, CO₂/O₂ is termed the “respiratory
quotient.” Its value varies, of course, with diet. In a herbivorous
animal, whose food consists of carbohydrates, it departs but little
from unity; in a carnivore, which eats fat and nitrogen-containing
food, it is about 0·8.
The respiratory exchange is very much smaller in cold-blooded animals
than in animals which maintain the temperature of the body at a fixed
level. In warm-blooded animals it rises as the temperature falls, falls
as it rises, the increased oxidation warming the body, the diminished
oxidation allowing it to cool; whereas in cold-blooded animals it
increases as the temperature rises, owing to the greater activity
induced by warmth, and falls as the temperature falls.
The respiratory exchange is increased by muscular activity. If the
amounts of oxygen absorbed and carbonic acid given out are measured
while a man is at rest, and again while he is doing hard physical work,
it is found that during work the respiratory exchange is twice as great
as during rest. During periods of starvation the respiratory exchange
remains unaltered, since heat has to be constantly produced if the
temperature of the body is to be kept from falling.
Since the purpose of respiration is to give to the blood the
opportunity of renewing its supply of oxygen, and of getting rid of the
carbonic acid with which it is charged, it might be supposed that the
respiratory exchange would be increased, so far as the intake of oxygen
is concerned, by breathing oxygen gas instead of air; but it appears
that under normal conditions nothing is gained. When an animal is
breathing air, its blood takes up all the oxygen that it wants—all the
oxygen, that is to say, for which its tissues are asking. Offering it
pure oxygen in place of mixed oxygen and nitrogen does not induce it to
take up more. The hæmoglobin is almost saturated with oxygen when the
blood leaves the lungs under ordinary conditions. In certain diseases
of the lungs, however, in which the blood becomes unduly venous, the
respiration of oxygen may be beneficial; but even in these cases the
results are disappointing, because the system is suffering much less
from deficiency of oxygen than from accumulation of carbonic acid.
Substituting oxygen for air does not facilitate the escape of carbonic
acid.
=The nervous mechanism of respiration= has been the subject of
much investigation and of many experiments, without, it must be
confessed, the development of a quite complete or satisfactory theory.
Respiration is a rhythmic process. About seventeen times in a minute
the intercostal and diaphragmatic muscles contract. Inspiration is
immediately followed by expiration, the falling movement being due, as
already explained, to the elasticity of the lungs, which are stretched
during inspiration. A slight pause intervenes between the end of
expiration and the commencement of the next inspiratory movement.
Tranquil respiration is a succession of reflex inspiratory movements,
the depth of which varies according to the needs of the body—that
is to say, according to the condition of the blood. If the need for
aeration of the blood becomes urgent, the depth of inspiration is
increased, and expiration also becomes an active movement, certain
muscles, especially those of the abdomen, being called into play. In
this condition two sets of reflex actions alternate. A large number
of nerves are concerned even in tranquil respiration. If a man in
falling “breaks his back” at the junction of the cervical and thoracic
regions, costal respiration ceases. The series of intercostal nerves
which arises from the dorsal spinal cord below the level at which
it is injured are thrown out of action. Diaphragmatic respiration
still continues, because the nerve of the diaphragm, the phrenic,
arises from cervical roots. The lungs are supplied by the vagus
nerve. This nerve joins the medulla oblongata as one of a group of
three—glosso-pharyngeal, vagus, and spinal accessory—which by a large
number of roots enter the groove between the olive and the restiform
body. The vagus is the channel along which afferent impulses from the
lungs enter the medulla. Such impulses call for respiratory movements.
Cutting both vagi, however, does not put an end to respiration.
Inspiratory movements continue, but they are much deeper and separated
by much longer pauses. Such a form of respiration is inefficient. The
blood is not properly aerated. The animal falls into a condition of
dyspnœa, which ends in death. When the central end of the cut vagus is
stimulated, the movements become more natural. Clearly, the respiratory
reflex is not dependent upon the vagus, since it continues after the
nerve is cut, although the impulses which pass up this nerve regulate
its rhythm. They govern the length of the inspiratory movements, cut
them short at the right moment, and secure their succession at proper
intervals.
The transfer of afferent impulses into efferent channels occurs in
the medulla oblongata. Long ago it was found that if the brain above
this level be removed, part by part, respiration is not interfered
with until the medulla oblongata is injured. When a cut is made into
the floor of the fourth ventricle not far to one side of the middle
line, the respiratory movements on that side of the body cease. If the
injury be bilateral, even though very limited in extent, respiration
stops. This spot was therefore spoken of as the “respiratory centre.”
Flourens, who first discovered it, believed that it was a mere spot.
He gave to it the fanciful name of _nœud vital_. It is the place at
which the afferent nerves which call for respiration are brought
into connection with all the various motor nerves which bring about
the respiratory movements of nostrils, larynx, chest, and diaphragm.
Possibly the knife in Flourens’ incision divides the tract of fibres
which distributes afferent impulses, but whether the junction be a
defined tract or no, injury to this region of the medulla throws the
nervous mechanism of respiration out of gear. At this particular spot
lies the “centre” for respiration—the one part of the nervous system
which must be intact if the movements of respiration are to be carried
out. There is no reason for thinking that respiratory impulses are
generated at this spot. It is a centre in the same sense in which Crewe
is a centre for distributing the goods of Lancashire and other parts
of England to North Wales. The use of the term “nerve-centre” has been
very much abused. Centres were supposed to be collections of cells,
each group of which had some prerogative of initiation. Reasoning
from the analogy of human institutions, it was thought necessary that
the nervous system should be organized into departments severally
responsible for the administration of the activities of certain sets
of muscles: one centre controlled respiration, another the beat of
the heart, another deglutition. The centres were dependent one on
another; each regulated lower centres, and was governed by those above
it, in this bureaucratic scheme. We know nothing of any function of
nerve-cells other than that of transmitting impulses. All that we know
about nerve-cells is that they place afferent and efferent routes in
communication, and interpose resistance into nerve-circuits. Every
nerve-cell of the grey matter of the brain and spinal cord gives
off processes which ramify. The ultimate twigs into which a branch
divides are in connection with other sets of twigs derived from the
end-branchings of nerve-fibres or processes of other nerve-cells. A
nerve-fibre is but the axis-cylinder process of a nerve-cell. Impulses
encounter resistance in passing along the neuro-fibrillæ (_cf._ Fig.
22) contained in the twig-connections of the ramifying processes of
nerve-cells. There is no reason for supposing that anything like
the same resistance is offered to the passage of impulses along the
fibrillæ where they lie within the stout branches of the cell-processes
or within the body of the cell. It is easy to make a pictorial
representation of such a mechanism. Imagine a model of the stem of a
tree made by binding together a large number of wires; its branches as
containing small groups of wires; the ultimate twigs as separate wires.
Carry wires from the roots of one tree to the branches of another.
Trees so constructed might be taken as representing nerve-cells. We
have not as yet succeeded in demonstrating the isolated neuro-fibrillæ
as they pass over from the end-twigs of a nerve-fibre to the end-twigs
of a nerve-cell branch, but we have abundant reason for believing
that they do so pass, and that the resistance to the passage of a
nerve-impulse is interposed in this neutral or junctional zone. This
resistance has to be overcome. It is overcome by the summation of
impulses. All nerve-impulses are vibratory. The first vibrations may
fail to get through; but if the vibrations continue, they exert a
cumulative effect. After a time they overcome the resistance; sensory
impulses flow through the centre into motor channels. In this way we
endeavour to explain the rhythmic discharge through the respiratory and
other centres. It has not been found possible to determine the source
of all the afferent impulses which reach the centre. Respiration
continues after all accessible nerves have been cut, including even
the posterior roots of the cervical nerves. Probably it is a mistake
to look for definite afferent channels in the medulla and the rest of
the brain. All parts of the body need aerated blood. From all parts,
including nerve-tissue itself, arises the demand for respiration.
Possibly nerve-centres have the power, as it were, of storing impulses,
and discharging them after the stream of fresh arrivals has ceased to
flow. They may acquire a habit.
The resistance in the centre is profoundly affected by the condition
of the blood. As the blood becomes more venous, impulses pass across
the nerve connections with ever-increasing force. Kept in the first
instance to definite channels, they spread as the centre becomes more
excitable farther and farther afield, reaching one group of muscles
after another, and pressing them into the service of respiration.
When, in dyspnœa, every muscle which can in any way help the movements
of the chest is doing its best, others which are useless for this
purpose receive the reflected impulses and join in, producing general
convulsions. The increased activity of the respiratory centre which
is produced by slight venosity of the blood is shown in the rapid
and deep inspirations which are caused by violent exercise. Perhaps
it is justifiable to go a step farther, and to assert that there is
something in blood which has been rendered venous by muscular activity
which is specially exciting to the respiratory centre. If the blood
from a limb be prevented from returning to the general circulation,
by compressing or tying its great veins, and if the muscles of the
limb be strongly stimulated by an electric current, their activity,
so long as the passage through the veins is blocked, has no influence
upon respiration. But, on relaxation of the pressure on the veins,
respiration may become twice as deep and twice as frequent as it was
before the muscles were stimulated, although the limb is now in a
condition of perfect rest.
What is the special action of the vagus nerve? Its superior laryngeal
branch checks inspiration and induces expiration, as already said. The
impulses which pass up its main trunk bring about ordered movements.
They are not dependent for their generation upon the condition of the
blood in the lungs. When the chest is filled with nitrogen, inspiration
and expiration alternate in the usual way, although the blood is
growing steadily more venous. The failure of inspiration to bring
about aeration of the blood does not lead to a prolongation of the
inspiratory effort. Inspiration is cut off and expiration established
in regular sequence. In performing “artificial respiration” (_cf._ p.
184) for the purpose of saving life, in cases in which respiration
has ceased owing to the lungs being filled with water, or for other
reasons, the chest is enlarged by raising the arms above the head,
and diminished by pressing the elbows against the sides. Enlargement
promotes a tendency to expiration, compression a tendency to a natural
inspiratory effort. Evidently there is a connection between the
movements of the chest and the stimulation of the respiratory centre.
If respiration is being carried on artificially, by forcing air from
a bellows into the trachea, the nostrils dilate as the chest is
distended, and contract as it is emptied, so long as the vagus nerve
is intact, just as they do in normal respiration. This shows that,
when the chest is emptied, a message is sent through to the nucleus
of origin of the nerve which supplies the dilator muscles of the
nostril. When the lungs are full, a message calls upon the nostrils to
contract. The only factor which is common to pressing in and pulling
out the ribs, and filling and exhausting the lungs with a bellows, is
the alteration in the form of the lungs which is produced by the two
methods. It is impossible to resist the conclusion that the stretching
of the tissue of the lungs stimulates the nerve-endings of the vagus.
The impulses thus induced automatically stop inspiration, and lead to
an expiratory effort.
There are many indications that the nervous mechanism of respiration
is a double one, certain stimuli inducing expiration, with inhibition
of inspiration, others inhibiting expiration and inducing inspiration.
There are, however, many difficulties in the way of formulating a
satisfactory theory of the relation of these antagonistic actions.
We may frequently observe indications of such an antagonism between
the two phases of the respiratory mechanism. Cold water dashed on the
back of the head (when the head is being shampooed) induces a long
inspiration with inhibition of expiration. A blow in the pit of the
stomach “knocks all the wind out of a man.” Expiration is prolonged
until the lungs are unusually empty, and yet the victim of the blow
feels as if he would never again be able to draw breath.
=Modified Respiratory Movements.=—The object of coughing is to expel
foreign matter from the windpipe or larynx; of sneezing, to clear
the nose. The former action consists of a long deep inspiration;
the closure of the glottis; a forcible expiration. The blast of air
encountering a closed glottis acquires considerable pressure. When
the resistance of the glottis is overcome, the blast rushes through,
carrying with it mucus or bread-crumb, or whatever the substance
may be which irritated the endings of the superior laryngeal nerve.
In sneezing, the back of the tongue is thrust against the palate,
closing the aperture of the fauces. Inspiration is prolonged. A strong
expiration follows. The blast rushes through the nasal cavities. This
reflex is usually provoked by a tickling of the endings of the fifth
nerve in the nasal mucous membrane. It is also caused in many persons,
through the optic nerve, by a bright light; an apparently purposeless
reflex about which we shall have something more to say in a subsequent
chapter. Laughing and crying are modified respiratory movements as
useless, so far as any immediate purpose is accomplished, as sneezing
in response to a bright light. As means of expressing emotions they
have been cultivated by the human race. Possibly a case for crying
might be made out on physiological grounds. Under certain circumstances
it relieves a feeling of distress which, while it lasts, is detrimental
to the proper functions of the body. Laughing undoubtedly is
beneficial. The rapid movements of the chest quicken the circulation.
The shaking of the midriff favours the discharge of digestive
secretions, accelerates the movements of the alimentary canal, and
generally is beneficial to digestion. But “laugh and grow fat” is not
necessarily the order of cause and effect. An efficient digestion and
a good capacity for assimilation lead to a sense of _bien-être_ which
predisposes to a merry view of life.
Yawning is a deep inspiration with open mouth and larynx. It commences
usually at the end of a normal inspiration, a slight pause being
followed by further inspiration, deep and prolonged. Its commencement
seems to be due to impulses generated by the relaxation of the tone of
the muscle which holds up the lower jaw. The masseter goes off duty for
a moment, allowing the jaw to fall. A reflex contraction of the muscles
which open the mouth immediately follows. Muscles of the neck and head
also come into play. Not improbably the yawn ends in a general stretch.
If the origin of this reflex is obscure, its usefulness is marked. The
circulation is quickened, the blood is changed, nervous system and
muscles again become alert.
“Apnœa” is the condition of arrested respiration. If a man about to
dive into the water breathe deeply and rapidly half a dozen times, he
abolishes for a while the desire to breathe. One is naturally inclined
to explain this as due to a surplus of oxygen taken into the blood, but
a moment’s reflection shows that this cannot be the cause. In the first
place, as we have already pointed out, the blood which leaves the lungs
in tranquil respiration is very nearly saturated with oxygen. It can
take up but little more. Again, the deep inspirations do not change the
air in the air-chambers; time is required for the renewal by diffusion
of their gaseous contents. It is improbable that the constitution of
the air in the alveoli is sensibly altered by a few deep breaths.
Probably the explanation is to be found in the effect upon the
nerve-centre of distention of the chest. Stretching the nerve-endings
of the vagus in the lungs inhibits inspiration. If the stimulation be
excessive, inspiration is inhibited for a considerable time. That this
is the right theory of apnœa is proved by repeatedly inflating the
lungs of an anæsthetized animal with a pair of bellows. The same arrest
of inspiration is induced whether the lungs are inflated with air or
with a neutral gas, such as nitrogen, so long as the vagus nerve is
intact. If this be cut, inflation with a neutral gas no longer produces
apnœa.
“Dyspnœa” is the term applied to the complex conditions and movements
which result from deficient aeration of the blood, or, rather, from
the distribution of insufficiently aerated blood to the centres in
the medulla oblongata. The blood of the rest of the body may be in a
satisfactory condition, but if, owing to ligature of the carotid and
vertebral arteries or other causes, the blood supplied to the brain be
inadequate to its proper nutrition, the phenomena of dyspnœa are as
marked as they are when air is prevented from entering the lungs. That
the excitability of the nerve-centres in the brain is greatly increased
when this organ is supplied with venous blood, and that their tendency
to transmit impulses which call for respiration is consequently
exaggerated, is remarkably shown by the following experiment: Two
rabbits—A. and B.—are placed under the influence of chloroform. Their
carotid arteries are cut, and a crossed circulation established by
connecting the proximal ends of A.’s arteries with the distal ends of
B.’s, and _vice versa_. The head of each rabbit is now supplied with
blood from the heart of the other, the rest of its body by blood from
its own heart. A.’s chest is now opened, so that its lungs collapse and
cease to take part in respiration. The animal continues to make the
movements of respiration in a tranquil manner, whereas B. is thrown
into violent dyspnœa. The animal whose brain is receiving aerated blood
remains normal, notwithstanding the fact that its lungs and the rest
of its body are poisoned with venous blood. The animal whose brain is
supplied with venous blood becomes dyspnœic, although its lungs and
body are receiving pure arterial blood.
There is a regular sequence in the phenomena of dyspnœa leading up
to the final stage termed “asphyxia.” If the trachea be suddenly
blocked, so that no air can pass, the respiratory movements at once
become deeper and more rapid. This condition is termed “hyperpnœa.”
In a comparatively few seconds the system appears, as it were, to
find out that inspiration is not needed. Expiratory efforts begin to
preponderate. They increase in violence. All accessory muscles are
brought into play. The cry for air is heard even by muscles which
cannot help. Muscles of the limbs contract, although their contraction
has no effect upon the capacity of the chest. Every expiratory effort
is accompanied by convulsions of a flexor type. At the end of two
minutes there is usually a sudden change. Attempts at expiration cease.
Slow, deep, infrequent inspirations take their place, accompanied by
convulsions of extensor muscles. Pupils are widely dilated, mouth
open, head thrown back. The subject is absolutely insensitive to every
kind of stimulus. The pulse shows a high arterial tension. The beating
of the heart is slow and strong. In about four minutes from the time
at which the windpipe was blocked respiratory movements cease. The
arterial tension falls. The heart’s action grows rapidly weaker,
although for two or three minutes longer it may still continue to
flicker. Recovery is possible until it finally gives up. After death
the right side of the heart is found gorged with blood, the left side
empty, showing that the heart had been unable to force the blood
through the capillaries of the lungs.
Under all ordinary conditions the sequence of phenomena of asphyxia
is the same—a stage of exaggerated breathing (hyperpnœa), a stage
marked by the co-operation of muscles which are not called into
action in tranquil breathing (dyspnœa), followed by the condition of
asphyxia properly so termed. An animal whose supply of fresh air is
cut off passes through these three stages, whether it be enclosed in
a small space or in a very large one. It must, however, be noted that
in asphyxia several factors combine in varying degrees. Carbonic acid
is in excess in the blood, oxygen deficient. The nervous mechanism
which regulates respiratory movements is thrown out of gear. Motor
and inhibitory impulses are in conflict. It is important, if these
complex phenomena are to be analysed, that one factor only should be
altered at any given time. For example, carbonic acid may be allowed
to increase in the air while a constant oxygen tension is maintained.
Under these circumstances the dyspnœic contractions are much less
marked. No convulsions follow. The paralysing action of carbonic
acid predominates. Anæsthesia passes into complete unconsciousness.
Death is tranquil. And this, speaking generally, is what happens in
disease of the lungs. Asphyxia comes on slowly. The supply of oxygen
is undiminished, but carbonic acid accumulates in the blood, acting as
a narcotic poison which lowers the excitability of the nervous system,
suspends consciousness, and slowly brings the vital activities to a
standstill.
In cases of drowning, when the lungs are filled with water, the
resistance to the passage of blood through their capillary vessels is
greater than it is when they are still filled with air. The heart is
sooner beaten in its effort to drive the blood through them. Usually it
stops in about four minutes. Yet it is difficult to say for how long
after a person has been immersed in water it may be still possible to
resuscitate him. Reports vary, owing in large measure to uncertainty
as to the exact time at which the immersed person sank and his lungs
filled with water. It is a wise precept that artificial respiration
should be tried in every case, without waiting a single instant to
ascertain whether the heart still beats. The first thing to do is to
empty the chest of water. Then place the subject on his back. Kneel on
the ground behind his head. Grasp an arm just below the elbow, in each
hand. Draw the arms up above the patient’s head, so that the pectoral
and other muscles drag on the ribs, enlarging the chest; then lower
them, and press them into the sides. This must be done with the natural
rhythm of respiration, and not more frequently than twenty times in a
minute. It is well if an assistant draws the tongue forward, to give
free admission to air. Presumably the slight exchange of air brought
about by mechanical expansion and compression of the chest favours
the passage of blood through the capillaries of the lungs; but the
real object of artificial respiration is to stretch the endings of the
vagus nerve, and in this way to originate impulses which will call
the respiratory centre into action. Perhaps it may not be superfluous
to point out that the failure of the pulse must not be taken as an
indication that the heart has ceased to beat. Owing to the obstruction
to the circulation through the lungs, the left side of the heart is
almost empty. Very little blood is pumped into the aorta. None reaches
the wrist.
=Exchange of Gases in the Lungs.=—In the lungs each red corpuscle
takes from the air a charge of oxygen which it carries to the tissues.
In the tissues the plasma of the blood receives carbonic acid, which
escapes from it when it reaches the lungs. Water dissolves oxygen and
carbonic acid. Towards animals and plants which live in it, water plays
the same rôle as the atmosphere towards dwellers on land. The quantity
of a gas which will dissolve in water is proportional to the pressure
to which it is subjected. If water were the circulating fluid, some
oxygen would enter it in the lungs; some carbonic acid would be taken
up in the tissues and liberated in the lungs. But it is clear that
the small quantity of fluid which the vascular system will hold would
be incapable of serving as an efficient medium of exchange between
the tissues and the lungs. When a given quantity of venous blood is
agitated with air, five times as much oxygen is taken up as the blood
could carry if the gas were simply dissolved. Both oxygen and carbonic
acid are held by the blood in chemical combination.
The condition in which oxygen is carried was discovered in 1864 (_cf._
p. 68). From all time it had been noticed that the blood which flows
from a vein is darker and of a more purple tint than the blood which
spurts out of a cut artery. Shortly before the date mentioned above,
the spectroscope had begun to be used to distinguish more accurately
than the eye can do the groups of rays which a coloured solution
transmits. The colour of a ray of light depends upon its wave-length.
The light of the sun, when its rays are sorted by a prism, according
to their wave-lengths, shows all colours from the long waves of red to
the short rays of violet, with certain gaps. At intervals where rays
are missing, the spectrum exhibits dark bands—Fraunhofer’s lines.
The colour of a solution is measured by placing a flat-sided vessel
containing it in the course of a beam of the sun’s light, on its way
to a prism. When the rays are spread out, it is observed that certain
groups have been absorbed by the coloured fluid. The colour of the
solution is due to the rays which it transmits. It had been pointed
out in 1862 that blood diluted with water absorbs parts of each end
of the spectrum, and also two groups of rays lying between the fixed
bands of Fraunhofer which spectroscopists had labelled D and E. Stokes
observed that this is true only of arterial blood. Venous blood absorbs
a broad band in this part of the spectrum in place of the two narrow
bands. He showed that, “like indigo, it is capable of existing in two
states of oxidation, distinguishable by a difference of colour and a
fundamental difference in the action on the spectrum. It may be made
to pass from the more to the less oxidized condition by the action of
suitable reducing agents, and recovers its oxygen by absorption from
the air.” The reducing agents of which Stokes made use were alkaline
solutions of ferrous sulphate or of stannous chloride containing some
citric or tartaric acid. These sub-salts of iron and tin very rapidly
absorb oxygen from the air or from any chemical substance which parts
with it readily. With these solutions Stokes replaced the tissues.
He abstracted the oxygen from the oxyhæmoglobin; then, shaking the
solution of reduced hæmoglobin with air, he reproduced the action which
occurs in the lungs.
If the hand be held between a spectroscope and the source of light,
in such a position that the beam passes through the thin tissue of
two fingers where they are in contact, the spectrum of oxyhæmoglobin
is obtained. If now the circulation through the fingers be impeded by
putting strong indiarubber bands round them, the blood becomes venous,
and the two narrow bands of oxyhæmoglobin give place to the broad band
of reduced hæmoglobin.
Although very soluble, hæmoglobin may be obtained in crystals, the form
of which varies in different animals. When obtained from human blood,
the crystals are rhombic prisms; from the guinea-pig, tetrahedra; from
the squirrel, hexagonal plates. Yet it is unlikely that the hæmoglobin
of one animal differs chemically from that of another in any proper
sense of the term. Probably the form of the crystals depends upon
the amount of water of crystallization. The apparent polymorphism of
hæmoglobin may be associated with the great size of its molecules
(_cf._ p. 66).
Even when in the crystalline form, hæmoglobin can take up oxygen; but
the difficulties which attend its purification and crystallization
render somewhat uncertain the amount of oxygen which a gramme of
crystallized hæmoglobin can absorb. In solution, 1 gramme can take up
1·34 cubic centimetres. The whole of the hæmoglobin of the body would,
therefore, if it were all in the oxidized condition, hold about 4
grammes of oxygen.
It is not with oxygen alone that hæmoglobin can combine. It can
absorb the same volume of carbonic oxide or of nitric oxide gas. Both
of these gases it holds more firmly than oxygen. Neither carbonic
oxide-hæmoglobin nor nitric oxide-hæmoglobin is of any use to the
tissues. If the blood becomes charged with the fumes of carbonic oxide
(CO) given off by a coke-fire, this gas proves extremely poisonous.
The blood does not lose it in its circuit through the body, nor is it
exchanged for oxygen in the lungs.
The instability of the compound of hæmoglobin and oxygen is shown under
the air-pump. The pressure of air in the open equals 760 millimetres of
mercury. When the pressure falls to about 250 millimetres, the oxygen
is rapidly given off. This is a matter of considerable interest in its
bearing upon the question of the height to which it is possible for a
human being to ascend. An animal placed in a chamber from which the air
is pumped dies when the pressure falls to 250 millimetres of mercury.
It has been ascertained that a man under the same circumstances can
bear with impunity a reduction to 300 millimetres. How much lower
must the pressure fall before it proves fatal? Of three aeronauts
who ascended in the balloon _Zenith_ to a height of 8,600 metres
(26,500 feet), two died. The third, Tissandier, became unconscious,
but recovered during the descent. The pressure of the atmosphere at
such a height is 260 millimetres. The greatest mountain heights yet
attained are 23,100 feet (Aconcagua, in the Southern Andes), reached by
Fitzgerald, and 23,400 feet (Trisul, in the Garhwal Himalayas), reached
by Dr. Longstaff and his companions. The pressure at this height was
320 millimetres. From these facts it is clear that mountaineers have
just about reached the limit; but since they have not as yet mounted to
a height at which the barometric pressure is less than 300 millimetres,
it is possible that slightly higher mountains are still waiting to
be conquered. At 23,000 feet the oxygen contained in arterial blood
does not exceed 10 volumes per cent. (_cf._ p. 190). It is therefore
about half the normal amount. Hence the breathlessness and sense of
feebleness experienced by climbers. The least exertion leads to the
consumption of all the circulating oxygen. But since the effects of
want of oxygen are felt at altitudes much lower than those to which
reference has been made, it is clear that the question cannot be
regarded as simply one of physics. The nervous system suffers when an
attempt is made to do work with a deficient oxygen-supply. Violent
headache and nausea attack most persons long before a level is reached
at which the combination of hæmoglobin with oxygen ceases to be
possible. The occurrence of this “mountain sickness” reminds us that
we must not take for granted that the nervous system will continue
to do its work right up to the altitude at which oxyhæmoglobin is
dissociated. Still, the figures show that, apart from these nervous
symptoms, which disappear after a time, no serious disturbance occurs
even though the atmospheric pressure be but little higher than the
absolute minimum at which hæmoglobin combines with oxygen.
The capacity of the blood for rapidly absorbing oxygen in the lungs
and readily parting with it to the tissues is easily and completely
explained by the property which hæmoglobin possesses of forming an
unstable compound with this gas.
It is quite otherwise with regard to the liberation of carbonic acid.
The problems presented by the solution of this gas in blood and its
elimination in the lungs are difficult to solve. Less than one-tenth
of the volume of carbonic acid which can be extracted from blood by
the air-pump is simply in solution. The remainder is in loose chemical
combination, the chief agents in holding it being the alkaline
carbonates which the plasma contains. With an excess of carbonic acid
they form acid carbonates, which give up carbonic acid and again become
normal carbonates in the lungs. About one-third of the carbonic acid
is, however, held by the blood-corpuscles—partly in virtue of their
alkaline carbonates and phosphates, partly in combination with their
globulin. The affinity of these several vehicles for carbonic acid is
sufficient to enable them to take it from the lymph, and to hold it
while the blood is in the veins. When they reach the capillaries of the
lungs, they part with their burden of carbonic acid to the air. It is
in connection with this renunciation that certain difficulties remain
to be explained. The carbonic acid is given up with greater readiness
than our knowledge of the chemistry of the compounds into which it
enters in the blood would lead us to expect.
Why does oxygen enter blood as it circulates through the lungs, and
carbonic acid leave it? We have referred to the immense surface which
the lungs expose to air. If a soap-bubble be filled with a mixture of
oxygen, nitrogen, and carbonic acid, and if the oxygen be in smaller
proportion, and the carbonic acid be in greater proportion, than in
the air of the room, oxygen will enter the bubble, and carbonic acid
will leave it, by diffusion. If, instead of filling a bubble with gas,
we fill a bladder with water charged with carbonic acid, but destitute
of dissolved oxygen, a similar exchange with the gases of the air will
take place. It is merely a question of “gaseous tension.” The tension
of the gases in the lungs is measured by passing a small tube down
the trachea, and along one of the two chief bronchi until it becomes
blocked in a bronchus just large enough to admit it. Respiration is
carried on under normal conditions in the remainder of the lung; but
in the lobe which the catheter blocks diffusion from stationary air
to tidal is no longer allowed. At the same time, since the circulation
is not interfered with, the gases in the blood of the occluded lobe of
the lung are not in markedly different proportions from those in the
air-chambers of other parts. If at the end of a sufficient interval
the air of the occluded lobe is drawn off and its gases measured,
their tensions can be compared with the tensions of gases in specimens
of arterial and of venous blood. If from 10 c.c. of fluid 1 c.c. of
gas can be removed by the air-pump, the volume of gas dissolved is
10 per cent. of the volume of the fluid which dissolved it. Commonly
this is written “10 volumes per cent.” To ascertain experimentally
the tension of a particular gas in a particular fluid when dissolved
to the amount of 10 volumes per cent. at the ordinary pressure of the
atmosphere and at the temperature of the body, it would be necessary
to place it in an open vessel in air containing a sufficient admixture
of the gas to prevent its escape from the fluid. Suppose that it were
found that, when the fluid containing the dissolved gas was placed in
air mixed with the same gas to the extent of one-tenth of its volume,
the fluid neither gave up gas nor absorbed more gas, the tension
of the gas would be equal to one-tenth of an atmosphere. Since the
pressure of the atmosphere equals 760 millimetres of mercury, the
tension of the dissolved gas would be 76 millimetres. If more gas were
added to the air, more would dissolve in the fluid; if some of the
gas were removed from the air, gas would escape from the fluid. Gas
passes from the medium in which its tension is high to the medium in
which its tension is low. The tension of carbonic acid in tissues,
particularly in muscles and glands, is higher than in lymph; in lymph
higher than in blood; in blood higher than in air. Hence it passes by
these several stages from the tissues in which it is formed to the air
in the lungs. Much ingenuity has been devoted to perfecting methods
for the determination of the tension of carbonic acid in lymph and
in venous blood. Frequently results have been obtained which seemed
opposed to the doctrine that carbonic acid progresses from one medium
to another in accordance with the law of pressures; but such perplexing
results were probably due either to imperfections in method or to the
establishment of abnormal physiological conditions during the course
of the observations. When, for example, it was found that the tension
in lymph was less than the tension in blood, the specimen of lymph
examined was probably not in the same condition as the lymph in the
tissue-spaces where the exchange occurs. The experimenter in such
a case was in error in supposing that the specimen of lymph which
he examined contained as much carbonic acid as did the lymph in the
tissue-spaces from which the blood which he compared with it received
its supply of this gas.
We have already given the figures for the composition of the air in the
air-chambers of the lungs. The figures commonly accepted as correct for
the percentages of the several gases in the blood are, at 0° C. and 760
millimetres of mercury pressure:
Carbonic
Oxygen. anhydride. Nitrogen.
In 100 vol. of arterial blood 20 39 1-2
In 100 vol. of venous blood 8-12 46 1-2
This table shows the gain in oxygen and the loss in carbonic acid which
results from the passage of blood through the capillaries of the lungs.
The aerated blood returned to the heart by the pulmonary veins contains
8 to 12 volumes per cent. more oxygen, and about 7 volumes per cent.
less carbonic acid, than the blood which the pulmonary artery carries
to the lungs.
As to the physics of this exchange, the air in the recesses of the
lungs contains about 16·36 per cent. of oxygen, and an amount of
carbonic acid variously estimated at from 2·57 per cent. to 3·84 per
cent. Of the 760 millimetres of mercury which the atmosphere holds up
in a barometric tube, the oxygen in the alveoli of the lungs supports
(760 × 16·36)/100 = 124·33 millimetres; the carbonic acid, at the lower
figure quoted (2·57 per cent.), 19·5 millimetres.
The tension of gases in arterial blood is ascertained by opening an
artery into a closed vessel which contains nitrogen mixed with oxygen
and carbonic acid at about the tensions which it is computed that they
have in the blood. If the amounts of these gases are exactly right,
no exchange occurs between the blood and the mixture of gases. The
mean of many observations made in this way by various physiologists
is, for oxygen in the blood 72·2 millimetres mercury pressure, for
carbonic acid 20·5 millimetres mercury pressure. At a glance it is
seen that, since the tension of oxygen in the blood never exceeds 72
millimetres, whereas its tension in pulmonary air never falls beneath
124 millimetres, there is no difficulty in accounting for its passage
from air to blood. The position is somewhat otherwise with regard to
carbonic acid. Aeration continues in the lungs until the tension of
this gas in the blood returning to the heart does not exceed 20·5
millimetres; whereas the tension in pulmonary air, even accepting
the lowest figure obtained by experimental means, is as high as 19·5
millimetres. This leaves a very small margin of pressure to account for
the escape—and it is undoubtedly a rapid escape—of carbonic acid from
blood as it circulates through the lungs. As was said regarding the
fixation of carbonic acid in the blood, it is somewhat doubtful whether
the problem has been completely solved.
The carbonic acid exhaled contains all the carbon of the digestible
food, with the exception of a comparatively small quantity given off in
urea. It amounts to about 900 grammes per diem.
How are we to determine the quantity of air which an individual
requires? We can but make the general statement that it must be
sufficient to dilute the carbonic acid exhaled to an extent which
precludes poisoning. It is impossible to fix a limit. Breathing becomes
embarrassed, and frontal headache and other symptoms make themselves
felt when 10 per cent. of pure carbonic acid is mixed with air. Even in
so large a proportion as this, carbonic acid is not fatal to life. Yet
an atmosphere in which there is present a hundredth part of this amount
of carbonic acid, produced by respiration, is extremely injurious to
health under the ordinary conditions in which people live. It may be
asserted, therefore, that under ordinary conditions 0·1 per cent. is
the extreme limit for wholesome living. But again we are obliged to add
that air contaminated to this extent is not under all circumstances
injurious to health. The explorers on the recent Antarctic Expedition
were obliged at times to sleep three men in one sleeping-bag, with
the aperture of the bag tightly closed. The atmosphere must have been
heavily laden with carbonic acid. Dr. Wilson assures us that it was
impossible to keep a pipe alight inside the bag. Not that any man so
placed would desire, one would imagine, to add the combustion-products
of tobacco to those given off from the lungs! The survival of the
explorers proves that it is impossible to fix a limit of safety even
for the carbonic acid in air vitiated by respiration. It is, however,
a matter of common observation that air which is moist and warm,
owing to respiration, and tainted with the odours of humanity, is
extremely prejudicial to those who live in it. Such an atmosphere
is a favourable medium for the conveyance of germs, whether of the
common cold or of a more virulent type. At one time it was supposed
that the volatile emanations which can be condensed, along with water,
by hanging a vessel of ice to the ceiling of a crowded room, were
actively poisonous; but this statement has not been confirmed by recent
research. It is unnecessary to call any such evidence in support of the
thesis that human beings thrive better in fresh air than in foul. The
admirable results achieved by the “fresh air cure” show that there is
no degree of vitiation which can be pronounced innocuous. Nevertheless,
public opinion demands that sanitarians should give some figure as a
guide. Commonly they fix the maximum of carbonic acid compatible with
health at 0·06 per cent., the quantity of carbonic acid being taken
as the measure of all impurities present. An adult exhales about 0·6
cubic foot of CO₂ per hour. Fresh air already contains about 0·04 per
cent. If, therefore, the percentage is not to rise higher than 0·06
per cent., each adult must be supplied with 3,000 cubic feet of air
per hour. With good ventilation air may be changed four times an hour,
and therefore 800 cubic feet is regarded as sufficient space for each
occupant of a room. The figure may pass. It is a reasonable basis from
which to calculate the packing capacity of a dormitory. So long as a
man has 800 cubic feet of air to himself, he may safely feel that he
has room to stretch his lungs. Dwelling on this figure may make him
feel uncomfortable when he finds himself in a railway carriage, seated
five on a side, with the windows closed. In the theatre or in church
he may doubt whether he has all the fresh air to which his humanity
entitles him. But, as a philosopher rather than as a physiologist, he
reflects that, whether on the Antarctic icecap in a sleeping-bag or
standing on a summit in the Alps, he takes all that he can get, for
fresh air is one of the few good things of which one can never have
enough.
=Tissue Respiration.=—A frog will live for seventeen hours in an
atmosphere of nitrogen. Under these circumstances it is clearly
impossible for it to take up oxygen, yet for several hours it gives
off as much carbonic acid as it would do if it were living in air.
Such an observation as this proves that oxidation does not occur in
the lungs, but deeper in the body. At one time the blood was regarded
as the seat of oxidation; the products formed by the splitting up of
proteins in the tissues were supposed to be passed into the blood,
where they came in contact with the oxygen carried by hæmoglobin. A
certain amount of oxidation does take place in the blood, as in all
other tissues, for blood is a living tissue and needs to respire. But
the oxidation which occurs in the blood is small in amount as compared
with that in the organs which the vessels traverse. Muscle and other
tissues detached from the body and free from blood give off carbonic
acid. It is possible to wash the blood out of the vessels of a frog and
to replace it with a solution of salt. In an atmosphere of oxygen such
a “saline frog” lives for a day or two, taking in the same quantity of
oxygen and giving off the same quantity of carbonic acid as a normal
frog. The oxygen is chiefly absorbed through the skin, the carbonic
acid discharged from the lung. This experiment shows that blood is not
essential for oxidation. Oxidations do not occur in the salt solution
with which blood is replaced. Taking all the evidence together, it
seems to be safe to conclude that the tissues absorb the oxygen which
the oxyhæmoglobin brings into their neighbourhood, and that they have
some capacity of storing it. A piece of detached muscle which gives off
carbonic acid in an atmosphere of nitrogen would appear to be holding a
store of oxygen, much as hæmoglobin holds it. The proof is not quite so
definite as might be desired; but we are probably justified in holding
the belief that the main part of the respiratory exchange occurs in
the tissues. Lymph dissolves oxygen which it obtains from the blood.
The tissues take it from lymph. Tissues set free carbonic acid which
lymph dissolves. Its tension being higher than in blood, carbonic acid
diffuses from lymph, through the walls of the capillary vessels, into
blood, from which it passes into the air in the lungs.