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'^ Arch. f. exper. Path. u. Pharmakol., Leipzig, 1887, Bd. xxii. S. 39; 1888, Bd.
xxiv. S. 85.

^ Journ. Physiol., Cambridge and London, 1887, voL viii. p. 117.



650 THE SECRETION OF URINE.

Heidenhain, however, points out that we have no right to conclude that
the secretion of urine has ceased when the mercury column no longer
rises. This stage in fact corresponds merely to the point at which the
continued secretion of urine is balanced by the reabsorption of the urine
from the tubules, in consequence of the abnormal pressure withm
them.

It must be confessed that we have no very definite evidence that
such a reabsorption takes place. It is true that the kidney becomes
cedematous in consequence of the hgature, but the oedema fluid was stated
by Ludwig to consist of lymph and not of urine ; and it has been shown
that increased pressure in the urinary tubules causes them to press on
the adjoining veins, so that the escape of blood from the kidney is
hindered, and ordinary oedema results. Fresh investigations on this
matter are much to be desired, since the only analyses we have of the
oedema fluid and retained urine are those of Hermaim, one of the earliest
observers on the subject. The urine, which is secreted under pressure
and which distends the pelvis and ureter, is light in colour, of low
specific gra\ity, and contains very little urea. If, after some time, the
ligatm'e round the ureter be relaxed, the result is at once a copious
secretion of watery urine. In man a similar fluid is well known to be
excreted in cases where there is a chronic obstruction of the ureter.

The concentration of the urine. — We have now to consider the
second part of Ludwig's theory, according to which the dilute urine
transuded through the glomeruli is concentrated on its passage down
the tubules, by the absorption of its water. Tliis absorption takes place
in consequence of the fact that the lymph surrounding the tuljules is
more concentrated than the urine. A cogent objection to this hypo-
thesis was raised in 1859 by Hoppe (Hoppe-Seyler), who showed that,
if urine were separated by an animal membrane from blood serum of
the same animal, there was a flow of water from serum to urine.^ The
tendency of this urine, therefore, in passing down the urinary tubules,
would have been to become more dilute, in consequence of osmotic
interchanges between it and the serum. At this time our knowledge
of the factors and forces involved in the interchange of water and sub-
stances in solution across animal membranes was meagre and inexact ;
and it is only quite recently that we have acquired the necessary data
for testing the truth of Ludwig's hypothesis and the fitness of Iloppe-
Seyler's objections.

Pfeff'er ^ showed that the osmotic attraction of any solution for water
might be determined by measuring its osmotic pressure, and first pointed
out how enormous these pressures were in the case of even relatively
dilute salt solutions. Van t' Hoff later on pointed out that the osmotic
pressure of a solution was proportional to the number of molecules this
contained, and was therefore a colHgative property (Ostwald), hke certain
other properties of solutions — such as the diminution of the freezing
point and of the vapoin- tension and the elevation of the boiling
point.

Since these properties of a solution are proportional to one another,
we need only know one to determine any of the others. This fact is of
importance when we wish to determine the osmotic pressure of animal
fluids, since we can substitute for the diflicult and inexact determination

^ Virchow's ArcMv, 1859, Bd. xvi. S. 412 (quoted by Heidenhain).
^ "0.smotische Untersuchungen," Leipzig, 1877.



OSMOTIC PRESSURE OF URINE. 651

of osmotic pressures by Pfeffer's method a determination of the freezing-
point of the solution. As van t' Hoff has shown, if A is the depression of
freezing-point and T the absolute freezing-point of the solvent {i.e., for
water, 273°, and w the latent heat of fusion of ice = 79 cal.), then the
work A can be reckoned from the following formula : —

dA= ~-X dv.

Thus for 1 per cent, solution of cane-sugar (a = "055)

■, . •055-79 ,
^^ = -273-^^"-

To reduce this result to gravitation units we must multiply by 424,
and we thus find that to separate the volume dv of pure water as ice
from 1 per cent, cane-sugar solution, a force is necessary equal to the

■055 X 79 X 424
pressure of a column of water of ^^ metres in height.

A depression of a=— 1° corresponds therefore to an osmotic

79 x424
pressure of — 7^ — ; that is to say, to 122-7 metres of water. We

have therefore to multi]3ly a by 122-7, in order to obtain the osmotic
pressure in metres of water of any solution.

Now it is evident that, according to Ludwig's hypothesis, the osmotic
pressure of the urine might attain to but could never exceed that of
the blood plasma. On estimating the osmotic pressures of these two
fluids, we find that, under normal circumstances, the osmotic pressure of
the urine is considerably greater than that of the blood, so that work
must have been done in the separation of this concentrated fluid from
the more dilute blood plasma. Dreser ^ has estimated this work in a
case in which, during one night, 200 c.c. of urine were secreted with
A = 2-3. This was separated by the kidneys from the blood with
A = -56. In the production of this fluid Dreser finds that the work
done by the kidney amounts to 37*037 kilogramme metres. This figure
by no means represents the maximum force which can be exerted by the
kidney. From a cat which had been deprived of water for three days,
Dreser drew off some urine with A = 4-72 C. The blood at the same
time had an osmotic pressure corresponding to a = 0-66 C. These
differences in freezing point denote an osmotic difference of 498 metres
water, i.e. a pressure of 49,800 grms. per square centimetre. If this
work of concentration were carried out by the cells of the tubules,
these results would imply that these cells can exert a force six times
greater than the absolute force of human muscle (8000 grms. per square
centimetre).

Assuming that the whole work of the tubules is confined to the act
of concentration, Dreser seeks, moreover, to demonstrate that the
glomerular secretion also involves the activity of living cells. Since
the blood pressure of 200 mm. Hg = 2-72 metres water, and a 1°-0 C. =
122-7 metres water, the highest possible difference between dilute urine
and blood, assuming that no concentration had taken place, could only
be a = 0-°022 C. Dreser finds, however, that after beer drinking, and

1 Arch. f. exper. Path. u. PharmakoL, Leipzig, 1892, Bd. xxix. S. 307.



652 THE SECRETION OF URINE.

in diabetes insipidus, the urine secreted may have a = 0°-16 C, i.e. a
difference between a of blood and of urine of -^ C. Hence he con-
cludes that the production of urine by the glomeruli is also attended
with the doing of work, and must therefore be looked upon as a process
of secretion. We might, however, still adhere to the theory of glomer-
ular filtration, if we assumed either that the cells of the tubules could
absorb water or solids according to the needs of the organism, or that
they were able to secrete pure water and so dilute the glomerular
transudation.

Heidenhain's criticism of the theory of Ludwig. — The difficulties
in the way of accepting Ludwig's hypothesis have led Heidenhain, after
a long series of researches on the subject, to reject this theory abso-
lutely, in favour of one very similar to that put forward by Bowman.

Heidenhain sums up his objections to the mechanical theory under
the following headings : —

1. The hypothesis that a rise of arterial pressure causes increased
transudation through the vessel walls, is not confirmed by our experience
in other parts of the body (lymphatics of the limbs, salivary glands).

2. This hypothesis is rendered the more improbable for the kidney,
since in this situation the glomerular capillaries are covered by a second
layer of epithelium, and we know, from Leber's researches on the cornea,
that such a simple epithelial layer can afford great resistance to
filtration.

3. If we assume that all the constituents of the urine are filtered
off in the glomeruli, the small amount of urea in the blood renders it
necessary that in man about 70 kilos, of fluid should be filtered through
and reabsorbed, in order that the urea produced in the course of
the day may be excreted in the urine, — an amount which is highly
improbable.

4. According to the filtration hypothesis, the amount of urine formed
must always increase with increased capillary pressure, whereas we find
that, on increasing capillary pressure by ligature of the renal vein, the
urinary flow is abohshed.

5. The hypothesis that the glomerular transudate is concentrated by
a process of osmosis or diffusion, on its way through the glomeruli, is
rendered impossible by the fact that the osmotic pressure of the
urine may be, and generally is, much higher than that of the lymph
or blood.

6. The filtration hypothesis does not explain why the amount of
urine is increased by the presence of water or crystalloid {harnfdhig)
substances in the blood.

From his own researches on the subject, Heidenhain comes to the
following conclusions with regard to the mechanism of secretion : —

1. In the kidney, as in all other glands, the secretion depends on the
active intervention of special secretory cells.

2. The first type of these cells is represented by the simple layer
of . epithelium covering the glomerular loop of capillaries. The office of
these cells is to secrete water and such salts of the urine as are found
in all other parts of the body in watery solution {e.g. sodium chloride).

o. Another system of secretory cells, forming the lining investment
of the convoluted tubules and ascending tubule of Henle, secrete the
specific constituents of urine (urea, uric acid, etc.). Under some con-
ditions they may at the same time secrete a certain amount of water.



HEIDENHAIN'S THEORY. 653

4. The activity of the two kinds of secretory cells is determined —
{a) By the amount of water or urinary constituents contained in the

blood ;

(5) By the velocity of the blood-flow through the capillaries of the
kidney, inasmuch as on this factor depends the supply of oxygen, and
of substances to be excreted, to the cells.

5. The great variability in the constitution of the urine may be
explained by differences in the secretory activities of these two types of
cell.

The most important part of these conclusions of Heidenhain is a
revival of Bowman's theory, that the specific urinary constituents, urea
and uric acid, are secreted by the tubules, and that the office of the
tubules is secretory rather than absorbent. What evidence have we of
the secretory activity of the cells in the tubules ?

The great solubility and diffusibility of urea render it impossible to
trace this substance on its way through the kidney by micro -chemical
means. A better prospect of success would seem to be afforded by
the more insoluble uric acid and urates ; and both Bowman ^ and v.
Wittich ^ have described the presence of uric acid crystals in the cells
of the convoluted tubules of birds. Semicrystalline deposits of guanin
have been demonstrated with certainty in the cells of the excretory
organ of molluscs, but later researches by Adolph Schmidt ^ have shown
that the observations of Bowman and v. Wittich must have been due
to faulty methods of preparation. Deposited urates were frequently to
be seen in the urinary tubules of birds, but never in the cells them-
selves. In order to throw light upon this point, Heidenhain had
recourse to a method, devised by Chrzonzsczewsky,* i.e. the injection of
sodium sulphindigotate (indigo-carmine) into the blood, and the tracing
of this coloured substance through the cells of the kidney.

It is found that this substance is excreted in any quantity by two
glands only of the body, namely, the liver and the kidney. If 5 c.c. of
a saturated watery solution of the sulphindigotate be injected into the
veins of a rabbit, within a few minutes the urine becomes a deep blue,
and on killing the animal the kidneys are found to be stained blue, the
colour being best marked towards the apex of the pyramid. In order to
find out in what portion of the secreting substance of the kidney the
colouring matter is turned out, the flow of urine must be checked, since
otherwise the excreted pigment is at once washed down into the lower
parts of the tubules and ureter. To this end Heidenhain ^ divided the
spinal cord in the neck. The flow of urine being thus stopped, 5 c.c. of
the saturated solution of indigo-carmine is injected into the blood vessels ;
ten minutes later the animal is killed, and the blood vessels of the
kidney washed out with absolute alcohol. By this means the pigment
is precipitated in situ. On cutting into the kidney, it is at once seen to
differ widely in appearance from that of an animal in which the cord
was intact. Instead of being diffusely stained, the kidney now is coloured
a deep blue in the cortex, the medulla presenting the normal appearance.
On examining a section under the microscope, it is seen that the blue
colour is due to the deposition of pigment granules in the lumen and
in the striated cells lining the convoluted tubules and the ascending

'^ Log. cit. ^ Arch. f. miJcr. Anat., Bonn, 1875, Bd. xl. S. 81.

^Arch.f. d. ges. Physiol., Bonn, 1890, Bd. xlviii. S. 34.

■* Virchows Archiv, 1866, Bd. xxxv. S. 158. 5 £0^^ ^^^



654 THE SECRETION OF URINE.

limb of Henle's loop, the capsules and the collecting tubules as well as
the descending loop of Henle being quite free from pigment.

A very interesting appearance is offered by the kidney, if, previous
to the injection, its surface has been cauterised over a small area with
silver nitrate, the cord being intact. In the cauterised zones, the secre-
tion of water is stopped, but the excretion of indigo-blue is not affected,
so that in these zones the blue colour is confined to the cortex, whereas
in the rest of the kidney the coloration is diffuse. Heidenhain con-
cludes from these observations that the excretion of indigo-blue is due
to the specific secretory activity of the striated cells lining the con-
voluted tubules and ascending loop of Henle. Since these cells are the
only cells of the kidney which have the power of excreting indigo-
carmine, an abnormal constituent of the blood, it is natural to assume
that they may also possess the specific function of secreting the urea of
normal urine.

These conclusions of Heidenhain's have not, however, passed un-
challenged. Various observers have pointed out that, in order to obtain
the results described by Heidenhain, it is necessary to repeat exactly all
the details of his experiments. If we inject larger doses of the
sulphindigotate and kill the animal ten minutes after the injection, it
will be found that, in addition to the staining of the striated cells of the
convoluted tubules, and the deposition of precipitated pigment in the
lumen of these tubules, there is also a shght staining of Bowman's
capsule and the glomerular epithelium. It has been suggested^ that
Heidenhain's results might be equally well explained on Ludwig's
hypothesis, according to which a dilute solution of the dye would be
exuded into Bowman's capsules, and would be concentrated by absorption
of fluid on its way through the convoluted tubules. Indigo-carmine is
soluble in water and in very weak salt solution, from which it is
precipitated on concentration. Moreover, indigo-carmine is liable to
reduction in the living tissues with the formation of a colourless
product, and these two factors, i.e. reduction of the pigment and the ex-
treme dilution of the glomerular exudation, have been held to explain
the absence of glomerular staining in Heidenhain's experiment. By
increasing the dose injected into the veins and killing the animal soon
after the injection, these two factors are minimised and a staining of the
capsules is brought about. Sobieranski ^ points out that the staining or
deposition of granules in the cells of the convoluted tubules is confined
to the parts of these cells bordering on the lumen — a fact which seems
to indicate that the pigment has been taken up by these cells from the
lumen rather than from the surrounding lymph spaces.

These observations are to a certain extent confirmed by the effects of
the injection of carmine. This substance, which has a much more com-
plicated composition than sodium sulphindigotate, enjoys the correspond-
ing advantage of smaller diffusibihty, so that it can be more easily traced
on its way through the tissues of the body. Moreover, it undergoes no
reduction in contact with the hving cells. The circulatory disturbance
which often accompanies the injection of this substance may be almost

' Pautynski, Vircltoiu' s Archiv, Bd. Ixxix. S. 393 ; Henschen, Akad. Afhandlung f.
inedieinsJca Graden, Stockholm, 1879 (quoted by Sobierauski) ; v. Sobieranski, Arch. f.
exper. Path. u. Pharinakol., Leipzig, 1895, Bd. xxxv. S. 144. (The two first papers are
the subject of a critical paper by Gi'litzner, Arch. f. d. ges. Physiol., Bonn, 1881, Ikl. xxiv.
S. 441.)

'^ Arch. f. exper. Path. u. J'Jiarviakol., Leipzig, 1895, Bd. xxxv. S. 144.



EXPERIMENTS OF N US SB A UM. 6 5 5

entirely avoided by using a solution of carmine in very weak soda, and
carrying out the injection slowly (10 c.c. in five minutes). If we kill
the animal thirty to forty minutes after the injection, and wash out the
kidney from the renal artery with absolute alcohol, we find the glomeruli
stained, the nuclei being red, the glomeruli themselves being of a fainter
reddish tinge. The epithelium of the convoluted tubules contains fine
granules of pigment towards the inner part of the cells, and here and
there deposits of carmine are seen in the straight tubules. Under no
circumstances are the pigment granules ever found in the basal parts of
the epithelial cells. There can be no doubt that these appearances
suggest that the pigment has been taken up by the cells from the lumen
rather than that it is in the act of excretion by the cells. In neither of
these two experiments do the facts at our command allow us to come to
a definite conclusion with regard to their interpretation. In order to
decide the relative functions of the glomeruli and convoluted tubules, it
would be necessary to separate in some manner the activities of these
two parts of the kidney, so as to obtain the action of one or other of
them in an isolated form.

Bxperiments of Nussbaum. — A method for attaining this object
was devised by ISrussbaum,^ and promised at first to be of crucial
importance for the physiology of urinary secretion. The kidneys of
amphibians possess, as Bowman pointed out, a double vascular supply,
i.e. from the renal artery and from the renal portal vein. From the
former vessel are derived the vasa afferentia to the glomeruli, whereas
the latter breaks up into capillaries which anastomose round the
tubules, in conjunction with the capillary ramifications of the efferent
vessels of the glomeruli. Nussbaum imagined, therefore, that the
glomerular activities might be altogether excluded by ligature of
the renal artery. Carrying out a number of experiments of this
description, he obtained results which seemed to decide absolutely in
favour of Heidenhain's hypothesis. Thus, after ligature of the renal
arteries in frogs, the urinary flow was abolished. A flow of urine
might, however, be evoked by the injection of urea into the blood,
proving, according to Nussbaum, that the substance was not excreted
by the glomeruli but by the tubules, and also that the latter struc-
tures could, under the influence of diuretics, secrete part of the water
of the urine. In a normal frog the injection of peptone, egg-albumin,
or sugar into the blood is followed by the excretion of these substances
in the urine. If, however, the renal arteries be previously tied, none
of these substances appear in the urine, even when a urinary flow is
produced by the injection of urea. Carmine also, which is acknow-
ledged by all observers to be excreted by the glomeruli, does not
appear in the urine of the frog, if the renal arteries be hgatured.
Nussbaum concluded, therefore, that the excretory apparatus of the
kidney consisted of two parts, namely, the glomeruli, which excreted
water and salts as well as egg-albumin, peptone, and grape-sugar ; and
the tubules, which excrete urea and probably uric acid, together with
a certain proportion of water.

These experiments are so definite that they would seem to decide
the question as to the part played by the various structures of the
kidney, were it only possible to place reliance on them. This un-
fortunately is not the case. A careful repetition of Nussbaum's ex-

1 Arcli.f. d. ges. Physiol., Bonn, 1878, Bd. xvii. S. 580.



656 THE SECRETION OE URINE.

periments by Adami/ working in Heidenhain's laboratory, has shown
that in the frog it is impossible to cut off the blood supply to the
glomeruli by ligaturing the renal arteries. In fact, after this operation,
fully half of the glomeruli may be injected from the aorta, owing to
the free anastomoses between the renal artery and the branches of
the ovarian arteries and the renal portal vein, and it is difficult to
understand how iSTussbaum can have obtained the very definite results
described by him. These, therefore, in spite of the ingenuity of the
methods employed, must be discredited in any discussion concerning the
functions of the various parts of the kidney tubule.

Experiinents of Ribbert. — A bold attempt to experimentally disso-
ciate the acti\'ities of the two portions of the urinary tubule was made
by Eibbert,- who adopted the method of excising as far as possible the
medulla of the kidney, so as to obtain the glomerular secretion after
it had passed through only the first convoluted tubules. This opera-
tion is only possible in animals such as the rabbit, in which the renal
medulla is made up of one Malpighian pyramid. It was carried out
in the following way: — One kidney havmg been exposed from the
back, was cut in two by an incision at right angles to the long diameter
of the organ, extending into the pehds. By means of a gouge, as much
as possible of the pyramid internal to the boundary zone was removed.
The two halves of the kidney were then placed together and secured
by sutm-es, and the other kidney totally excised. Eibbert found that
such animals dm-ing the next twelve to twenty-four hom-s secreted a
much larger quantity of urine than they had previously done. The
urine was more dilute and much lighter in colour than the urine of
rabbits under normal conditions. Xo analyses, however, of the fluid
were made. Eibbert interprets these results as confirming Ludwig's
hypothesis. But apart from the increased quantity, which does not
seem to me to be definitely estabhshed by Eibbert's experiments, the
production of a more dilute urine would be expected on either
hypothesis, whether we assume with Ludwig that the tubules absorb
water from the urine, or with Heidenhain that they excrete solid
substances into the urine.

Experiments of Bradford. — The very insufficient description of
his experiments given by Eibbert might inchne us to discredit them
altogether, were it not that somewhat analogous results have been
obtained by Bradford.^ This observer found that extirpation of one
kidney, combined with excision of a large wedge-shaped piece from the
other kidney, might bring about one of two results —

1. If the amount of kidney substance left amounted to one quarter
of the weight of the two kidneys, the animals (dogs) hved a considerable
time, but suffered from hydruria, i.e.. the quantity of urine excreted was
largely increased, but the excretion of urea remained unciianged, so
that the urine was much more dilute than before.

2. If the amount of kidney left was less than one-sixth of the total
kidney substance, polymia was produced, i.e. a large increase in the
excretion of water as well as of urea. This increased production of urea
was due to a rapid wasting of the proteid constituents, and especially of
the muscles of the body, so that the animals died in a short time in a

^ Journ. Physiol., Cambridge and Loiidon, 1885, vol. vi. p. 382.
- Virchoio's Archiv, 1883, Bd. xciii. S. 169,
^ Proc. Roy. Soc. London, 1892, vol. li.



REACTION OF THE URINE. 657

state of extreme emaciation. This latter result is difficult to explain,



Online LibraryE. A. (Edward Albert) Sharpey-SchäferText-book of physiology; (Volume v.1) → online text (page 92 of 147)