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substance imbibed may be of very different order, for gelatin takes up a more
concentrated solution of methyl- violet than that in the dye-bath ; while, on the
other hand, a f errocyanide of copper membrane will take up water while almost
absolutely indifferent to dissolved cane sugar.

Such "affinities" are not purely mechanical, since they vary with the
chemical nature of the substances, and yet are not of the nature of chemical
affinity in the usual sense of the term, since the "compounds" do not obey the
laws of constant and multiple proportion. Ostwald has introduced the term
mechanical affinity to meet the case.

In the complex known as protoplasm there may be imbibing substances of
different nature, permeated by a solution of substances whose chemical nature
may, directly and indirectly, affect the imbibition of a solution brought in
contact with the mass ; and, furthermore, undissolved particles may themselves

^Ztschr. f. rat. Med., 1849, Bd. viii. S. 1 ; Ann. d. Phys. u. Chem., Leipzig, Bd.
Ixxviii. S. 307.

^ Untersuch. z. Naturl. d. Mensch. u. d. Thiere, 1857, Bd. iii. S. 294.

^ Ann. dechim., Paris, 1854. Ser. 3, tome xliii. p. 420.

•» Tamman, Ztschr.f.physikal. C7ie??i. , Leipzig, 1892, Bd. x. S. 255 ; Walden, ihid.,^. 699.



2 76 DIFFUSION, OSMOSIS, AND FIITRATION

exert surface action, so that the possibilities for purely physical absorption are
quite unknown, and so-called vital elective action may be the result of specific
adsorptive affinity. Hofmeister ^ has shown that gelatin has an " elective " action,
for common vsalt, the concentration of the solution imbibed exceeding that of
the surrounding solution ; and, further, that the combination of sodic chloride
with the gelatin favours the uptake of water. Again, gelatin takes up more
water from "5 to 2 per cent, solution of ethyl-alcohol in water than from pure
water.

With salts that undergo electrolytic dissociation in solution, permeability
must be a function of ions. Thus, according to Ostwald,^ copper ferrocyanide
is permeable to potassium chloride, because both chlorine and potassium ions can
pass ; it is impermeable to barium chloride, because the barium ion is stopped ; and
impermeable to potassium sulphate, because the sulphuric acid ion cannot pass ;
and, under ordinary circumstances, on account of opposite electrical charges, if
one ion is stopped, so must be the other. There are, however, conditions
under which an ion, stopped on account of the impermeability of the membrane
to its fellow in a salt, may pass the membrane.

If the negative ion of a salt is prevented from passing through the
membrane, only because it is impermeable to its positive fellow, the addition of
another salt, wdiose positive ion can pass the membrane, will allow the negative
ion of the first salt to pass in company with it. Or a salt whose negative ion
can pass the membrane may be placed on the opposite side, the two negatives
exchanging with their positive fellows across the membrane, and equal numbers
of the two negative ions passing in opposite directions in a given time. This is
of interest to the physiologist, since it opens a possible physical explanation of
the fact that a cell may hold back a substance under certain conditions, while
under others, when surromided by a differently constituted fluid, the same
substance may be given up.

Koeppe^ has attempted to apply this to the formation of hydrochloric acid
in the stomach from sodium chloride, maintaining that the stomach wall is
impermeable to chlorine ions, but that the sodium ions are exchanged for
hydrogen ions from the blood. That free hydrogen ions are present in the
alkaline blood is, however, hardly possible.

Whether permeabihty be a function of physical or chemical nature, it
is obvious that in the case of a hving membrane the complex to which
the term " physiological condition " is applied must affect the property,
so that one and the same membrane in the body may, under diU'erent
circumstances, be more or less permeable by the same substance.

The simplest living membrane with which experiments can be made
is probably the differentiated outer layer of the protoplast of the vege-
table cell {Plasmalumt). There is no doubt that the permeability of this
membrane for different chemical substances is very variable. It is pene-
trated by some dye-stuffs but not by others, very unpermeable to
many simple salts, though easily permeable by certain complex organic
substances.^ Since this membrane is in its living condition so
slightly permeable to salts, the osmotic pressure within vegetable cells is
high (3 to 4 atmospheres). This special relative impermeability to salts
is obviously regulated in some manner by the " physiological condition "
of the membrane. Jansen ^ found that the cell sap of the alga, Chccto-

^ Arch. f. exjKr. Path. u. PharmakoL, Leipzig, 1891, Bd. xxviii. S. 210.
" Ztschr. f. physilcal. Chem., Leipzig, 1890, Bd. vi. S. 71.
^ Arcli.f. (I. ges. Physiol., Bonn, 1896, Bd. Ixii. S. 567.

'^VkSe.x, Ahhandl. d. onath.-phys. CI. d. k. siichs. Gesellsch. d. Wisscnsch., 1890, Bd.
xvi. S. ] 49.

^ Vcrhandl. d. k. Akad. v. JVetcnsch., Amsterdam, 1888, vol. iv. p. 345.



OSMOSIS. 277'

morpha, growing in sea water, is practically isosmotic with that of
Sinrogyra, growing in fresh water, though the osmotic pressure of sea
water is some 240 times that of fresh. Thus the osmotic pressure of the
cell sap of Chmtomorpha is far below that of the water in which it lives,
while that of the sap of Spirogyra is far above that of fresh water.

One can investigate the permeability of the living protoplast by the
plasmolytic method already alluded to above. The cell is plasmolysed
with a solution of some substance indifferent to the protoplasm and known
not to penetrate (sugar). A solution of the substance to be tested is now
prepared of the same osmotic pressure as the solution of indifferent
substance which just causes plasmolysis. If the solution so prepared has
exactly the same effect as the standard, it cannot pass through the proto-
plast, for, if it did, there would no longer be equality of osmotic pressure
on the two sides thereof. If the substance to be tested is only slightly
soluble in water, or is poisonous to the protoplasm, a small amount of it
is added to the standard indifferent solution, and the effect of the
addition on the plasmolysis noted. If it does not pass the membrane,
then, by virtue of the higher osmotic pressure due to its addition, the
mixture will produce more plasmolysis than did the standard solution,
and the effect will be lasting. If no effect results from the addition, it
must pass quickly through the membrane ; if a passing effect, with
subsequent recovery, it must pass slowly.

In this way Overton^ has investigated the permeability of the
protoplast by a number of chemical substances, and finds that salts
much dissociated in solution hardly pass the membrane, while many
complex organic bodies rapidly penetrate, and that the presence of
certain radicles in these markedly affects the result.

In animal cells investigations are rather limited (by the fact that
there is no plasmolysis) to shrinkage and swelling and escape of
hsemoglobin (in red corpuscles), as indices of permeability, under
conditions of variation of osmotic pressure of surrounding solutions.
More, therefore, is known about the permeability of the red corpuscle
than any other cell. A table of substances is given by G-ryns,^ and we
here confine ourselves to stating that red corpuscles are permeable
to urea,^ glycerin, ethyl- and methyl-alcohol, and most ammonium salts
(not to sulphate, phosphate, and thiocyanate), impermeable to sugars,
sodium and potassium salts, barium and calcium chlorides, glycin and
asparagin.

Thus, as regards action on red blood corpuscles, dilution of an isosmotic
sodic chloride solution with urea solution produces the same effect as
dilution with water, because the urea diffuses at once into the interior of
the corpuscle, while, on the other hand, addition of sugar at once causes
contraction of the cell.

Obviously, therefore, a so-called " hyperisotonic " solution does not
necessarily extract water from a cell, and absorption of water from such
a solution by the blood may be a purely physical action, if the substance
in solution can permeate the wall separating it from the blood. Again,
a drug, by making the wall of a cell less permeable by virtue of its
chemical action on the protoplasm, may markedly affect the " water
extracting power " of a salt solution. Possibly the fact that some salts

1 Ztschr. f. physiTcal. Cliem., Leipzig, 1897, Bd. xxii. S. 189.

'^ Loc. cit., p. 102.

^ See also Schondorff, Arch. f. d. ges. Physiol,, Bonn, 1896, Bd. Ixiii. S. 192,



278



DIFFUSION, OSMOSIS, AND FIITRATION



in the intestine purge (soclic sulphate), while others do not (sodic
chloride), may in the end find its explanation in a permeability of the
membrane by the latter, but not by the former. A universal
" physiological salt solution," then, if by such a term is meant a salt
solution in which tissues neither lose nor take up water, and the dis-
solved substance of which does not enter the cells, is not a possibility ;
each tissue must in fact have its own " normal solution," ^ and this may
possibly be in some cases a solution of some other substance than a
salt.

The effective osmotic pressure, therefore, exerted against membranes
such as those in the body which are, as a rule, partially permeable to
dissolved substances, is far below that measured by a semipermeable
membrane, and freezing-point determinations of osmotic pressures
(determinations which give a gauge of the full osmotic pressure as it
would be exerted against a semipermeable membrane), are of but
orienting value to the physiologist, except in cases where the per-
meability of the membrane to the substance in solution is known.

The following table from Pfeffer ^ is illustrative of the diminution in
the estimate of the full osmotic pressure caused by substituting a
permeable membrane (bladder or parchment paper) for copper ferrocy-
anide, and it is evident that the eifect is far more marked in the case of
the crystalloids (saltpetre and sugar) than in that of the colloid (gum).



Six per cent. Solution of


Parchment Paper.


Bladder.


Copper Ferrocyanide.


Gum arable .


17-9


13-2


25-9


Cane siigai' .


29-0


14-5


2877


Saltpetre


20-4


8-9


700-0
(not directlj' estimated.)



The pressures are In cms. of mercury.

The conditions, then, for the interchange of water and the con-
stituents of solutions through membranes in the body, are evidently
exceedingly comyjlex, and it is at present practically impossible to assess
the value of all the factors. Broadly stated, the following factors are
concerned : —

1. The quantitative composition of the solutions separated by the
membrane, and consequently the partial osmotic pressure exerted by the
several constituents.

2. The coefficients of diffusion of the various constituents.

3. The permeability of the membrane in its physiological condition
to the constituents.

4. The circumstances affecting the relative concentrations of a
constituent on the two sides of the membrane with time, e.g. circulation
and stirring.

5. The hydrostatic pressure on the two sides of the membrane.

6. The temperature.

The partial osmotic pressure of the constituents of a solution is
obtainable from a quantitative analysis, if the molecular weight and

1 Koeppp, Arch.f. d. cjes. Tliysiol., Bonn, 1897, Bd. Ixv. S. 492.
- " Osrnotisclie Untersuch.," Leipzig, 1877, S. 73.



OSMOSIS. 279

dissociation coefficient (in case of electrolytes) is known. If the
molecular weight is not known, as in the case of proteids, the substance
must be removed from the solution, and the difference in the total
osmotic pressure so produced estimated.

The coefficients of diffusion must be obtained under the special
conditions {e.g. diffusion into serum, etc.).

The permeability of the membrane to dissolved substances, one of
the most important factors, and one generally not capable of accurate
estimation, will not only affect the passage of water and dissolved sub-
st'ances across the membrane by osmotic action, but also the hydrostatic
pressure necessary to cause filtration.

We shall here content ourselves with considering a simple but usual
case of absorption of a solution by blood, namely, one in which the
osmotic pressure of the solution is lower than that of the blood, and the
membrane separating the two permeable to the substance in solution,
and to one at least of the constituents of the blood, but impermeable to
others. For convenience the dissolved substance is called x, and that
constituent of the blood to which the membrane is permeable, y. The
blood, by virtue of its superior osmotic pressure, tends to take up water
from the solution, and at the same time x diffuses through the membrane
into the blood, and y into the solution. If the blood be first supposed
to be stationary, a time is arrived at when the partial pressure of x and
y is the same on either side of the membrane ; in other words, this
solution of X and y is now the " solvent " in an osmotic experiment, and
the substances in the blood to which the membrane is impermeable are
the " dissolved substances." The whole of x, of y, and the water of the
original solution, must therefore in the end be absorbed.^ If the blood,
however, is circulated, the conditions for absorption are at once improved,
for the diffusion of x into the blood is favoured by the fact that its
partial pressure in the blood is kept low by renewed supplies of blood,
by the stirring action of the corpuscles preventing the formation of
" wall layers," and by the fact that cells in other parts of the body are
enabled to take up the substance as it is brought round. It is also
evident from the above that if, as a rare case, the solution had a higher
osmotic pressure than the blood, provided only the membrane separating
the two is permeable to the dissolved substance, and impermeable to
some constituents of the blood, when once the solution has taken up
enough water from the blood, and lost enough of its dissolved substance
to the blood, to lower its osmotic pressure to that of the blood, the
process described above is gone through, and it is in the end all
absorbed.

For such absorption to be carried out completely, it is evident that
the osmotic pressure of those constituents of the blood to which the
membrane and capillary wall are not permeable, must exceed the
pressure necessary to cause filtration across the same structures, for if
the available osmotic pressure on the inner side of the capillary wall is
less than the difference between the hydrostatic pressure on the two
sides of the membrane, filtration must occur, and the solution can never
be totally absorbed.

The assumption is here made that the resistance to the passage of
fluid across the membrane is the same in both directions. It must be

^ For this explanation to hold good, the substances in the blood to which the membrane
is impermeable must be in true solution; and capable therefore of exerting osmotic pressure.



28o DIFFUSION, OSMOSIS, AND FILTRATION.

noted, however, that animal membranes are known in which the
resistance to the passage of fluid is quite different in opposite directions.
The most familiar example is the shell membrane of the egg, which
permits filtration far more easily from within outwards than in the
reverse direction,^ and the same is true of the skin of the frog.^

FlLTKATION.

By filtration is meant the passage of fluid through a membrane, as a
result of a difference of hydrostatic pressure on the two sides. To filtfer
water from a solution across a membrane into another solution, the
difference between the hydrostatic pressures on the two sides must
exceed the difference between the osmotic pressures of the solutions, in
the case where the higher osmotic and hydrostatic pressures are on the
same side. If a porous pot bearing a semipermeable membrane is
filled with a solution and immersed in the pure solvent, the pressure
necessary to produce filtration of the solvent from the solution is one
just exceeding the full osmotic pressure of the solution ; but where we
deal with permeable membranes, as those in the body, the necessary
pressure is far less, because the difference of osmotic pressure on the
two sides of the membrane is reduced by the diffusion of some of the
dissolved substance.

Experiments on filtration through animal membranes appear to have
given very contradictory results, which seems to be due to the fact that
not only does continued pressure upon such membranes vary their
permeability, but a certain amount of " recovery " takes place in the
intervals between use ; hence the conditions of the membranes have been
by no means uniform in the experiments of different observers.

If the passage of fluid through an animal membrane is by more or
less tortuous paths, and if the walls of these are more or less elastic, it
is obvious, not only that by continued pressure must the resistance to
filtration rise, but that on removal of the pressure a slow " recovery " will
take place. Deformation of the membrane, if simply tied over a tube,
must also, unless it is properly supported, tend to distort channels and
so increase resistance to filtration. To these sources of difference in the
experiments must be added, previous drying of the membrane or not,
the condition of imbibition of the fibrous tissue, and temperature
variations. All these sources of differences are, moreover, accentuated
by the fact that most of the experiments have been made with thick
membranes, such as pericardium, bladder, intestine, and ureter.

Nearly all who have studied filtration have found that at constant
pressure the amount of the filtrate falls off with time,^ but, as pointed
out by Tigerstedt and Santesson,* this falling off is far more rapid in the
earlier hours of an experiment than later, so that it is advisable in all
such experiments on filtration to expose the membrane to pressure for

1 Meckel quoted by Ranke, " Physiologie des Mensclien," 1872, S. 122.

- Cima, MeTn. d. Accccd. di Torino, 1853, vol. xiii. ; Reid, Journ. Physiol., Cambridge
and London, 1890, vol. xi. p. 312.

^ Liebig, " Untersuch. ii. einige Ursachen der Saftebewegiing im tliierischen Organismus,"
Braunschweig, 1848, S. 7 ; Beitr. z. Anat. u. Physiol. {Eckhard), Giessen, 1858, Bd. i. S.
95 ; Runeljcrg, Arch. d. Heilk., Leipzig, 1876, Bd. xviii. S. 58 ; Gottwalt, Ztschr. f.physiol.
Chem., Strassburg, 1880, Bd. iv. S. 423 ; v. Regeczy, Arch. f. d. ges. Physiol., Bonn, 1883
Bd. XXX. S. 544.

■^ Bijhang. tillk, Svcns. Vct.-Akad., Stockholm, 1886, Bd. xi., No. 2,



FILTRATION.



many hours before the oljservations are conducted, and the stage of
relatively constant rapidity of filtration (a uniform rate is never actually
attained) is reached quicker at higher than at lower pressure. This
phenomenon is not due to any stopping of pores by particles suspended
in the fluids, since it is not noted when filtration is effected through
unglazed porcelain, and is probably simply a result of compression of
tortuous channels.

The quantity of filtrate rises with the pressure, but in lower ratio.

Thus, in an experiment by Tigerstedt and Santesson ^ of filtration of
distilled water through goldbeater's skin (serosa of ox-gut), which had
previously been exposed for 95 hours to a pressure of 80 cms. of water,



Pressure.


Filtrate per Minute
Observed.


Filtrate per Minute if
proportional to Pressure.


20 cm.
40 ,,
60 „

SO ,,


•046 grni.
•081 „
•110 ,,
•148 ,.,


•046 grm.
•092 ,,
•138 „
•184 ,,



The experiments of Wilibald Schmidt ^ showed a contrary result,
i.e. that the filtration rapidity increased at a higher ratio than the
pressure (possibly due to using dried membranes, the pores of which
were opened during experiment), as also did those of v. Eegeczy.^

A period of rest, interpolated between two filtration experiments at
the same pressure, is found to often cause an increase of the permeability
of the membrane above the value it possessed at the time of dis-
continuing the first experiment (Eckhard, Kuneberg, Tigerstedt and
Santesson).

Thus Tigerstedt and Santesson,* in a filtration of distilled water
through gold-beater's skin at 40 cm. pressure, observed a filtrate of
•490 grm. per minute, but after a resting period of 530' the filtrate at
the same pressure was "577 grm. per minute. But whether or not this
phenomenon is observed, is probably due to whether or not the elastic
limits of the fibres have been passed, " recovery " not being possible if
the membrane has been excessively stretched. The interpolation of a
period of filtration at lower pressure of course produces the same effect.^

The rapidity of filtration rises with the temperature,^ and, according
to Schmidt, the temperature coefficient is nearly that of Poiseuille, for
the flow of fluids in capillary tubes.

The nature of the solution to be filtered must obviously affect
both the rapidity of filtration, from differences in viscosity, and also
the quantitative composition of the filtrate in relation to that of the
original solution.

The following experiment from Tigerstedt and Santesson '^ may be
quoted in evidence of the first point : —

^ Loc. cit.. p. 31.

-^Ann. d.'Phys. u. Chem., Leipzig, 1856, Bd. xcix. S. 337 ; 1861, Bd. cxiv. S. 337.
^ Loc. cit. * Loc. cit. , p. 30. ® Runeberg, loc. cit.

^ Schmidt, loc. cit. ; Eckhard, loc. cit. ; LcJwy, Ztsclir. f. pliysiol. Chem., Strassburg,
1885, Bd. ix. S. 537.
■^ Loc. cit., p. 42,



28.2



DIFFUSION, OSMOSIS, AND FIITRATION



1. Erjcj albumin — 4'14 per cent, proteids ; pressure, 32'5 cm.; goldbeater's skin.

(Filtrate per minute in grms., -820, -051, -008, '003, -002, -002.)

2. Ox serum — 4"5 per cent, proteids ; pressure, 30*5 to 34 cm. ; goldbeater's skin.

(Filtrate per minute in grms., 2-586, 1-594, I'OOO, -812.)

In neither case had the membrane been previously stretched.

According to G-ottwalt,^ the serum albumin of blood serum filters
through a ureter more easily than the globulin. Martin ^ has shown
that homogeneous membranes of gelatin and gelatinous silicic acid form
filters impermeable to solutions of many colloids, but as permeable
to certain crystalloids as to water.

The relation of the concentration of the filtrate to that of the
original solution is perhaps the most important point to the physiolo-
gist in the matter of filtration through animal membranes. There is
general agreement that in the filtration of crystalloids the concentration
of the filtrate is very nearly that of the original solution, and this
appears to obtain at very various filtration pressures.^ There is also
general agreement that in filtration of colloids the concentration of the
filtrate is always less than that of the original solution.* But as regards
the effect of pressure on the concentration of a colloid filtrate, the
results of different observers are not in accordance. Euneberg^ has
maintained that the concentration of the filtrate is higher at lower
than at higher pressures, and the following table, taken from his later
paper j*^ is illustrative : —



Fresh Sheep's Intestine — Ascitic Fluid {circulated) holding
3-72 'per cent, of Proteids.



Time.


Pressure in Cm.
of Fluid.


Filtrate per hour
in Grms.


Per Cent. Albumin
in Filtrate.


8 P.M. to 8.15 A.M.
8.15 A.M. to 2.15 P.M. .
2.15 P.M. to 8.15 P.M. .


90 \


1-84
1-85
1-97


2-34
1-86
1-60


8.50 P.M. to 8.50 A.M. .
8.50 A.M. to 3.0 P.M.


} '' {


1-29
1-52


2-02
2-12


4.0 P.M. to 7.30 P.M.
7.30 P.M. to 9.30 A.M. .


} '' {


7-30
3-60


1-44
1-26


10.15 A.M. to 2.15 P.M. .
2-15 P.M. to 6.15 P.M. .


} '-'' {


2-89
3-56


2-42
2-60


7.0 P.M. to 9.30 P.M.
9.30 P.M. to 8.15 A.M. .


} '' {


6-70
6-21


1-84
1-68



^ Loc. cit. - Journ. Physiol., Cambridge and London, 1896, vol. xx. p. 364.

" Schmidt, urea, sodic chloride, and potassium nitrate, Ann. d. Phys. u. Chem.,
Leipzig, 1861, Bd. cxiv S. 391.

■* Schmidt, loc. cit., gum and albumin through ox-pericardium ; Hoppe-Seyler, Virchow's
ArcMv, 1856, Bd. ix. S. 245, blood-serum through ureter ; Runeberg, loc. cit., gut,
ureter, and pleuiul mendirane, with serum, ascitic;, and ]ileuritic fluids ; Gottwalt, loc. cit.,
egg albumin, hydrocele lluid, serum, and parovarian cyst iluid through ureter.

^ Loc. cit. ^ Zischr. f. physiol. C'hcm., Stras.sburg, 1882, Bd. vi. S. 508.



Online LibraryE. A. (Edward Albert) Sharpey-SchäferText-book of physiology; (Volume v.1) → online text (page 39 of 147)