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5 See F. Hof'meister, Z/schr. /. 2>hysiol. Chcm., Stras.sburg, 1882, Bd. vi. S. 69.

" See " Digestion and Absorption of Fats," p. 457.



ABSORPTION OF CARBOHYDRATES AND PROTEIDS. 433

There are thus two channels of absorption leading to the systemic
blood stream. One by the capillaries of the villus, passing through the
liver ; the other by the lacteals, md the abdominal lymphatics, to the
thoracic duct leading directly to the subclavian vein.

Ahsorption of ivater. — It has been shown by Heidenhain ^ that by
far the greater share of the water absorbed from the small intestine
is taken up by the capillaries of the villus and not by the lacteals.
When large quantities of dilute saline solution (0-3 per ce^t.) are injected
into the small intestine, the rate of lymph flow in the thoracic duct
is not markedly increased, unless so much salt solution is injected at one
time that the intestine becomes forcibly distended. Zawilsky^ also
found that even during active fat absorption there was no great increase
in the amount of lymph flowing from the thoracic duct; the lymph
became charged with an exceedingly fine emulsion of fat, but was not
largely increased in quantity.

The considerable absorption of water which commences in the lower
end of the ileum, and goes on throughout the entire length of the large'
intestine, causing the thin chyme of the upper part of the small intestine
to become semi-solid, and finally to assume the consistency of the fseces,
is also carried out by the agency of capillary blood vessels, so that
practically all the water absorbed from the intestine is taken up by the
blood stream. The blood is not diluted to a corresponding extent in the
process ; in fact, even with the absorption of an excessive amount of
water, as in Heidenhain's experiments above quoted, the composition of
the blood is little altered. The absorbed water in such a case of
excessive absorption passes at first into the lymph which bathes the
tissues, to be afterwards brought out and gradually eliminated by the
kidneys as the excess in the blood diminishes.

Absorption of soluble constituents.^ — All those substances which leave
the epithelial cell in solution, are also carried away from the lymph
spaces of the villus by the capillaries.* This has been shown chiefly by
observations made during active absorption of these several constituents,
on the rate of flow in the thoracic duct, the constitution of the lymph so
flowing, and the effects of ligature of the duct or diversion of the stream
to the exterior. Direct analyses of the blood of the portal vein, as com-
pared -with the systemic blood, do not yield trustworthy results ; partly
because of the difficulty of making very exact determinations in such a
complex fluid as blood serum ; still more because of the very small change
in composition which is sufficient to account for the carriage of a great
weight of absorbed substance, by reason of the copious flow which takes
place through the capillaries, especially when active digestion is in
progress.

If a cannula be inserted into the upper end of the thoracic duct, and
the rate of flow of the lymph stream measured, as well as the amount of
proteid contained therein, neither of these is found materially to alter,
whether the animal (dog) be fasting, or active proteid digestion be going

^ Arch./, d. ges. Physiol., Bonn, 1888, Sapp. Heft, Bd. xliii. S. 53; 1894, Bel. Ivi.
S. 579.

^ Arh. a. d. pliysiol. Anst. zu Leipzig, 1876, Bd. xi. S. 161.

^ For the Absorption of fats and fatty acids, see p. 443.

■* It is often stated that all the dissolved intestinal contents are so absorbed, but if, as
is probable, fats are absorbed in soluble form, such a statement is obviously incorrect. Only
those constituents which remain soluble, after the action of the absorbing cells, pass into
the capillaries.

VOL. I. — 28



434 CHEMiSTR Y OF THE DIGESTIVE PROCESSES.

on. This could obviously not be the case if any appreciable part of the
proteid were absorbed by the lacteals.

Again, if the thoracic duct be ligatured, and an hour after the opera-
tion the animal (dog) be given a rich meal of proteid food, absorption
goes on in a normal manner. If the animal be killed after the lapse of
about forty-eight hours, it will be found that all the proteid has been
absorbed, while a corresponding amount of nitrogen has been eliminated
in the urine.^

A similar proof has been given for carbohydrate absorption l)y the
blood vessels. In this case the animal is fed with carbohydrate food
instead of proteid ; and the amount of sugar in the lymph which flows
from a fistula of the thoracic duct is estimated. The percentage of sugar
lies between 0-6 and 1-6 per thousand, and does not vary in the least
with the state of digestion, this being the usual percentage of sugar
found in lymph or blood serum.-

A dkect proof has also been given of the absorption of sugar by the
capillaries, as it has been shown that on injection of sugar into the
intestine the percentage of sugar in the portal vein may rise as high
as 4 per 1000, while in a fasting condition the amomit of sugar
contained in the l^lood of either portal or hepatic veins does not essentially
differ from that in the blood of the remainder of the circulation.^

It may be taken, then, that, under normal conditions, all the soluble
constituents which leave the epithelial cell are taken up by the capillaries.
But when excessive absorption is taking place, as when large quantities
of sugar in concentrated solution are injected into the intestine, this is
not the case. Here the work of absorption becomes too great for the
capillaries, a part of the dissolved foodstuff passes the region of their
action and is absorbed by the lacteals, probably in a passive fashion.

Conditions of absorption of carbohydrates.— There is no doubt
that a considerable share of the carbohydrate food is taken up from
the intestine by the absorbing cells as simple sugars (mainly as dex-
trose and Itevulose), otherwise the reason of the ferment actions which
have been previously described would be diificult to see. But we
possess no experimental evidence to show that all the carbohydrate
is absorbed in such a form. Indeed, it is probable that the absorbing
cells are capable of taking up not only saccharoses, but even colloidal
carbohydrates, such as dextrin and starch, and converting these into
simple sugars before turning them into the blood stream.

We have already seen, in discussing the digestion of starch and
glycogen, that it is impossible, in experiments carried out m vitro, to
further convert all the dextrin formed into maltose or other form of
sugar. Sheridan Lea's ^- experiments show, mdeed, that the rapidity of
diastatic action is much increased by dialysis, and the quantity of dextrin
left unchanged into maltose largely diminished. Lea argues from this
result that, under the more favourable conditions for removal of digestion
products existing in natural digestion, the conversion of dextrin into
maltose may become complete. Contrary to this view, there is the
experience of Musculus and Gruber,^ that the unchanged dextrin remain-
ing after a prolonged digestion of starch, with a diastatic ferment, is not

1 Schmidt-Mulhf4m, Arch. f. Anal. u. Plujsiol., Leipzig, 1877, S. 549.

2 Von Mcriiig, ihid., 1877, S. 379.

"^ Journ. Physiol., Cainliridge and London, 1890, vol. xi. p. 226 ; see also }ip. 321 and 394.
* Zlschr. f. 'physiol. Chcm., Strassburg, 1878, Bd. ii. S. 177.



ABSORPTION OF CARBOHYDRATES. 435

convertible into maltose by a fresh addition of diastatic ferment after
complete removal of the maltose produced l^y the first digestion. So
that the failure of the ferment to convert the last portion of dextrin into
maltose cannot ]je wholly due to the stoppage of its action by the presence
of excess of maltose.

There is no very evident reason why soluble materials like the dex-
trins should not be absorbed as such by the epithelial cells. The argument
that dextrin is not directly assimilable, because, when injected subcutane-
ously or intravenously, it is eliminated by the kidneys, is not valid against
its absorption as dextrin ly the qnthelial cell. For there is no reason to
suppose that the cell must turn it into the lymph in exactly the same
form in whicli it takes it up from the intestine ; the chances are, in fact,
all against such a supposition. It may be taken as probable, then, that
the digestive enzymes of the alimentary canal are incapable of con-
verting all the starch of the food into maltose, and hence into dextrose,
and that a portion is absorbed as dextrin, and changed into something
else before reaching the blood stream.

What has been said above concerning dextrin applies also to the
double sugars. The intestinal juice, as we have seen, contains enzymes
capable of converting maltose and cane-sugar into simple sugars, and it is
probable that such a change does take place to a very large extent. Still
it cannot be concluded that the double sugars undergo complete conversion
before absorption. Lactose appears not to be acted upon by any of the
digestive enzymes, and so far as it escapes lactic acid fermentation this
double sugar must be absorbed by the epithelial cell unchanged. Again,
Brown and Heron ^ found that the dried mucous membrane acted much
more energetically on maltose than did any extract of it, which tends to
show that this action takes place in part within the cell.

Eohmann ^ has also shown that not only sugar, but even starch solution
disappears from a Thiry-Vella fistula with considerable rapidity ; and as
the succus entericus possesses only an exceedingly feeble diastatic action
on starch, it seems that here the starch must be directly taken up by the
intestinal cell. Such a view is also supported by the fact that, after
removal of the pancreas, the secretion of which must produce the greater
part of the diastatic action which goes on within the intestine, one-half
to three-fourths of the starch of the food is still utilised.^ Under normal
conditions, however, the diastatic conversion by the pancreatic juice is so
rapid, that it is very improbable than any appreciable portion of starch is
absorbed as such.

Form in lohich carhohydrcdes reach the hloocl stream. — During active
carbohydrate absorption, traces of carbohydrates, resembling dextrin, are
said to be present in the blood of the portal vein,"^ but it is probable that
very little carbohydrate leaves the epithelial cells other than dextrose
or laevulose. These two sugars are capable of direct assimilation after
subcutaneous injection, and of forming glycogen in the liver, but no such
direct assimilation takes place in the case of cane-sugar or maltose.

1 Loc. cU. 2 Arch./, d. ges. Physiol., Bonn, 1887, Bd. xli. S. 411.

'^Minkowski and Abelmann (Inaug. Diss., Dorpat, 1890; Centralhl. f. Physiol.,
Leipzig u. Wien, 1891, Bd. iv. S. 522) found, after complete extirpation of tlie pancreas,
ail absorption of 57-71 per cent, of starcli ; the brothers Cavazzanni {Centralhl. f. Physiol.,
Leipzig u. Wien, 1893, Bd. vii. S. 217), under like circumstances an absorption of 47 per
cent.

*Otto, Jahresl. ii. cl. Forfschr. d. T/^Jcr-C'Acm., Wiesbaden, 1888, Bd. xvii. S. 138 ;
V. Mering, Arch. f. Anat. u. Physiol., Leipzig, 1877, S. 413.



436 CHEMISTRY OF THE DIGESTIVE PROCESSES.

Ingestion of large quantities of solution of sugar leads to the appear-
ance of sugar in the urine (ahmentary glycosuria), due to the assimilation
of the sugar not keeping pace with its absorption.^ Absorption itself
is also disturbed by the appearance of diarrhcEa. But when carbohydrate
is introduced into the alimentary canal, in the form of starch, immense
quantities can be rapidly and completely absorbed without any glycosuria
or other disturbance ensuing.

Thus Eiibner^ found that a man consumuig a daily ration of 508-670
grms. of carbohydrate contained in wheaten bread, left unabsorbed only
0-8-2-6 per cent. ; of the carbohydrate of peas (357-588 grms.) 3-6-7"0 per
cent, was unabsorbed; and of potatoes (718 grms.) 7'6 per cent. This
complete absorption and utilisation of carbohydrate, when taken in the
form of starch, is probably due to the rate of assimilation and storage as
glycogen in the liver, beiag alile to keep pace with that of absorption
from the intestine.

Conditions of absorption of proteids.— The power possessed by
the intestinal cells of absorbing various forms of proteid affords one
of the best illustrations that this process is not one of mere
physical diffusion. The products found towards the end of a proteid
digestion in vitro are distinguished from the proteids from which they
originate by being sMghtly diffusible. To this fact great importance
was at one time attributed, because it was thought that only proteids
in a diffusible form were capable of absorption, and hence that peptonisa-
tion was in all cases a necessary preliminary. It is now generally
admitted that many forms of native proteid are capable of entering the
epithehal cells without previous change by digestion or otherwise ; and
in those cases in which a proteid is incapable of du'ect absorption a much
less profound change than peptonisation is sufficient to render it so,
namely, conversion into acid or alkali albumin. Such an absorption of
soluble proteid, other than albumose or peptone, takes place not only in
the small intestine, but in the large intestine, and even in the rectum.

Voit and Bauer ^ cleared a loop of small intestine of its contents as
completely as possible by stroking, and separated it from the rest of the
gut by double hgatures at each end. Various forms of proteid, in solu-
tions of known amount and strength, were then injected uito this loop ;
the mtestine was replaced, and its contents examined on killing the
animal (cat or dog) some hours later. It was found that variable amounts
of these proteids had disappeared ; thus, in one to four hours, 16—33 per
cent, of white of egg had disappeared, and of syntonin from ox muscle
28-95 per cent. It might be supposed that the portion of proteid ab-
sorbed had been peptonised by traces of proteolytic enzyme which might
be present in the intestine ; but in the unabsorbed proteid remainiag at
the end of the experiment, no albumose or peptone w^as found. A^oit and
Bauer also injected solutions of white of egg and sochum chloride into the
rectum of man and animals in a fasting condition, and found a marked
increase (6 to 8 grms. in 24 hours) in the amount of nitrogen eliminated
by the kidneys ; in fact, an equilibrium of nitrogenous metabohsm may
even be maintained in this way. It has been shown by Eichhorst,^ who
confirms these results, that no appreciable amount of peptonisation takes

1 C. Voit, Ztschr.f. Biol., Miinchen, 1891, Bd. xxviii. S. 245.

'Hid., Bd. xix. S. 45.

=* Ibid. , Bd. V. S. 562.

* Arch. f. d. fjes. Physiol., Bom], 1871, Bd. iv. S. 570.



ABSORPTION OF PROTEIDS. 437

place in the large intestine. Finally, the objection that the action is due
to traces of enzymes, has been disposed of by the observations of Czerny
and Latschenberger,^ in a case of a fistula situated in the sigmoid flexure.
The rectum was thoroughly washed out from the fistula, yet from 60-70
per cent, of the injected proteid disappeared in 23-29 hours.

Assimilable and non-assiinilable proteids. — Some forms of proteid, such
as alkali albumin, prepared from white of egg, and acid albumin, prepared
from muscle, myosin, fibrin, or white of egg, are directly assimilable ; that
is to say, when injected into the blood stream they are not removed again
by the kidneys ; others, such as unchanged white of egg, caseinogen, and
glutin, are, when injected, at once excreted in the urine. The latter forms
must therefore, under normal conditions, be changed during absorption,
before passing into the blood ; but when excess of white of egg is present
in the intestine, absorption oversteps the rate at which this change can
take place, and a portion of the egg albumin reaches the circulation
unchanged. Under these circumstances, this portion is promptly re-
moved by the kidneys, and an " alimentary albuminuria " is the result,
just as an excessive amount of sugar in the intestine produces " alimentary
glycosuria."

Relative amounts of jj'^oteids absorhed in different forms. — It is
evident, then, that absorption can take place, either in the form of
albumose or peptone, of alkali or acid albumin, or even occasionally
in that of native proteid ; and the question arises, to what extent does
absorption take place under natural conditions in each of these different
forms ? Such a question is exceedingly difficult to answer by experimetit.
It is impossible to do so exactly by observation of the amount of each
form of proteid present in the intestinal contents during proteid absorp-
tion, because the absorption is selective, and a substance present only in
traces may be passing out of the intestine more rapidly as it is con-
tinuously formed than another which is present in much larger quantity.

A rough estimate of the relative amounts of proteid absorbed as
albumose and peptone, and that absorbed in other forms, may be obtained
from analyses of the intestinal contents during proteid digestion. Thus,
Schmidt-Miilheim ^ examined the contents of the stomach and intestine
at varying periods during digestion of flesh in dogs ; he found that the
am"ount of proteid in solution, both in the stomach and intestine, was
small at any given time, but that the amount present as albumose and peptone
was always somewhat greater than that present in other forms. When it is
remembered that albumose and peptone are absorbed more rapidly than
other proteids, this points to the greater part of the proteid being absorbed
as albumose and peptone.

It is not known with certainty to what extent amido -acids are formed
from proteids, in the natural course of intestinal digestion. The experi-
mental evidence on the subject is somewhat conflicting, but the majority
of observers are of the opinion that but little proteid is absorbed as leucine
and tyrosine, being nearly all absorbed as albumose or peptone, or even
at a still earlier stage.

The only positive evidence as to the formation of leucine and tyrosine
in natural digestion, rests on the amounts found in the intestinal contents

1 Virchoiv's ArcMv, 1874, Bd. lix. S. 174 ; see also Ewald, Ztsclir. f. Idin. Med., Berlin,
1887, Bd. xii. S. 407 ; Haber, Deutsches Arch. f. Min. Med., Leipzig, 1891, Bd. xlvii,
S. 495.

'^ Arch. f. Anat. u. Physiol., Leipzig, 1879, S. 39,



438 CHEMISTR V OF THE DIGESTIVE PROCESSES.

during proteid digestion. Such evidence can only give a reliable
estimate of the amount formed relatively to other proteid products,
v^heu the rate of absorption from the intestme of these various products
is known.

Kolhker and Miiller ^ found only microscopic traces of leucine and
tyrosine in the upper part of the small intestine of carnivorous animals,
and none in the lower part. Kiihne - subjected fibrin to tryptic digestion
in a tied-oft' loop of intestine, into which the pancreatic duct entered,
and subjected the residue to analysis after four hours. Leucine and
tyrosine were found among the products, but the yield w^as small, and,
moreover, the conditions in such an experiment are not quite comparable
to those of natural digestion. Indeed, Kiihne himself thinks it probable
that the greater part of the " peptone " being rapidly absorbed escapes
such a decomposition.

Schmidt-Mulheim ^ states that, in proteid digestion in the carnivora, leucine
and tyrosine are either not formed at all, or else in such small quantities that
their absorption is of no physiological importance as a means of removing from
the alimentary canal any appreciable amount of nitrogen derived from the
proteid foodstuffs.

On the other hand, Sheridan Lea ^ obtained from the intestinal contents of
a dog, six hours after a plentiful meal of lean flesh, what must he regarded as a
considerable amount of amido-acids to be found there at any given time (par-
ticularly when it is remembered that the amount of intestinal contents in the
dog at any given moment, even during active proteid digestion, is very scanty),
namely, 1 grm. of pure leucine, and "3 grm. of tyrosine. These figures equal the
total amounts obtainable of these products from 10 grms. of dried proteid, and
if it he assumed that leucine and tyrosine are absorbed with a rapidity equal to
that with which albumoses are taken up, indicate that a considerable percentage
of proteid was being converted into amido-acids, and absorbed as such.

The relative amount of proteid decomposed in the intestine into amido-
acids, as well as that absorbed in the various other forms, probably varies within
wide limits with the state of nutrition of the animal and the amount of proteid
food. It is possible, as Foster ^ states, that such a degradation of proteid in
the intestine may serve as a safety-valve to the economy, diverting from the
tissues the burden of an often unnecessarily large proteid metabolism. The
waste of energy to the animal economy caused by the disintegration of proteid
into amido-acids in the intestine is often advanced as an argument against the
occurrence of this process to any marked extent. Certainly the potential
chemical energy of the proteid is lost to the economy, as far as the performance
of some forms of physiological work is concerned, but it should not be forgotten
that the total amount of energy abstracted by the animal from its food is
measured by the chemical form in which it enters and that in which it leaves
the body, and a given portion of proteid entering the body and then leaving it
as urea, water, and carbon dioxide, will give up to the body exactly the same
store of energy, no matter what may be the intermediate steps by which it is
reduced from one form to the other. In one case the energy is set free in
the tissues, in the other in the intestine. In the second case, the heat
set free by chemical decomposition is communicated to the intestine and
carried off by the circulating blood, to keep up the temperature of the body,
thus sparing reserve chemical energy which would otherwise have to be used
for this purpose.

1 Verliandl. d. 2>tiys.-med. Gescllscli. zu JVilrzhurrj, 1S56. Bd. vi. S. 499.
- Virchoiv's Archiv, 1867, Bd. xxxix. S. 130. " Loc. cit.

■• Journ. Physiol., Cambridge and London, 1890, vol. xi. ]). 255.
•' "Text-Book of Physiology," 1889, 5tli edition, part ii. p. 476.



ABSORPTION OF PROTEIDS. 439

Changes in albumose and j^ej^tone during absorjJtion} — Although
there is no doubt that a considerable, if not the greater part of
the proteid of the food is absorbed as albumose or peptone, these
bodies are never found in appreciable amount in the blood. Schmidt-
Miilheim ^ stated that the maximum amount in serum is 0-028 per cent.;
but recent experiments by Neumeister^ have given an altogether
negative result, and, according to this observer, albumoses are not present
at all in blood, even in traces.

Injected directly into the blood, albumoses and peptone are treated
by the organism as foreign bodies ; they are not assimilable proteids, but
are promptly excreted by the kidneys, unless injected in large quantities,^
and in a short time practically all the peptone and albumose injected is
found in the urine, while not a trace is to be found in the blood.^
That albumose and peptone are foreign substances in the blood stream,
is shown not only by this rapid elimination, but by the fact that they
possess, besides, marked toxic properties, and cause the death of the
animal when injected in larger doses, producing an immense and rapid
fall in arterial blood pressure ; in addition, they so alter the nature
of the blood that on drawing it from the vessels it no longer coagulates,
or does so very slowly. These results, taken in conjunction with the
fact that normal urine never contains albumoses, even in traces, prove
that the albumoses and peptones absorbed from the alimentary canal
never reach the general circulation as such, but are somewhere on their
route converted into other substances which can harmlessly enter the
cu'culation. Positive experiments on the subject not only confirm this
indirect proof, but clearly indicate that the change takes place in the
lining epithelial cells.

Seat of the modification of albumose and peptone during absorj^tion. —
It might be supposed that the albumose and peptone disappeared as
such in the liver ; this is not, however, the case. Schmidt-Mlilheim ^



Online LibraryE. A. (Edward Albert) Sharpey-SchäferText-book of physiology; (Volume v.1) → online text (page 62 of 147)