We have now traced the food materials through the upper part of the small intestine. However various in appearance and in composition the ingredients of the diet may have been, they are now, in a healthy person, reduced to a uniform semifluid consistence, and their proteins, fats, and carbo-hydrates have suffered chemical change under the influence of the digestive juices and are, in large part if not entirely, converted into a number of simpler chemical bodies, suitable for absorption by the epithelium of the villi. This process of absorption is most active in the ileum, although it takes place to some extent along the whole of the small and large intestine.
The means by which the epithelial cells of the gut are able to take up food substances and transfer them to the lymph and blood are imperfectly understood, and the experimental study of the problem is hedged about by difficulties. In the first place it is clear that absorption is selective, some substances being taken up much more readily than others.
In the case of inorganic salts this may perhaps be explained to a considerable extent on physical grounds, for those salts which, when dissociated, liberate slowly moving ions, are slowly absorbed. This is the case with magnesium sulphate and is one explanation of its action as a purgative, the Mg and SO4 ions thus remaining in the intestine and attracting fluid by their osmotic tension until the gut is distended, and stimulated to pass the whole on. Solutions of salts in the intestine do, in great measure, follow the laws of osmosis through semi-permeable membranes, a hypertonic solution attracting water from the blood and lymph and a hypotonic losing water, with the result that the saline solution in the gut becomes isotonic with the blood and lymph; it is then gradually absorbed, presumably because the partial pressure of that particular salt is less in these fluids than in the solution, and because the epithelium is more permeable to that salt in the direction from the lumen to the blood. Osmosis through a semi-permeable membrane will not, however, fully explain the absorption, for salts which have similar physical properties, such as rapidity of movement of their ions, are not taken up at equal rates. The facts that an animal can take up water and salts from its own serum, and that the excised gut when placed in the defibrinated blood of the same animal will take up fluid into its mucous membrane, also indicate that even in the case of inorganic salts, and probably of water, the epithelium has some direct selective activity, and that there" is a general tendency for water and some salts to pass inwards. This does not imply that physical factors are not of great value. Nature uses all the means at her disposal. No one doubts that when water is taken into the intestine of a thirsty person the raised osmotic tension of the water-poor fluids of the body accelerates its absorption. Nevertheless the facts at present available as to the absorption of salts are not fully explained by physical conditions, and it is as well to speak plainly of a selective absorption as to shelter the same idea behind the phrase of a membrane possessing special permeabilities.
The absorption of the food-stuffs is a different matter, for it is probable that each of them is altered in its passage through the epithelium.
In the case of the proteins the former hypothesis that they are taken up in the form of albumoses and peptone is giving place to the view that the object of digestion is to break them down still further to the aminoacids which we have enumerated above, and that these bodies are absorbed, and either oxidized to furnish energy or rebuilt into fresh protein. It is still possible on this view that some protein is taken into the epithelial cells as albumose or peptone, for the ferment erepsin which is believed to complete the disintegration into aminoacids probably acts in the mucous membrane as well as in the lumen of the gut. It is important to note that protein loses but little potential energy in the course of its transformation into these amino-bodies.
If the assumption that proteins are absorbed in this form be correct, it should be possible to maintain nitrogenous equilibrium upon a mixture of the aminoacids; this has been shown to be the case. The nitrogenous needs of dogs and rats have been satisfied for considerable periods by the material obtained by the digestion of casein in vitro until no albumoses or peptones remained. In such a diet it is necessary that all the essential fractions required to build up the body protein shall be present. A diet of gelatine, for instance, cannot play the part of protein because, although gelatine yields many aminoacids on hydrolysis, it cannot furnish tyrosine, or tryptophane, or cystine, all of which are needed to form the higher proteins; if, however, those bodies be added there is evidence that gelatin can replace true protein in the diet. Zein, again, a protein obtained from maize, has been shown by Willcock and Hopkins to be more efficient as a food if tryptophane, which is missing from its decomposition products, be added to it.
If we conclude that the epithelial cells take up aminoacids from the gut, we have still to inquire whether these bodies are passed into the blood in this form. A portion of the amino-bodies at least must be built up somewhere into the body protein. Some authorities, for instance Abderhalden, have expressed the view that this is done in the wall of the intestine. Leathes has, however, brought forward evidence to show that there is a definite though small increase in the non-protein nitrogenous bodies in blood flowing from the absorbing intestine, and v. Bergmann has found amino-bodies in such blood. If the amino-bodies pass into the portal blood we should suspect that their elaboration might take place in the liver; this is supported by the fact that if the portal blood be led into the vena cava without going through the liver (Eck's fistula) symptoms of poisoning occur when the animal is fed upon meat, indicating that the liver plays an essential part in the assimilation of such food. It is also possible that some, or a great part, of the aminoacids may be taken, as such, directly to the various body cells. It is interesting to note that in plants nitrogen is absorbed into the sap in the form of aminoacids, formed by the breaking down of the seed proteins.
Nitrogenous material is taken up entirely by the blood and not by the lymph, for even on a rich protein diet the amount of nitrogen in the chyle is not materially raised.
Carbohydrates are absorbed entirely in the form of the monosaccharides and, on an ordinary diet, chiefly as dextrose, with small quantities of levulose, and as galactose. The conversion of maltose to dextrose is probably completed in the intestinal wall. The sugar is taken up by the blood, for it does not appear in the lymph of the thoracic duct. The passage of dextrose from the cell is, no doubt, aided by osmosis, for there will be a fall of pressure from the bowel to the blood. The portal blood carries the sugar to the liver where it is stored as glycogen.
We have seen that the fats are prepared for absorption by being emulsified in the mixture of juices in the small intestine, with the help of the bile; they are then more easily attacked by the fat splitting ferment of the pancreatic juice. The procedure is probably as follows. The ferment splits about five per cent of the emulsified fat into fatty acid and glycerine. In the presence of the alkaline juices the fatty acid combines with the base present, for example, sodium, and forms a soap; both soap and fatty acid are dissolved by the bile salts and are therefore presented to the epithelial cells in solution and taken up by them. In the usual case the contents of the gut are alkaline and the fat will be absorbed as soap; when the reaction is acid, as it is sometimes, the absorption will take place of the fatty acid. The removal of the fatty acid or soap will leave the steapsin free to split another five per cent of the fat in the intestine and this will go on until all is converted and absorbed.
The absorbed fatty radicle, whether acid or soap, is re-united with glycerine somewhere in the wall of the gut to form again a neutral fat (Munk). This is probably effected by a reversible ferment action working in the opposite direction to the lipase in the pancreatic juice, though no ferment could be separated by B. Moore from an extract of the mucous membrane. The same observer has shown that after a fatty meal the mucous membrane of the intestine contains 15 to 35 per cent of its fat in the form of fatty acid, whilst in the lymphatics of the mesentery only 5 per cent is in that form, the remaining 95 per cent being neutral fat which has, therefore, been re-synthesized before leaving the wall of the bowel. No loss of energy is involved in the transformation either way between fat and fatty acid. The microscopical appearances of the epithelial cells when stained with osmic acid do not enable us to distinguish between fatty acid and fat, as both of these take the stain.
Fat is absorbed by the lymphatics and passed into the thoracic duct, and thence into the subclavian vein, reaching, therefore, the general circulation without having been through the liver. About 60 per cent of that absorbed can be recovered from the duct; the fate of the remainder is not clear, but as the blood of the portal vein has not been found, during digestion, to contain more fat than that of the systemic arteries it is supposed that it is not taken up into the blood.
Water is absorbed along the whole length of the small intestine, but probably not in much greater quantity than is poured out in the intestinal juices, for the contents of the ileum remain in a semi-fluid condition until the caecum is reached. It is in the colon that the absorption of water chiefly takes place. If the food be mixed with bismuth the first part of it can be observed to arrive at the caecum four or five hours after the meal. As it passes out of the ileum it fills up the ascending colon, and this excites waves of contraction which pass backward from the transverse colon to the caecum and keep the semi-fluid mass from moving along : this delay in the colon enables water to be taken up, as well as the residue of food-stuffs.
As the faeces become drier and fill up the transverse colon, true peristaltic waves are from time to time excited which pass the distal portions of the mass into the descending colon and sigmoid flexure, whence they are periodically discharged in defaecation. Hertz found that food mixed with bismuth reached the hepatic flexure in 6 1/2 hours, the middle of the transverse colon in 8, and the splenic flexure in 9 hours after the meal. This refers to the daytime. During sleep the movements of the intestine are more sluggish.