This section is from the book "Diseases Of The Intestines", by Max Einhorn. Also available from Amazon: Diseases Of The Intestines A Text-Book For Practitioners And Students Of Medicine.
The object of digestion is to dissolve and partially change the food substances into such combinations as can be assimilated by the blood. Before assimilation can be effected absorption must take place. The main place for the absorption of nutritive material is the small intestine. It will be best to describe the process of absorption of the different food materials separately.
(a) The proteids are usually changed into albumoses and peptones before their absorption. Albumen as such, however, is also liable to be absorbed, although not so quickly as when its change into peptone has been accomplished. The absorption of albumoses and peptones takes place through the intestinal wall by way of the capillaries of the blood-vessels and not through the lacteals. Thus Munk and Rosenstein 1 observed in a patient with a lymph fistula that after a meal rich in albuminous food the lymph did not contain more proteids than before the meal. The peptones and albumoses do not reach the blood current as such, but are previously reconverted into albumin. This fact has been clearly shown by the experiments of Ludwig and Salvioli.2 These investigators tied a resected intestinal coil at both ends and injected into its lumen a solution of peptone, while the coil was kept alive with defibrinated blood. Although the peptone entirely disappeared from the intestinal coil, the blood did not contain even traces of peptone. It therefore must have become changed into another substance. This change of the peptones into albuminates before reaching the blood is of teleological importance.
For, as has been shown by Schmidt-Muhlheim3 and others, peptone introduced into the circulating blood is soon eliminated with the urine. Where the change of the peptones into albuminates takes place and by what mechanism are not as yet certain. Some seem to believe that the epithelial cells of the intestinal walls perform this office, others that the leucocytes are the means of its conversion.
1 Voit: Zeitschr. f. physiol. Chemie, Bd. 13.
The absorption of the albuminates appears to be more complete as regards animal than vegetable food. The reason for this is that the cellulose surrounding the legu-men partly renders its absorption more difficult. Again, the peristalsis being greater after vegetable food, the intestinal contents pass through the canal quicker, and thus less of the albumen is utilized. And again, according to Hammarsten,1 a part of the nitrogenous substances of the plant proteids appears to be indigestible.
1 Munk and Rosenstein: Virchow's Arch., Bd. 123.
2 Ludwig and Salvioli: Du Bois-Reyraond's Arch., 1880, Suppl.
3 Schmidt-Muhlheim: Du Bois-Reymond's Arch., 1880.
(b) The carbohydrates are absorbed principally as monosaccharides. Glucose, laevulose, and galactose are absorbed as such. Cane sugar and maltose are ordinarily changed first into glucose and laevulose. According to Voit and Lusk, sugar of milk is not converted, and is either partly-absorbed as such or else undergoes lactic-acid fermentation. The different kinds of sugar are absorbed through the capillaries of the villi and thus reach the circulation. They enter the liver through the vena porta and are here retained in great part as glycogen. In case, however, a large quantity of sugar is at once absorbed, it may occasionally reach the lacteals and thus enter the blood current outside of the liver. In such instances sugar appears in the urine, a condition which is known as alimentary glycosuria. The introduction of larger quantities of sugar into the intestinal tract occasionally gives rise to diarrhoea. Carbohydrates, however, even in large amounts in the form of starch, will be absorbed without difficulty and without giving rise.to any trouble.
(c) The fats. In the absorption of fats their emulsifica-tion seems to be of greatest importance. Although a small part is absorbed in the form of soaps, the greatest quantity of fat is taken up in the form of an emulsion. The latter comprises not only neutral fats but also fatty acids. These, however, undergo a change into ueutral fats after their absorption from the intestinal walls. It is generally accepted that fats after their absorption from the intestinal wall directly reach the lymphatics and thus enter the thoracic duct, whence they afterward find their way into the blood current. In a girl with a lymph fistula Munk and Rosenstein found that sixty per cent of the ingested fat appeared in the lymph. After giving the patient erucic acid (a fatty acid foreign to the organism) they could discover thirty-seven per cent of this particular substance in the form of neutral fats. Thus it appears to be proven that while the proteids and carbohydrates after their absorption directly reach the blood current, as mentioned above, the fats are an exception and directly enter the lac-teals. The ultimate way in which absorption takes place is not as yet known. It must, however, be accepted that the epithelial cells of the intestinal wall cause this process by some specific action.
The absorptive property of the small intestine for fat is very great. According to Rub-ner,1 a man can absorb over 300 gm. of fat per day. Not all kinds of fat, however, have the same coefficient of assimilation. Thus fats with a low melting-point (olive oil, goose fat, butter, etc.) are absorbed more quickly than those with a high melting-point (mutton fat and stearin). Moreover, free fats, like butter and lard, are assimilated more quickly and thoroughly than bacon, in which the fat is surrounded by connective tissue.
1 Olof Hammarsten: " Lebrbucb der physiologischen Cbemie," Wiesbaden, 1895, p. 293.
Besides the above-named three groups of food substances, water and different salts which are kept in solution are very quickly and thoroughly absorbed all along the intestinal tract. Aside from the salts, other soluble substances of the secretory juices are also absorbed. Thus the urine contains traces of pepsin and also urobilin, which shows that the biliary products must have been absorbed and eliminated through the urine. According to Schiff,1 the bile is absorbed from the small intestine and reaches the liver with the blood current in order to be eliminated again by this organ from the blood.
1 Rubner: Zeitschr. f. Biologie, Bel. 15.
The pancreatic juice being the principal factor in the digestion of the different kinds of food, it appears of interest to ascertain how much of these foods will be absorbed after the pancreas has been excluded from participation in the act of digestion. Minkowski and Abelmann2 experimented on dogs by extirpating the pancreas, and found that forty-four per cent of the proteids and from fifty-seven to seventy-one per cent of carbohydrates (amylaceous food) were absorbed, while the fats remained totally unabsorbed. The fat contained in milk, being emulsified, however, was absorbed to the extent of from twenty-eight to fifty -three per cent.
While the main place at which the absorption occurs is the small intestine, the large bowel is also able to serve-in this capacity. Thus aside from the absorption of fluids and salts which normally takes place in this organ, albuminates and carbohydrates can be absorbed in considerable amounts, and fats in small quantities. This function of the large bowel is of great practical importance, as it is utilized in some conditions for nourishing purposes (rectal alimentation).