A Text-book of medical practice for practitioners and students / edited by William Bain.
- Date:
- 1904
Licence: In copyright
Credit: A Text-book of medical practice for practitioners and students / edited by William Bain. Source: Wellcome Collection.
Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.
66/1042 page 38
![ABSORPTION FROM THE ALIMENTARY CANAL. In considering the (luestion of absorption one of the first jjoints to examine is how far our present physical knowledge can be used to explain the oliserved facts and how far we must be obliged to fall back on some unknown vital process as affording tlie only explanation. The study of the chemical processes occurring in the alimentary canal teaches us that the essential change thus effected is a cleavage of the complex food-stuffs into bodies of relatively small molecular size. With this smaller molecular size goes increased solubility and diffusibility, and hence there was, for a long time, a tendency to regard absorption as in the main a diffusion process. But that this is not so, is clearly shown in many experiments. Thus, rate of absorption is not proportional to the diffusion- coefficients of the several bodies, e.g., sugar is absorbed more rapidly than sodium sul- phate though its diffusion-coefficient is much higher. Again, absorption usually takes place from a very dilute solution and consequently the volume of fluid required would be immense, far more than we have any reason to suppose is available in the intestine, e.g., albumoses are never present in higher concentration than 2 per cent., and sugar is only found in traces. Thus the absorption and re-secretion of water would have to be enormous. More and more evidence accrues that absorjation of larger molecules than those produced ])y the digestive ferments can be quite readily eftected. Hence, when we incjuire what is the real significance of these ferments, we are probably correct in concluding that they split oft' bodies of the right size and conformation for rapid utilisation by the cells of the intestine. Though these cells can take up and utilise very large and complex molecules if required to do so, yet their work is greatly facilitated if they are supplied by material in the proj^er form. The possible seats of absorption are the stomach, the small intestine and the large intestine, and of these by far the most important is the small intestine. For the stomach, it has been shown that water is not absorbed but that alcohol can be. Many soluble substances, e.g., chloral or dextrose, are not absorbed if in aqueous solution, but are slowly taken in from a dilute alcoholic solution. Moreover, absorption from the stomach is greatly favoured by increased diffusion ; thus for sugar the rate of absorption increases with increasing concentration up to 20 per cent., whereas in the intestine absorption is most rapid at a concentration of 0'5 per cent. Proteid can also be absorbed in slight degree from the stomach. When we turn our attention to the intestine, the important absorbing surface, one of the first general questions is :—Through what channel are the various bodies absorbed, the capillaries, the lymphatics or both ? In all cases they must first reach the lymph- spaces below tlie epithelial cells before they can be passed on into the caijillaries. This problem has been investigated by collecting and examining the lymph issuing from the thoracic duct during the active absorption of the substance in question and comparing it with lymph collected during fasting. If the lymph is collected during the absorption of water, no increase in quantity and no change in constitution is observed. Hence it must be concluded that water is absorbed into the capillaries. But if we examine the blood, here too there is apparently no change, for no dilution can be detected. This is, however, only to be expected when we remember the rapid flow and consequently small dilution that would account for the whole quantity absorbed. Similar observations for the salts lead to the same conclusion in their case, and while, since we can collect the whole of the lymph flowing from the intestine, and, finding no excess present, can con- clude that it does not travel that way, we yet cannot decide from direct observations upon the blood that it has entered by this path. Wlien we come to other substances om- difliculties are greater because pf the increased difliculty of the analysis of such complex fluids as the lymph or blood. There is one other method possible, viz., to divert the whole of the lym]j]i of the thoracic duct to the exterior. As in such cases the nutrition of the animal does not suffer we can only conclude that the path via the thoracic duct is of minor importance. Absorption of Salts.—The first point we have to consider here is whether the process is one of simple diffusion, and the experiments that have been devised to test this have been to follow the absorption of solutions of different salts, some more and some less concentrated than the blood. Without entering into a long description of these experiments, it may be stated that for both concentrated and dilute solutions the rate ot absorption does not follow the ordinary laws of diffusion. We must therefore conclude that another factor, namely a physiological one, comes into play. , i a • i Absorption of Carboliydrates.—The carbohydrates are taken into the body mainly as dextrose or levulose for these are the only two carbohydrates normally found in t ie blood after a meal rich in carbohydrates. They are also the final sugars formed l.y the action of the ferments secreted in the intestinal canal, and, further, are the only sug^iis directly assimilable when injected into the blood-streiira. 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