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.
64/1042 page 36
![matioti of dextrose from the liver glycogen is a pathological process and only occurs in diseases (diabetes, etc.), in which an abnormal amount of dextrose appears in the Ijlood and consequently in the urine. This view is most strongly supported by his discovery of many proteids which on decomposition yield a carbohydrate molecule among the other simj)le products. Several of these are, for instance, to be found in the blood. The rule played by the liver in nitrogenous metabolism may be of a twofold nature, the first in taking part in the initial synthesis of proteid substances, the second in dealing with the final nitrogenous waste products of the tissues. As to the former function our knowledge is of the most meagre character. We have a certain number of facts which indicate that the liver is concerned with the fcjrmation of certain specific proteids, such, for instance, as those controlling the coagulability of the blood, the formation of certain anti-bodies, and, as already stated above, there are indications that glycogen is simply stored there to be used in building up new,proteids, or in reforming old proteid molecules, which in their passage through the tissues have been robbed of a part of their substance, which has gone to supply the tissue-cells with a necessary material. When we come to the study of the part the liver plays in dealing with nitrogenous waste products, we are on more certain ground. Here we are concerned more particularly with the formation of iu?ea. The older view was, that this substance was made by the kidney at the time that it was being eliminated, but it was shown that extirpation of these organs resulted in an accumulation of urea in the tissues and in the blood. Nitro- genous waste occurs in every cell of the body and it may well be that every tissue turns out some of its waste in the form of urea, but many facts prove that the main source of urea is the liver. This statement is based on the following evidence : (1) tlie results following removal of the liver, (2) the conditions observed in disease of the liver, and (3) on results obtained by perfusing the liver. Removal of the liver in frogs leads to an almost complete cessation of urea formation, its place being taken by ammonia. In mammals, the operation is so severe that the animals very quickly die, but the organ can be thrown almost completely out of action by uniting the portal vein to the inferior vena cava (Eck's fistula). Under these circumstances the aniount of urea secreted is gi-eatly diminished, and its place is taken by ammonia. In these experiments, if the animal be given a proteid meal, acute symptoms (convulsions, coma, etc.) are at once produced, the animal passing into a state which is practically that of acute ammonia poisoning. If the blood is now examined it is found to contain an excess of ammonia, thus giving ])roof that the explanation just given is the correct one. If in dogs m which an Eck's fistula has been established the portal vein is left unligatured, the alterations in the urine are only partial, though in the direction indicated above. In other animals in which the portal vein is ligatured but the hepatic artery left the symptoms are much more acute, especially if proteid is given in the food. Lastly, if the hepatic artery is also ligatured the animal dies within twelve to twenty-four hours with acute symptoms of ammonia poisoning. , . ■, • , • r • These conclusions are confirmed by observations on birds, m which the experiments are simpler, for they possess a natural communication between the portal vein and inferior vena cava, via the renal veins. Birds can therefore survive the operation of extirpation for several days, but, unfortunately for our present argument, the chief nitrogenous excretion in this class is uric acid and not urea. The result obtained is an almost complete disappearance of uric acid from the urine, its place being taken by ammonia. Such experiments clearly prove that the chief precursor of uric acid m the bird is ammonia. The urine in these cases also contained considerable quantities of lactic acid, thus mm- cating that the carbon compound which the liver employs in effecting this synthesis is this acid. This is confirmed by an examination of the structural formula for uric acid, by the fact that the acid has been synthesised from lactic acid and urea, and from the experiments in which uric acid formation has been studied m artificially perfused bu-ds livers. In the latter experhnents the formation of uric acid is found to be greatly favoured by the addition of lactic acid, though its presence is not an essential condition, for the liver can still form uric acid if supplied with ammonia only. , . , , Perfusion experiments upon the livers of mammals have also conclusively proved that the organ can form urea. Thus, experimenting upon^ dogs, it ^t.^^^Z liver ammonia disappeared from the blood, its place being taken by urea, and that the livei could in this way deal with a considerable amount of ammonia. In these experiments, as well as in those in which the liver has been short circuited or removed, there is evidence that the ammonia exists in the blood as ammonium carbamate, not as c'*^^^^- has further been shown that this compound of ammonia is also present in normal blood The difference between the two con.pounds is slight, yet it is importont, for ammon m carbamate is the intermediate compound between ammonium carbonate and urt.a. inus rammonimn carbonate loses one molecule of water it becomes ammonmm carbamat^ Irif a second is removed it is converted into urea. It is also known that if small ouantities of aminonfa gas be led tlirough a carbonic acid solution some of the ammoma unSs will the acid to form the carbamate while the rest appears as carbonate. We](https://iiif.wellcomecollection.org/image/b21510167_0064.jp2/full/800%2C/0/default.jpg)
