Physiology of the Alimentary Tract in Relation to Diarrhoea
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3. small anirn. Pract. Vol. 8, 1967, pp. 123 to 130. Pergamon Press Ltd. Printed in Great Britain.
Physiology of the Alimentary Tract in Relation to Diarrhoea*
A. CARLYLE Dejiartment of Physiology, University of Brisfol
Abstract-Criteria of diarrhoea are discussed. The general nature of alimentary processes are considered briefly and a more detailed description is given of the accretion of endogenous material within the lumen of the alimentary tract and of the activity of epithelial cells. Special features of the tract of the dog and cat are considered and examples given of the types of alimentary functional inadequacy associated with diarrhoea in these species.
I N T R O D U C T I O N IT IS not the purpose of this paper to present an overall account of the physiology of the alimentary tract but rather to select certain aspects which seem relevant to the understanding of the possible causes and consequences of diarrhoea. I n doing this it has not been thought necessary to confine exemplification to the dog and cat since many of the principles involved are of general applicability and not restricted to particular species.
An initial difficulty is in the definition of the term diarrhoea, since it is often used somewhat vaguely and to refer to different kinds of abnormality of the faeces or of the defaecation pattern. I t may connote an increase in the quantity of faeces passed per day, an increase in the frequency of defaecation, an increase in the fluidity of the faeces, or the presence of abnormal faecal constituents. All these features may be present together in exaggerated form, as in the type of scour associated with primary or secondary infections of the small and large intestines, or the features may appear singly or in any combination. There is generally implied or assumed to be an abnormal loss of nutrient material. I n any particular case it may be important to distinguish between these different meanings because of the different implications as to cause and consequence.
An illustration of this is seen in the figures given in Table 1 relating to the effect of transferring cattle to spring grass after winter stall rations. The result would be described as diarrhoea by the criteria cf total faecal bulk or of fluidity, but not on the
*To be presented at the 10th B.S.A.V.A. Congress, 1967.
124 A . CARLYLE
criterion of nutrient loss. Criteria of diarrhoea are of particular importance in species such as the dog and cat in which marked differences are found, as between one individual and another, of both faecal consistency and defaecation pattern.
TABLE 1. THE EFFECT OF NUTRIENT INTAKE ON THE WATER AND DRY MATTER CONTENT OF THE FAECES IN CATTLE
Faeces (Kg/Day) Water intake (Kg/Day) Total Water Dry matter As water Concealed in food
Winter stall rations 20 14.7 5 . 3 52 19 Spring grass 34 31.0 3.0 51 33
From ROOK, J. A. F. and BALCH, C. C. (1962) 3. agric. Sci. 58, 103.
Where diarrhoea, in any of the senses considered above, occurs, it may be attributable either to a failure of alimentary function to deal efficiently with normal ingesta, implying some impairment or abnormality of gut function, or to the inability of a normal gut function to cope with the stress imposed by an abnormal type or amount of ingested material.
NORMAL G U T FUNCTION The primary function of the gut is that of assimilation; the ingestion of nutrient
material; its passage along the tubular tract with a gradual conversion of insoluble to soluble material; a reduction in molecular size, and an absorption of the products of this process through the mucosal epithelium so that they become available as meta- bolic substrates; substances not being rendered absorbable, and not being absorbed, are rejected as the residual material in the faeces. I n the simple-stomached carnivore the breakdown of ingested material is brought about principally by enzymes liberated into the lumen of the gut, usually as components of glandular secretions. Both the breakdown of material (digestion), and its removal (absorption), may be completed or limited by time, due to the movement of material through the system. These three aspects of alimentary function-digestion, absorption and rate of passage- are each, separately, controlled by a variety of mechanisms which are often linked to the quality and quantity of luminal material. There are, in addition, interactions between the separate aspects. Rate of movement through the gut is controlled by the state of activity of its smooth muscle. The activity and reactivity of the inuscle is largely conditioned by the distension of the gut by the luminal fluid bulk which is, in turn, largely determined by the balance between secretion and absorption. Both these processes act in two ways, directly and indirectly through the osmotic conse- quences of their activities. The effectiveness of the secretory and absorbative pro- cesses are in turn, through the time factor, dependent on the rate of passage of luminal contents as influenced by fluid bulk.
Thus, there is a highly integrated relationship between the different aspects of alimentary function determining the efficiency of the assimilatory process. This tends, in the normal course of events, towards a homeostasis of gut function and a stabilization of its terminal phase, the formation of faeces. I t is, however, a relation- ship in which abnormalities of faeces may be attributable to a variety of causes which
T H E A L I M E N T A R Y T R A C T I N R E L A T I O N T O D I A R R H O E A 125
may be late or early in the sequential chain, and which may take on an escalatory character.
Regional separation of gut function Even apart from the special features associated with fermentative processes,
which are not prominent in the dog and cat, there is a considerable separation of function within the alimentary tract. Enzymatic digestion occurs only to a limited extent in the stomach, is most intense in the duodenum, less so in succeeding parts of the small intestine, and is again reduced to a low level in the large intestine. Absorption also only occurs to a limited extent in the stomach. Although it occurs throughout the small and large intestine, there are considerable differences both in the intensity and the nature of the absorption mechanisms at different levels. Absorption of nutrient organic material is maximal in the small intestine, especially in the duodenum, and is virtually completed before the large intestine is reached.
Absorption of inorganic ions occurs in both small and large intestines. I n the duodenum, the intensity of secretory processes results in a net gain in both water and sodium, an increase in fluid bulk, and a rise in cation concentrations towards that of blood. I n the ileum, absorption and secretion tend initially to be balanced and then fluid bulk and cation concentration both fall (Hindle and Code, 1962). I n the large intestine, absorption of both water and sodium continues with a progressive reduction in total fluid volume. The osmotic pressure of luminal fluids becomes hypertonic half-way along the small intestine, is isotonic by the time the caecum is reached, and hypertonic beyond this (Van Weerden, 1961).
The character of the luminal fluid in the different regions of the tract differs very much. The nature of the fluid leaving any one region is determined partly by the nature of the fluid presented to it, partly by the activity of the epithelial and glandular elements within the region and partly by the rate of passage of luminal fluid through the region. The predominance of secretory processes in the duodenum cause a rise in volume and tonicity which must be redressed by ileal absorption of organic and inorganic substances, and by further absorption in the large intestine of water and electrolytes. The faeces-forming function of the colon is influenced not only by its own absorbative capacity and motor reactivity, but also by the nature and amount of the material presented to it from the small intestine.
TABLE 2. DAILY INTAKE AND POSSIBLE ALIMENTARY LOSS OF WATER AND ELECTROLYTES IS MAN
Water Electrolytes in mxq. (1.) Na+ K+ HC03-
Daily intake 2 150 100 Alimentary secretions 8 600 50 net 900* Total at risk 10 750 150 900 f At risk as yo E.C.F. 125 62 400 380
Excess over hydrion secretion.
Modified froin THOMPSON, R. H. S. and KINC,.J. (Eds.) (1964) Biochemical Disorders in Human Disease 2nd ed. Churchill, London.
126 A . C A R L Y L E
Luminal accretions Luminal contents represent much more than ingested material. There is a
considerable admixture of endogenous material. An obvious contribution comes in the form of the secretions of the various glands associated with the alimentary canal, and of the cells in the epithelium lining the gut itself.
From the figures given in Table 2 it is clear that water and ions in the luminal fluid are only in part derived from the ingested material, that the endogenous accretion is large in relation to the ingested component and, also, that it is large in relation to the immediately available reserve in the extra-cellular fluids. In so far as the luminal contents are at risk (should absorption and re-absorption not occur under diarrhoea1 conditions) there may be not only a loss of potential nutrients but also a drain of body constituents, with dehydration, loss of electrolytes, and a loss of alkali reserve.
Nor is it only water and electrolytes which pass into the lumen with the secretion. Either in the form of mucus or of digestive enzymes there may be a considerable organic content. In man, pancreatic juice alone can account for as much as 10-20 g protein a day, compared with a recommended dietary