The large bowel--a supplementary rumen?

The rumen and the mammalian large intestine are similar in many respects. Microbial protein appears to be synthesized and degraded in the digesta of both organs in a comparable manner. The VFA end-products of carbohydrate fermentation are produced in similar concentrations. Digesta pH is maintained with buffer added by the saliva or ileal fluid, HCO3 released into the lumen and rapid absorption of the organic acids. VFA are absorbed at equivalent rates by rumen epithelium and large intestinal mucosa. Over-production of VFA produces similar adverse effects. There is a considerable amount of species variation in the relative length and volume as well as the extent of sacculation of the large intestine. The caecum is the primary site for retention of digesta and microbial fermentation in the large intestine of rabbits, rodents and a few other species. However, the proximal colon is the major site of retention and fermentation in most mammals. Absorptions of Na and VFA appear to account for absorption of most of the water removed during passage of digesta through the large intestine. A relatively slow rate of Na absorption and release of HCO3 appears to provide the fluid and buffering capacity needed for efficient microbial digestion in the rumen and in the large intestine of some species. A more rapid absorption of Na by the large intestine of other species would aid in the conservation of Na and water. The many similarities between the large intestine and the rumen suggest that further comparison can provide additional information on both the function and diseases of these two organs. The rumen has proved to be accessible to a variety of procedures useful for the study of microbial digestive processes and its epithelium has provided a non-glandular tissue for studies of inorganic ion transport as well as the transport and metabolism of VFA. Comparative studies of the large intestine also can provide a better understanding of the functions and malfunctions of the human large intestine. The pig and some species of monkey would appear to be most suitable for examining the effects of diet and carbohydrate malabsorption. The equine large intestine provides a good system for studying events, such as those associated with the recycling of nitrogenous compounds. Furthermore, as demonstrated by comparative studies of the kidney and other organs, a great deal of information can be gained through examination of the various mechanisms that have been developed to serve similar functions.