METABOLITES, WATER AND MINERAL EXCHANGES ACROSS THE RUMEN WALL - MECHANISMS AND REGULATION

In ruminants, the forestomachs, and especially the reticulorumen, have walls with anatomical and histological properties that permit the exc hanges of various metabolites, water and minerals between the rumen co ntents and the blood. The development of papillae on the walls and the local blood circulation favour these exchanges. They depend to varyin g degrees on the food supply and the concentrations of volatile fatty acids (VFA) produced by the microbial catabolism of polysaccharides. T he absorption of VFA and ammonia occurs essentially by a process of pa ssive diffusion of their non-ionised form through the epithelial cell membranes. For each of these substances, the existence of a transport system for the ionised forms is also suspected, but its relative impor tance is unknown. Shortterm modifications in the absorption of these t wo substances are thus primarily determined by variations in their int raruminal concentrations and pH. Other factors may also be implicated, and it is known in particular that the absorption of ammonia is enhan ced when the intraruminal concentration of VFA or the carbon dioxide ( CO2) level increases. The movement of urea through the wall occurs fro m the blood towards the rumen content according to the concentration g radient. The main factors liable to influence the transepithelial flux of urea seem to be the blood urea levels and factors that act on the contact surface between the blood compartment and the epithelium (CO2, VFA). Ruminal ammonia concentration also affects the net urea transfe r across the rumen wall but the mechanisms involved in this regulation are not clearly understood. The absorption of water through the rumen wall results from an osmotic pressure gradient between the rumen and the plasma. This is modified not only by factors that modify the blood flow rates at the wall, but also by electrolyte concentrations. The a bsorption of minerals from the rumen has also been demonstrated (Mg, C a, Na, Cl, K, sulphur acid certain metals). This occurs by mechanisms of varying complexity according to the element involved.