Review article Metabolites, water and mineral exchanges across the rumen wall: mechanisms and regulation*

Summary — In ruminants, the forestomachs, and especially the reticulorumen, have walls with anatomical and histological properties that permit the exchanges of various metabolites, water and min-erals between the rumen contents and the blood. The development of papillae on the walls and the local blood circulation favour these exchanges. They depend to varying degrees on the food supply and the concentrations of volatile fatty acids (VFA) produced by the microbial catabolism of polysaccharides. The absorption of VFA and ammonia occurs essentially by a process of passive 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 importance is unknown. Short-term modifications in the absorption of these two substances are thus primarily determined by varia-tions in their intraruminal concentrations and pH. Other factors may also be implicated, and it is known in particular that the absorption of ammonia is enhanced when the intraruminal concentration of VFA or the carbon dioxide (C02) level increases. The movement of urea through the wall occurs from the blood towards the rumen content according to the concentration gradient. The main factors liable to influ-ence the transepithelial flux of urea seem to be the blood urea levels and factors that act on the con-tact surface between the blood compartment and the epithelium (C02, VFA). Ruminal ammonia con