<P> The main substrates of digestion in the reticulorumen are non-structural carbohydrates (starch, sugar, and pectin), structural carbohydrates (hemicellulose and cellulose), and nitrogen - containing compounds (proteins, peptides, and amino acids). Both non-structural and structural carbohydrates are hydrolysed to monosaccharides or disaccharides by microbial enzymes . The resulting mono - and disaccharides are transported into the microbes . Once within microbial cell walls, the mono - and disaccharides may be assimilated into microbial biomass or fermented to volatile fatty acids (VFAs) acetate, propionate, butyrate, lactate, valerate and other branched - chain VFAs via glycolysis and other biochemical pathways to yield energy for the microbial cell . Most VFAs are absorbed across the reticulorumen wall, directly into the blood stream, and are used by the ruminant as substrates for energy production and biosynthesis . Some branched chained VFAs are incorporated into the lipid membrane of rumen microbes . Protein is hydrolysed to peptides and amino acids by microbial enzymes, which are subsequently transported across the microbial cell wall for assimilation into cell biomass, primarily . Peptides, amino acids, ammonia, and other sources of nitrogen originally present in the feed can also be used directly by microbes with little to no hydrolysis . Non-amino acid nitrogen is used for synthesis of microbial amino acids . In situations in which nitrogen for microbial growth is in excess, protein and its derivatives can also be fermented to produce energy, yielding ammonia . </P> <P> Lipids, lignin, minerals, and vitamins play a less prominent role in digestion than carbohydrates and protein, but they are still critical in many ways . Lipids are partly hydrolysed and hydrogenated, and glycerol, if present in the lipid, is fermented . Lipids are otherwise inert in the rumen . Some carbon from carbohydrate or protein may be used for de novo synthesis of microbial lipid . High levels of lipid, particularly unsaturated lipid, in the rumen are thought to poison microbes and suppress fermentation activity . Lignin, a phenolic compound, is recalcitrant to digestion, through it can be solubilized by fungi . Lignin is thought to shield associated nutrients from digestion and hence limits degradation . Minerals are absorbed by microbes and are necessary to their growth . Microbes in turn synthesize many vitamins, such as cyanocobalamin, in great quantities--often great enough to sustain the ruminant even when vitamins are highly deficient in the diet . </P> <P> Microbes in the reticulorumen include bacteria, protozoa, fungi, archaea, and viruses . Bacteria, along with protozoa, are the predominant microbes and by mass account for 40 - 60% of total microbial matter in the rumen . They are categorized into several functional groups, such as fibrolytic, amylolytic, and proteolytic types, which preferentially digest structural carbohydrates, non-structural carbohydrates, and protein, respectively . Protozoa (40 - 60% of microbial mass) derive their nutrients through phagocytosis of other microbes, and degrade and digest feed carbohydrates, especially starch and sugars, and protein . Although protozoa are not essential for rumen functioning, their presence has pronounced effects . Ruminal fungi make up only 5 - 10% of microbes and are absent on diets poor in fibre . Despite their low numbers, the fungi still occupy an important niche in the rumen because they hydrolyse some ester linkages between lignin and hemicellulose or cellulose, and help break down digesta particles . Rumen Archaea, approximately 3% of total microbes, are mostly autotrophic methanogens and produce methane through anaerobic respiration . Most of the hydrogen produced by bacteria, protozoa and fungi is used by these methanogens to reduce carbon dioxide to methane . The maintenance of low partial pressure of hydrogen by methanogens is essential for proper functioning of the rumen . Viruses are present in unknown numbers and do not contribute to any fermentation or respiration activity . However, they do lyse microbes, releasing their contents for other microbes to assimilate and ferment in a process called microbial recycling, although recycling through the predatory activities of protozoa is quantitatively more important . </P> <P> Microbes in the reticulorumen eventually flow out into the omasum and the remainder of the alimentary canal . Under normal fermentation conditions the environment in the reticulorumen is weakly acidic and is populated by microbes that are adapted to a pH between roughly 5.5 and 6.5; since the abomasum is strongly acidic (pH 2 to 4), it acts as a barrier that largely kills reticulorumen flora and fauna as they flow into it . Subsequently, microbial biomass is digested in the small intestine and smaller molecules (mainly amino acids) are absorbed and transported in the portal vein to the liver . The digestion of these microbes in the small intestine is a major source of nutrition, as microbes usually supply some 60 to 90% of the total amount of amino acids absorbed . On starch - poor diets, they also provide the predominant source of glucose absorbed from the small intestinal contents . Under conditions of ruminal acidosis, when the environment of the reticulorumen has become too acidic (usually due to excessive fermentation of starches and sugars into VFA and lactate), microbes that favor a lower pH may start to dominate the ecosystem of the reticulorumen . This gives rise to rumen acidosis and often feed intake of the ruminant will drop . </P>

Where is the rumen located on a cow