The rumen forms the larger part of the reticulorumen, which is the first chamber in the alimentary canal of ruminant animals. It serves as the primary site for microbial fermentation of ingested feed. The smaller part of the reticulorumen is the reticulum, which is fully continuous with the rumen, but differs to it with regard to the texture of its lining.

The lining of the rumen wall is covered in small finger like projections called papillae, which are flattened, approximately 5 mm in length and 3 mm wide in cattle. The reticulum is lined with ridges that form a hexagonal honeycomb pattern. The ridges are approximately 0.1 - 0.2 mm wide and are raised 5 mm above the reticulum wall. The hexagons in the reticulum are approximately 2-5 cm wide in cattle. These features increase the surface area of the reticulorumen wall, facilitating the absorption of volatile fatty acids. Despite the differences in the texture of the lining of the two parts of the reticulorumen, it represents one functional space.

After being swallowed, ingesta travels down the oesophagus and is deposited in the dorsal part of the reticulum. Contractions of the reticulorumen mix the recently ingested feed with the other contents of the reticulorumen. Within the reticulorumen, the uppermost area, the headspace, is filled with gasses (such as methane, carbon dioxide, and, to a much lower degree, molecular hydrogen) that are the result of the fermentation and anaerobic respiration of feed. These gasses are regularly expelled from the reticulorumen through the mouth, in a process called eructation. Below the gasses is a mat of partially degraded long fibrous material that is liable to be regurgitated and chewed again. This process is called rumination and reduces the particle size of digesta. The digesta in the mat is subject to great amounts of fermentation. As fermentation proceeds, digesta particles first increase in buoyancy due to entrapment of fermentation gases produced from digesta then decrease in buoyancy as fermentable substrate is depleted and gas production is lowered. Below the rumen mat, the particles gradually become smaller in size and less buoyant, and the digesta more liquid as the process of microbial degradation of the feed becomes more complete. Once the particles are sufficiently small (<1 mm in size) and dense (>1.2 g/ml), the digesta is swept out of the reticulorumen with liquid through the reticulo-omasal oriface, which leads to the next chamber in the ruminant animal's alimentary canal, the omasum. The reticulo-omasal orifice is located close to the oesophageal entrance to the reticulorumen (approximately 20-30 cm away in adult cattle).

Protein, non-structural carbohydrates (including starch, sugar, and pectin), and structural carbohydrates (including hemicellulose and cellulose) are hydrolyzed and fermented by ruminal microbes for the duration of their residence in the reticulorumen. Protein is hydrolyzed to peptides and amino acids and fermented to ammonia, and these are used as a source of amino nitrogen by the microbes in the reticulorumen so that they can reproduce to create more microbes. Peptides, amino acids, ammonia, and other sources of nitrogen present originally in the feed can also be utilized directly by microbes with little to no hydrolysis. In cases in which nitrogen for microbial growth is in excess, protein and its derivatives can also be fermented to produce energy. Carbohydrates are fermented to the volatile fatty acids (VFAs) acetate, propionate, butyrate, and other branched-chain VFAs via glycolysis and other biochemical pathways, which allows the microbes to capture energy. These VFAs are mostly absorbed across the reticulorumen wall, directly into the blood stream, and are used by the ruminant as a supply of energy. Some branch chained VFAs are incorporated into the lipid membrane of rumen microbes. Lipids are hydrogenated and glycerol, if present in the lipid, is fermented, but lipids are otherwise inert in the rumen. High levels of lipid in the rumen are thought to poison microbes and reduce fermentation activity. Lignin, a phenolic compound, can be digested to only a small degree and only by fungi. Microbes in the reticulorumen eventually flow out into the omasum and the remainder of the alimentary canal, where they are digested and absorbed by the ruminant.

Microbes in the reticulorumen include bacteria, protozoa, fungi, archea, 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, amolytic, and proteolytic bacteria, which preferentially digest structual carbohydrates, non-structural carbohydrates, and protein, respectively. Protozoa (40-60% of microbial mass) mainly prey on bacteria and other protozoa via phagocytosis, though some varieties they also can use carbohydrates as a food source. Ruminal fungi make up only 5-20% of microbes, but their capability to degrade recalcitrant lignin gives them an important niche in the rumen despite their low numbers. Rumen archea, whose numbers are not well-defined, are mostly methanogens and produce methane through anaerobic respiration. 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.

Digestive system

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