The rumen, also known as a paunch, is the largest stomach compartment in ruminants and the larger part of the reticulorumen, which is the first chamber in the alimentary canal of ruminant animals. The rumen's microbial favoring environment allows it to serve 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 from it with regard to the texture of its lining.
The rumen is composed of several muscular sacs, the cranial sac, ventral sac, ventral blindsac, and reticulum.
The lining of the rumen wall is covered in small fingerlike projections called papillae, which are flattened, approximately 5mm in length and 3mm wide in cattle. The reticulum is lined with ridges that form a hexagonal honeycomb pattern. The ridges are approximately 0.1–0.2mm wide and are raised 5mm 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.
Digestion in the reticulorumen is a complex process. Digestion occurs through fermentation by microbes in the reticulorumen rather than the animal per se. The reticulorumen is one of the few organs present in animals in which digestion of cellulose and other recalcitrant carbohydrates can proceed to any appreciable degree.
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.