Rumen, Reticulum, Omasum & Abomasum

The primary difference between a ruminant and non-ruminant (called monogastrics, such as humans, dogs, and pigs) is that ruminants have a four-compartment stomach. 

The four parts of the stomach are rumen, reticulum, omasum, and abomasum. 

In the first two chambers, the rumen and the reticulum, the food is mixed with saliva and separates into layers of solid and liquid material. Solids clump together to form the cud or bolus. The cud is then regurgitated and chewed to completely mix it with saliva and to break down the particle size. 

Fiber, especially cellulose and hemi-cellulose, is primarily broken down into the three volatile fatty acids (VFAs), acetic acid, propanoic acid and beta-hydroxybutyric acid, in these chambers by microbes (mostly bacteria and well as some protozoa, fungi and yeast). 

Protein and non-structural carbohydrate (pectin, sugars, starches) are also fermented. Even though the rumen and reticulum have different names they represent the same functional space as digesta can move back and forth between them. Together these chambers are called the reticulorumen. 

The degraded digesta, which is now in the lower liquid part of the reticulorumen, then passes into the next chamber, the omasum, where water and many of the inorganic mineral elements are absorbed into the blood stream. After this the digesta is moved to the true stomach, the abomasum. 

The abomasum is the direct equivalent of the monogastric stomach (for example that of the human or pig), and digesta is digested here in much the same way. Digesta is finally moved into the small intestine, where the digestion and absorption of nutrients occurs. Microbes produced in the reticulorumen are also digested in the small intestine. Fermentation continues in the large intestine in the same way as in the reticulorumen. 

Only small amounts of glucose are absorbed from dietary carbohydrates. Most dietary carbohydrates are fermented into VFAs in the rumen. The glucose needed as energy for the brain and for lactose and milk fat in milk production, as well as other uses, comes from non-sugar sources such as the VFA propionate, glycerol, lactate and protein. 

The VFA propionate is used for around 70% of the glucose and glycogen produced and protein for another 20% (50% under starvation conditions).