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The rumen is the largest compartment of the ruminant digestive system. It serves as a fermentation vat where microbes digest fibrous plant materials. This fermentation produces volatile fatty acids (VFAs) that the animal uses for energy. It also generates gases like carbon dioxide and methane, which the animal expels through belching.
The rumen contains a diverse population of bacteria, protozoa, and fungi. These microbes break down cellulose and hemicellulose, which are difficult for the animal to digest on its own. The rumen's mucosal surface has papillae to increase the surface area for absorption. An efficient rumen is crucial for converting indigestible plant fibers into usable nutrients.

The omasum is the third compartment of the ruminant digestive system. It has a distinctive appearance with many layers of tissue, which look like leaves or pages in a book. This structure allows the omasum to grind down food particles more finely and absorb water and nutrients.
The function of the omasum is essential for water conservation: it significantly reduces the water content of the digestive matter before it proceeds to the next compartment. This helps ruminants thrive in environments where water may be scarce. Furthermore, the omasum helps absorb minerals and VFAs, crucial for the animal’s energy requirements.

The abomasum is often referred to as the 'true stomach' of the ruminant. It functions similarly to the monogastric (single-chambered) stomach found in animals like humans and pigs. The abomasum secretes hydrochloric acid and digestive enzymes like pepsin, which break down proteins and other nutrients from the food.
This compartment is essential for the final stages of enzymatic digestion before nutrients are absorbed in the intestines. The acidic environment of the abomasum also helps kill bacteria and microbial residues from the previous compartments, ensuring that the animal absorbs safe and clean nutrients. This compartment completes the digestive process initiated by the microbes in the rumen and reticulum.

The reticulum is the second compartment of the ruminant digestive system, closely associated with the rumen. The two compartments work together, and the reticulum has a honeycomb structure that traps large particles. These particles need further breakdown and are either sent back to the mouth for re-chewing (cud) or retained for additional fermentation.
This process of regurgitating and re-chewing is known as rumination. It helps break down fibrous plant material more thoroughly, which increases the efficiency of microbial digestion. The reticulum also plays a role in trapping foreign objects that the animal might ingest accidentally, preventing them from causing harm or moving further down the digestive tract.