Functions of various parts of the digestive tract: Stomach

The stomach serves as a reservoir for feed where active digestion begins. The structure and function of the stomach vary greatly in ruminants and non-ruminants.
Ruminants (Polygastrics): Compared to non-ruminants, ruminants have three additioonal stomach compartments and are therefore known as compund stomach animals. The different compartments of the uminant stomach are the rumen, reticulum, omasum and abomasum.
Rumen and Reticulum:  The first two parts are closely associated and sometimes reffereed to as the reticulo-rumen. The reticulo-rumen and omasum develop from the distal part of the oesophagus; thus they are also called the "forestomach".
The rumen serves as a big fermentation vessel divided into dorsal and ventral sacs. The reticulo-rumen in adult animals occupies about 50% of the total capacity of the digestive tract and 75% of the abdominal cavity. The internal surface of the reticulo-rumen is non glandular and raised into folds. The reticulum has a honeycomb like structure, while the rumen is internally covered with papillae of varying shapes and sizes. Reticulo-rumen contractions, which increases from 47 - 80 at resting and 79 - 100 during eating, are responsible for the breakdown and mixing of feed particles.
Rumen Contents: The rumen contents have a large amount of water (850 - 930 g/Kg). The proportion of dry matter in the rumen contents varies from 7 - 15% depending on the nature of the diet. The rumen never empties, but the contents become more watery with fasting. In regularly fed animals, the rumen contents can be divided into a liquid phase and particulate phase. The liquid phase occupies the ventral sac, while the particulate phase, having suspended feed particles, is limited to the dorsal sac.
Rumination: As stated earlier, the mixing and breakdown of rumen contents is accomplished by strong ruminal movements. These movements force part of the rumen contents through the oesophagus and back into the mouth. This is called regurgitation. The regurgitated matter in the form of boluses is re-masticated and swallowed, and this process is called rumination. Rumination involves regurgitation, re-mastication, re-salivation, and re-swallowing of ingested feed. It enables the animal to take its feed at one time and chew it slowly later on while resting. Rumination stimulates saliva secretion which is important for proper rumen function. Normally, cattle and buffaloes spend up to one-third of their time (about eight hours) ruminating through out the day. It has been estimateed that one rumination cycle takes about a minute, of which four seconds are used for regurgitating and re-swallowing, and the remaining time is used for re-masticating. The coarseness of feed affects rumination time. Animals with a diet of concentrate have a shorter rumination time (two to three hours) compared to those with a roughage-based diet (more than eight hours).
Rumen micro-organisms: Fermentation in the rumen is a continuous process taking place under anaerobic conditions. Ruminant digestion does not require enzymes produced by the animals but occurrs a result of the combined action of bacteria, protozoa, and fungi. The population density of bacteria inhabiting the rumen is about 10(9) per ml of rumen contents. More than 60 species of rumen bacteria have been identified to date. They can be broadly classified in to cellulolytic (cellulose-digesting), amylolytic (starch-digesting), and proteolytic (protein-digesting). The important rumen bacterial species are listed in Table. These bacteria secrete various enzymes for digestion of feed in the rumen. The relative population of these bacterial species largely depends on the composition of feed. For example, amylolytic bacterial will predominate when animals are consuming a large quantity of cereal grains in the diet. On the other hand, a high roughage diet will support a cellulolytic bacterial population in the rumen. Rumen bacteria are very sensitive to dietary changes, therefore frequent and sudden changes in the diet may disturb rumen functions.
Rumen bacteria, under suitable conditions, multiply very rapidly. They may be present either in the liquid phase (free floating bacteria) or attached to feed particles (adherent bacteria). Free floating bacteria generally live on soluble carbohydrates and protein and can be quickly washed out of the rumen with liquid flowing to the small intestine. In contrast, adherent bacteria are retained in the rumen for a longer time. Microbial cell lysis and multiplications in the rumen is continuous process which maintains their population in a steady state.
Rumen microorganisms serve two main purposes:

  1. They digest feed and convert it to end products, which are easily assimilated by the host animal, and
  2. They serve as a source of high quality protein which leaves the rumen and is digested in the small intestine. 
Rumen protozoa are larger in size but fewer in number (10(6)/ml rumen content) than rumen bacteria. They can be divided into two groups:
  1. Holitrichs, covered with cilia
  2. Entodinomorphs, having no cilia on their body.
Protozoa have the ability to engulf small food particles and rumen bacteria. Their numbers also increase rapidly; however, unlike bacteria, they usually adhere to feed particles and the rumen wall (sequestering), and so do not flow rapidly to small intestine. Although predation of bacteria by protozoa is considered a harmful effect on the bacterial population, protozoa have been shown to complement feed digestion in the rumen. The interaction of bacteria and protozoa is considered a harmful effect on the bacteria population, protozoa have been shown to complement feed digestion in the rumen. The interaction of bacteria and protozoa in the rumen is very complex and a subject of great interest these days. The number and species of protozoa in the rumen depend on the availability of the substrate and conditions prevailing in the rumen. Low rumen pH associated with high grain feeding considerably reduces protozoal population. 
The role of anaerobic fungi in rumen fermentation is not fully understood and is the subject of recent interest. it has been suggested that they play a complementary role in fibre digestion and are therefore considered beneficial in roughage-based diets, particularly tropical forages.
Substrate and fermentation products: Different substrates available for microbial fermentation in the rumen include dietary carbohydrates, proteins and lipids. Carbohydrate sources in ruminant diets include forages (cellulose and hemicellulose), cereal grains and their by-products (starch), and molases (soluble sugars). Leguminous forages (berseem, lucerne, and cow pea, etc.) also serve as a good source of protein. Oilseed cake and oilseed meal are commonly used as protein supplement and in addition, these feeds may contain a substantial amount of lipids. Compared to monogastrics, ruminants have a limited ability to digest lipids, and therefore feeding of large amounts of fats often adversely affect rumen function, particularly fiber digestion. The major end product of microbial fermentation of these substrates in the rumen are volatile fatty acids (VFA's), gases, ammonia, peptides, amino acids and microbial proteins.
Volatile fatty acids and gases: The principle VFA's produced in the rumen are acetics, propionic, and butyric acids. The proportions and concentrations of these VFA's depend on the type of substrate and microbial species in the rumen. Propionic acid, a glycogenic VFA is predominant in forage based diets. the concentration of butyric acid is usually high with readily soluble carbohydrates such as molasses. rumen gases include carbon dioxide, methane and hydrogen. The VFA's and a part of the gases are absorbed through the rumen wall and metabolized. A large part of the gases is lost through eructation.
Ammonia: Ammonia in the rumen is largely produced from protein fermentation. Protein in the diet is hydrolysed by rumen microorganisms via peptides to free amino acids which are further deaminated to produce ammonia. The concentration of ammonia in the rumen depends on the quantity and quality of dietary protein. Highly soluble protein results in a high concentration of ammonia in the rumen. Non-protein nitrogen such as in urea is almost instantly dissolved and converted to ammonia by bacterial ureases. Ammonia is one of the most important nutrients required by rumen bacteria for growth and multiplication. Deficiency of ammonia in the rumen generally depresses microbial growth and activity. Under conditions of ammonia excess, the surplus is lost from the system. However, very high concentrations of ammonia in the rumen may cause ammonia toxicity. Ruminants have the unique capability of recycling a part of the absorbed ammonia through saliva and the rumen wall. This nitrogen conservation system is more efficient when diets are low in protein.

Note: In next post, details about Omasum and Abomasum will be described.