Ruminant Gastro Intestinal Tract’s Peculiarity and Salient Features
In Veterinary science, there is one of the classification of animals on the basis of their types of the stomach:
Simple stomach Animals (non-ruminants) i.e., Horse, Dog, Pig, etc. and
Compound stomach Animals (ruminants) i.e., Cattle, Buffalo, Sheep, Goat.
Ruminant GIT includes;
Mouth
Ruminant animals have no upper incisor or canine teeth. They depend on the rigid dental pad in front of the hard palate, the lower incisor teeth, the lips, and the tongue to take food into their mouth.
Esophagus (Accessory parts of GIT)
This is a tubelike passage from the mouth to the stomach. The esophagus, which opens into the stomach at the junction of the rumen and reticulum, helps transport both gases and cud.
Volume(gm)
Volume(%) Large ruminants Small ruminants
Rumen+Reticulum 80-85 60-100 9-18
Omasum 5-8 6-10 1-2
Abomasum 5-8 5-8 2
Full capacity of the reticulorumen is only used in animals fed low quality roughages. Only 60 to 70% of the total capacity is used in animals fed high quality roughages.
Capacity in large ruminants(in liter)
Rumen : 184 liter
Reticulum : 16 liter
Omasum : 8 liter
Abomasum : 27 liter
Small intestine : 76 liter
Large intestine : 33 liter
Capacity in small ruminants(in liter)
Rumen : 11-22 liter
Reticulum : 1-2 liter
Omasum : 1 liter
Abomasum : 4 liter
Small intestine : 9-10 liter
Large intestine : 4-6 liter
Structures within reticulorumen
Folds
Areas of tissue dividing the reticulum and rumen into different compartments
Functions: Mixing and sorting particle
Prevent fluid from reaching the cardia during eructation
Pillars
Highly muscled areas of the rumen that form grooves on the outside of the rumen
Contains blood vessels, lymph, and nerves
Functions: Contractions
Rumen(interior view)
Papillae
Finger-like structures (10mm x 2 mm) covering the rumen wall
Particularly well-developed in ventral portion of the rumen
Few present on pillars in roughage-selecting species, but more evenly distributed across rumen in concentrate-selecting species
Function: Increase absorptive surface for VFAs.
Reticulum
The reticulum is cranial to the rumen at ribs 6-8.
It is located from cardia to the diaphragm.
It lies above the xiphoid process of the sternum.
This compartment is attached to the rumen and has a honeycomb structure to its wall.
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.
The main function of this compartment is to trap large feed particles.
First to make sure they don’t enter the next section of the stomach before they are digested and second to collect for additional rumination.
These particles are then regurgitated, rechewed, salvia added and then reswallowed (otherwise known as chewing their cud).
Saliva is important to the rumen as it functions as a buffer.
A cow will produce 160-180 liter of saliva per day.
This system has allowed cattle to eat forage rapidly and then store later for digestion.
Salient features of Reticulorumen
As there is no sphincter between the rumen and the reticulum and they function to a large extent as a single organ, they are usually considered together.
Feed, after being chewed during eating, enters the reticulorumen where it is subjected to microbial attack and to the mixing and propulsive forces generated by coordinated contractions of the reticulorumen musculature.
Musculature activity is coordinated not only to mix the digesta but also to allow the removal of fermentation gases by eructation, the regurgitation of digesta for rumination, which is largely responsible for the physical breakdown of digesta particles and the passage of digesta out of the reticulorumen through the reticulo-omasal orifice.
The microbial activity in the reticulorumen gives the host the ability to eat and utilize forages.
Most of the material digested in the rumen yields short-chain fatty acids, known as Volatile Fatty Acids (VFA), which are absorbed through the rumen wall.
Acetic acid is produced in the greatest quantities, around 20-50 moles per day in dairy cows, while propionic acid is usually produced at about one-third of the rate of acetic acid.
Butyric acid accounts for around 10% of the total acid production, while Valeric and Isovaleric acids each form about 1% to 2%.
Omasum
The omasum is right of the midline.
The Rumen and reticulum are located to the left and the liver and body wall to the right.
The omasum is covered by lesser omentum and is bilaterally flattened. It is located at ribs 8-11.
The lower pole has an extensive connection to the fundic region of the abomasum.
The omasal canal is formed between the end of the laminae and the omasal groove, part of the gastric groove between the cardia and the pyloric sphincter.
The omasal canal passes between the reticulum and the abomasum.
Its mucosa is smooth except for particularly large papillae around the reticulo-omasal opening.
The omaso-abomasal opening is a large and oval, partly covered by overhanging abomasal folds.
Functions:
Filter large particles
VFA absorption
43 to 77% of VFAs entering or 10% of total produced
85% of VFAs produced are absorbed before abomasum
Exchanged for Cl-
Improves efficiency of absorption
Prevents buffering of the abomasum
VFAs are strong buffers with a pK of 4.6
Abomasum functions at pH 2
Water absorption
30 to 60% of water entering
Magnesium absorption
Species difference:
Small Ruminant: The small ruminants have a smaller omasum, which is bean shaped.
Large Ruminant: The lower pole of the omasum contacts the abdominal floor below the costal arch.
Abomasum
Structure
The abomasum lies upon the abdominal floor. The cranial part is split into the pylorus and body. There is also a caudal part. It is covered by the lesser omentum. It has 15-20 folds inside.
The torus is at the pyloric exit. The outflow is fairly constant. There is motility at the pylorus (peristalsis) and some control at the pyloric sphincter.
The abomasum is large in newborn animals. The proximal ends of the abomasal folds form a plug preventing reflux into the omasum. It has thin walls and a serosa covering.
Function
The function of the abomasum is the chemical breakdown of food.
It secretes hydrochloric acid and pepsinogen. It has some intrinsic motility.
Impaired motility can cause distension.
The movements are slow, contractions occur first in the proximal part and are more forceful at the pyloric part.
Salient features of Omasum and Abomasum:
Omasum: Digesta pass from the reticulum to the omasum via a sphincter, the reticuloomasal orifice. The omasum is filled with about 100 tissue leaves (the laminae), which almost completely fill the lumen.
Abomasum: From the omasum, digesta pass to the abomasum, the compartment equivalent to the monogastric stomach.
As in monogastrics, acid and enzymes are secreted in the abomasum and are mixed with the digesta by the muscular activity of the organ.
However, whereas in monogastric animals there is a circadian rhythm (a period or cycle of 24 hrs) in this activity associated with the feeding pattern, abomasal motor activity exhibits an ultradian rhythm (a recurrent period or cycle repeated throughout a 24-hour circadian day) as a consequence of the relatively continuous passage of digesta from the reticulorumen.
Distension of the abomasum inhibits reticulorumen emptying but is the main stimulus for emptying of the abomasum.
Small intestine
The increases in digesta flow that occur with increasing intake are the result of increases in the amount of digesta propelled per contraction rather than in the number of contractions.
Digestion in the small intestine is similar to that in simple-stomached animals.
Large intestine
The flow of digesta to the caecum and proximal colon from the ileum is intermittent and can be followed by periods of quiescence, which may range from 30 min to 5 hrs.
Digesta in the caecum and proximal colon are subjected to both peristaltic and antiperistaltic contractions so that digesta are mixed as well as being moved towards the distal colon.
There is further VFA production and absorption in the large intestine but its main function is probably the absorption of water.
The flow of digesta through the distal colon differs between Sheep and Cattle.
