Digestive System

The process through which large feed particles are broken down into smaller units that are absorbed and undigested material is excreted out of the body is called digestion. The process of digestion is carried out through digestive system. There are two types of animals on the basis of variations in structure and function of digestive system i.e. ruminants and non ruminants. Ruminants which are also called polygastrics and include cattle, buffalo, sheep and goats while non ruminants which are also called monogastrics and include horses, donkeys, rabbits, dogs and cats. The key difference between digestive systems of ruminants and non ruminants is the structure of stomach. Non ruminants have simple stomach while the stomach of ruminants consists of four compartments i.e. rumen, reticulum, omasum and abomasum.

The digestive system of ruminants consists of following organs: 

• Mouth – with teeth, tongue and pharynx  

• Esophagus – a muscular tube extending from the back of the mouth to the stomach

• Stomach – consists of rumen, reticulum, omasum and abomasums

• Small intestine – consists of duodenum, jejunum and ileum

• Large intestine – consists of caecum, colon and rectum

• Anus

As accessory organs or glands, salivary glands, liver, gall bladder and pancreas also take part in the process of digestion. The process of digestion includes prehension, deglutition, grinding or mastication, digestion of feed, absorption of nutrients and excretion of waste products.

Prehension:
First step in digestion is prehension. It is the act of bringing food into mouth. Tongue is main prehensile organ in ruminants which also plays role in the selection of feed with the help of taste buds.

Mastication:
Mastication is the act of chewing feed which involves the physical grinding and tearing of the feed with the help of teeth in addition to the admixture of saliva. Saliva lubricates the feed as well as initiates a limited amount of enzymatic digestion. Feed after mastication is formed into a small compact ball called bolus which is passed into digestive tract.

Deglutition:
Deglutition is the act of swallowing in which bolus is lifted by the tongue and moved to the back of the mouth. The bolus passes through the pharynx during which respiration is temporarily inhibited by the reflex closure of larynx and then passes down the esophagus to the stomach through peristaltic movements. Peristaltic movements consist of alternate relaxation and contraction of muscles in the wall of esophagus.

Role of saliva:
Saliva is released by salivary glands. There are three pairs of salivary glands:

●  Parotid Glands

●  Submaxillary Glands  

●  Sublingual Glands

Salivary secretions act as an aid in mastication, formation of bolus, and swallowing. It provides a means for recycling the nutrients back to the rumen. It contains considerable amounts of urea, mucin, phosphorus, magnesium and chloride all of which can be used by bacteria and protozoa in the rumen. It, acting as a surfactant, helps to prevent the problem of gas accumulation in the rumen and development of serious bloat. It also solublizes several of the chemicals in the feed and thus helps their detection by the taste buds. It provides moisture to keep the membranes in the mouth moist and thus viable. The most important for the lactiating dairy animal perhaps is the large quantity of sodium and other cations that are secreted in saliva, thus serving as a buffer in the ingesta. The buffering capacity of saliva is critical since over 170 literes of saliva can be secreted by cattle daily into the rumen.

Digestion in Stomach:
From mouth bolus enters stomach through esophagus where active digestion starts. The stomach of ruminants is a complex structure consisting of four morphologically distinct compartments; rumen, reticulum, omasum and abomasum. 

Rumen & Reticulum- The rumen and reticulum are collectively called fore stomach as they are closely related in physiological function. The rumen is a large physiological fermentation vat which does not secrete enzymes but causes the mixing and breakdown of feed particles through strong movements or contractions. The feed in rumen move back to the mouth due to these movements which is called regurgitation. The regurgitated matter in the form of bolus is remasticated and again swallowed; this process is called rumination. Due to rumination animal becomes able to take feed at one time and chew it slowly later on while resting.

In rumen the process of digestion occurs in an anaerobic environment with the help of rumen microorganisms. Such type of digestion is termed as fermentative digestion. Carbohydrates in feed are digested by rumen microbes and as a result carbon dioxide and volatile fatty acids are produced. Among several volatile fatty acids (VFAs) the primary VFAs are acetic acid, propionic acid and butyric acid. VFAs are absorbed directly from rumen and supply much of the energy required by the animal. These are also used in the synthesis of milk fat in lactating animals.  Protein digested by ruminal microbes is converted into ammonia which is utilized by microorganisms for their growth. These microorganisms are a major source of dietary protein for ruminants as they passes down the abomasums and digested there. These microorganisms have also the ability to utilize non protein nitrogen source like urea because they can convert them into ammonia with the help of urease enzyme. The protein which is degraded by ruminal microbes is called ruminal degradable protein. The protein which is not degraded by ruminants and escape rumen is called ruminal undegradable protein or bypass protein which is digested in abomasum. Vitamin B complex is also synthesized by ruminal microbes.

Sometimes nails, tones, and various foreign objects along with the bolus of food enter rumen. The churning movement of the rumen causes these heavy objects to be driven to the front portion of reticulum. The wall of reticulum is also non secretary like that of rumen as it does not secrete any enzyme. Its functions are to assist the passing of bolus up the esophagus and to regulate passage of the food from rumen to the omasum and from the rumen to the esophagus.

Omasum & Abomasum – From rumen and reticulum bolus enters omasum where absorption of water occurs and the size of feed particles is reduced. From omasum food is passed on to the abomasum which has the similar function as that of the stomach of the non ruminants. It is the only glandular part of the ruminant stomach means it secretes enzymes which play role in the process of digestion. The secretion of abomasums is called gastric juice which contains water, hydrochloric acid, mucus, intrinsic factord, pepsinogen, and rennin. Gastric juice has low pH as 2 or less which is protective for animal as most of the foreign microbes ingested with food cannot survive in such an acidic environment. The low pH is also essential for the functioning of enzymes in abomasum like pepsinogen is an inactivated form of enzyme pepsin which is activated by low pH. Pepsin plays its role in the digestion of protein due to which protein is broken down into simpler compounds mainly peptides which are short chains of amino acids as pepsin cannot cause the complete digestion of protein into amino acids. Rennin is another enzyme which coagulate milk and reduce its rate of passage through gastrointestinal tract which is important in young calves.

Digestion in Small Intestine:
The small intestine is divided into three portions which include duodenum, jejunum and ileum. The first portion of small intestine is duodenum which originates at the pyloric sphincter of the stomach. Both the secretion of liver which is called bile and the secretion of pancreas which is called pancreatic juice are released in this portion. The next portion is jejunum and last is ileum. Throughout the surface of the small intestine there are fingerlike projections which are called villi which play their role in absorption. When feed enters small intestine from abomasums, it is called chyme which is semi digested feed and further digested and absorbed here through the action of various enzymes. These enzymes are secreted by intestinal glands and also supplied by pancreatic juice (secretion of pancreas). Maltase, amyase, lactase and sucrase are the major enzymes involved in the digestion of carbohydrates. Trypsin, chymotrypsin and peptidases take part in the digestion of protein. Functions of digestive enzymes are given in table below:.

Role of Accessory Organs in Digestion:
The pancreas, a glandular structure, plays its role in digestion by the secretion of pancreatic juice in small intestine through pancreatic duct. Pancreatic juice primarily consists of a variety of digestive enzymes and sodium bicarbonate. Sodium bicarbonate raises the pH of chyme because small intestinal epithelium is not protected against acidic solution. The higher pH is also better for the action of pancreatic digestive enzymes. There are three major groups of pancreatic enzymes including proteases (proteolytic enzymes), amylases (amylolytic enzymes) and lipases (lipolytic enzymes). Proteolytic enzymes include trypsin and chymotrypsin which are secreted as inactive form, trypsinogen and chymotrypsinogen and activated by enterokinase enzyme. These two enzymes cause the digestion of protein and convert it to the peptides which are further hydrolyzed completely into amino acids for hydrolysis by peptidases secreted from the cells of small intestine. Amylase is secreted in its active form and digests starches to oligosaccharides which are further digested to monosaccharides by the action of maltase and sucrase enzymes secreted by small intestine. Lactase enzyme secreted by small intestine of young animals digests lactose. Lipase hydrolyzes triglycerides into fatty acids and glycerol. This action is mostly effective after the fats have been emulsified by bile.

The liver is the largest gland in the body and the secretion of it is called bile which is stored in gall bladder and released per requirement. Bile causes the emulsification of large globules of fats entering into small intestine due to which pancreatic lipases become able to hydrolyze them. Bile contains calcium and potassium salts of glycocholic and taruocholic acids which are required for maintaining alkaline pH and emulsification of fats. Bile facilitates the solublization and absorption of dietary fats and also aids in excretion of certain waste products such as cholesterol and the by products of hemoglobin breakdown.

Role of Large Intestine in Digestion:
After digestion and absorption in small intestine digesta enters large intestine. Large intestine is a major site for the absorption of water and salts like sodium and chloride and consists of three parts as caecum, colon, and rectum. Some absorption of VFAs occurs in caecum while considerable amounts of water and electrolytes are absorbed in colon. The last part of large intestine is rectum which is a dilatable tube serving as a storage place for feaces until it is excreted out through anus.  The feaces consist of the undigested residue of the feed, the remains of the digestive secretions, waste material resulting from wear and tear of the digestive tract, certain excretory products and the bacterial flora.

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