A typical gastrointestinal system is composed of four basic regions:
a. Receptive region
b. Storage region
c. Digestive and absorptive region
d. Excretive and water absorptive region
a)The receptive region is basically the mouth of the animal. It is the organ of prehension, mastication, insalivation and rumination (in case of ruminants). Prehension (food gathering) is assisted by the rough tongue and teeth of a ruminant. It is followed by a preliminary chewing called mastication and mixing of the saliva. Enormous amounts of saliva (100-200 liters per day in cow) are secreted by the three pairs of glands located in the mouth region:
a. The parotid gland – lies below the ear and behind the border of the lower jaw.
b. Submaxillary gland – lies within the angle of the lower jaw, ventral to the parotid.
c. Sublingual gland – lies at the side of the root of the tongue.
The salivary glands are composed of collections of secreting acini (minute sacs) held together loosely by a certain amount of connective tissue. The do not possess a distinct capsule.
-Prehension is the act of gathering food and directing it into the GI tract. To ingest food, quadruped animals must first grasp it with their lips, teeth or tongue. This involves highly coordinated activity of small voluntary skeletal muscles of the face, lips and tongue. The exact method of food prehension varies greatly among different species. For example, horses use their lips extensively, whereas cattle often use their tongues for grasping food. In all domestic animals however, prehension is a highly coordinated process involving direct control by the central nervous system.
-The teeth serve as organs of prehension and mastication and are used also as weapons of offence and defense. According to their form and location in the mouth, the teeth are classified as incisors, canines, premolars and molars. Premolars and molars are also called cheek teeth. The incisors are situated in the front. The incisors are prehensive organs in all animals. They are absent from the upper jaw of the ruminant animals. In the lower jaw there are 8 incisors. In the non-ruminants, canines are behind the incisors and are typical of carnivorous and omnivorous animals and are used mainly for fighting purposes. In ruminants, the upper jaw has no canines, and in the lower jaw they have moved forward, assuming the function and shape of the incisors. Cheek teeth are 24 in number, 6 on each side of each jaw. The anterior 3 of the 6 are premolars and the posterior 3 are the molars.
-Saliva is an aqueous solution of mucin, a mucopolysaccharide. The primary function of saliva is the lubrication of feed in order to facilitate swallowing. Depending on species, saliva may have antibacterial, digestive, and evaporative cooling functions. Lysozyme is an enzyme in saliva that has antibacterial properties. It aids in keeping in check the large thriving population of bacteria in the mouth. Animals without salivary function are prone to infectious diseases of the oral cavity. Omnivorous animals such as rats and pigs contain a starch-digesting enzyme – salivary amylase. This enzyme is absent from canine saliva e.g. dog and cat. Some species of animals have a fat-digesting lingual lipase, usually in young animals such as calves which are on milk diet. The enzyme disappears as the animal mature. Ruminant saliva, compared to canine saliva, has a relatively higher concentration of bicarbonate and phosphate ions and a higher pH. This well-buffered solution is necessary to neutralize acids formed by fermentation in the rumen.
-The esophagus is the tube that connects mouth to the stomach. It has no digestive function and may thus be considered as part of the receptive region. Its peristaltic contractions are responsible for bolus movement to the stomach.
The stomach is basically the storage region, although some digestive processes take place in the stomach. Ruminants have one stomach, divided into several compartments. True ruminants, including sheep, cattle, goats, deer and antelope have four compartments (i.e. rumen, reticulum, omasum, abomasums). Pseudo-ruminants – camels, ilamas and related species have a 3-compartmented stomach, the omasum being absent. Non-ruminants like canines, felines, equines and swine have one-compartmented stomach (simple stomach).
In simple stomached mammals like horse, pig and dog gastric juice is secreted from the digestive glands when food enters the stomach. The juice contains pepsin, an endopeptidase which acts on protein constituents. In suckling animals, the juice contains rennin (chymosin) which is also an endopeptidase. Endopeptidases break proteins at internal points along the amino acid chains, resulting to the production of short chain peptides from complex proteins. Pepsin and chymosin are luminal phase enzymes involved in protein digestion in the stomach. Gastric hydrolysis of protein is important to the physical as well as chemical digestion of protein because most connective tissue of animal origin is protein. Digestion of connective tissue helps to break food down to particles small enough to pass the pylorus. Although stomach action is important in initiating protein digestion, it is not essential; animals without stomachs can digest protein, provided they have a functional pancreas and are fed small, frequent meals of soft, moist food. Luminal-phase digestion of protein is completed in the small intestine by the action of pancreatic enzymes.
The luminal-phase pancreatic enzymes which are involved in protein digestion in the small intestine include the following: trypsin, chymotrypsin, elastase, carboxypeptidase A and carboxypeptidase B. The latter two are exopeptidases, meaning they release individual amino acids from the ends of the peptide chains. The complete digestion of peptides to free amino acids takes place in the membranous-phase by protein-digesting peptidases which are present on the surface membrane of the small intestine.
The small intestine is divided into three parts: duodenum, jejunum and ileum. The duodenum is the first part and ducts from the pancreas and liver open into it. The jejunum is indistinctly separated from the duodenum and is continuous with the ileum. The ileum, which is the last part, enters the large intestine at the ileo-ceco-colic junction. The small intestine is the chief site of absorption in most of the domestic animals. Its mucous membrane consists of numerous finger-like projections known as villi. Each villus is further surrounded by innumerable finger-like projections known as microvilli for even greater surface area for absorption. The rhythmic contractions and pendulum movements of villi is controlled by a hormone villikinin which thus aids in absorption. The duodenum receives both bile from gall-bladder and pancreatic secretions from the pancreas via a duct which at the point of entry into the duodenum is common to both organs.
The pancreatic enzyme alpha-amylase is involved in luminal-phase digestion of starch in the small intestine. It breaks down long starch chains into short chains known as dextrins. The chains continue to be attacked until di- and trisaccharide units are formed: maltose and malto-triose. The digestion of maltose and malto-triose is completed in the membranous phase by enzymes (maltase) that is chemically-bound to epithelium of small intestine. Glucose is the end-product of starch digestion. For sucrose and lactose, their digestion takes place in the membranous-phase through the actions of the enzymes sucrase and lactase . Glucose and fructose are the end-products of sucrose digestion while glucose and galactose are the end-products of lactose digestion.
The pancreatic juice also contains the fat-splitting enzyme lipase which acts on tiny globules of fat which have been emulsified by bile to produce glycerol and fatty acids. Fats are absorbed by lymph vessels which occupy the center of intestinal villi. They are then passed over to blood stream. Sugars , salts and soluble proteins pass directly into small blood vessels of intestinal walls and are carried to liver and then to general circulation.
marto answered the question on April 16, 2019 at 12:07