Digestive Tract Physiology PDF

LESSON 2 DIGESTIVE TRACT PHYSIOLOGY There are several variations in digestive systems that limit what an animal can or cannot use for nutrition. Many of the organic components of food are in the form of large insoluble molec...

LESSON 2 DIGESTIVE TRACT PHYSIOLOGY There are several variations in digestive systems that limit what an animal can or cannot use for nutrition. Many of the organic components of food are in the form of large insoluble molecules, which have to be broken down into simpler compounds before they can pass through the mucous membrane of the alimentary canal into the blood and lymph. The breaking down process is termed ‘digestion’, and the passage of the digested nutrients through the mucous membrane is called ‘absorption’. The processes important in digestion may be grouped into mechanical, chemical and microbial activities. The mechanical activities are mastication and the muscular contractions of the alimentary canal. The main chemical action is brought about by enzymes secreted by the animal in the various digestive juices, though it is possible that plant enzymes present in unprocessed foods may in some instances play a minor role in food digestion. Microbial digestion of food, also enzymic, is brought about by the action of bacteria, protozoa and fungi, microorganisms that are of special significance in ruminant digestion. In monogastric animals, microbial activity occurs mainly in the large intestine, although there is a low level of activity in the crop of birds and the stomach and small intestine of pigs. DIGESTIVE TRACT PHYSIOLOGY  Nutritional requirement of animals and their ability to utilize feedstuffs are greatly dependent on their digestive tract anatomy and physiology.  Livestock can be divided into three groups according to their digestive tracts: simple non-ruminants, ruminants, and non-ruminant herbivores. Simple Non-Ruminants  Include swine, poultry, dogs, cats, rats, and humans  Often referred to as MONOGASTRIC animals - They have pouch-like, non-compartmentalized stomach - Does not depend much on microbial digestion in any part of the gut but instead, relies on chemical digestion of food thru secretion of digestive enzymes in the gut. Functions of Each Major Segment: 1. Oral cavity  Physical digestion occurs thru mastication of food to smaller particles to increase the surface area for exposure to digestive enzymes.  Starch is hydrolyzed to maltose by enzymatic digestion of salivary amylase 2. Stomach  Stores ingested feed and meters ingesta into small intestine in amounts that intestinal digestion can accommodate.  HCL secreted into the stomach kills most of the bacteria ingested with feed. It also hydrolyzes proteins due to its acidic pH and activates pepsin.   In nursing animals, RENIN is secreted to coagulate CASEIN (a milk protein) to prevent rapid passage of milk out of the stomach, therefore promoting proteolytic digestion of milk. 3. Small intestines  Major site of digestion and absorption in simple non-ruminants  Consists of three segments: duodenum, jejunum (major site of nutrient absorption) and ileum  Pancreas serves as a major source of digestive enzymes that degrade the following: - CHONS: trypsin, chymotrypsin - CHO: amylase - LIPIDS: lipase  It also secretes buffers (bicarbonates) that neutralize stomach acid Primary Enzymes of the Digestive Tract Food Source (substrate) Enzyme Origin Product of Digestion Carbohydrates  Starch, glycogen, dextrin Amylase Saliva, pancreas Maltose, glucose  Maltose Maltase Small intestine Glucose  Lactose Lactase Small intestine Glucose, galactose  Sucrose Sucrase Small intestine Glucose, fructose Fats and Oils  Lipids Lipase Gastric mucosa, Monoglycerides Pancreas Glycerol, fatty acids Proteins  Milk proteins Rennin Gastric mucosa Coagulates milk (young calf) proteins  Proteins Pepsin Gastric mucosa Polypeptides  Protein breakdown products Trypsin Pancreas Peptides,proteoses Chymotyrpsin Pancreas Peptides Carboxypeptidase Pancreas Peptides,amino acids Aminopeptidase Small intestine Peptides,amino acids Dipeptidase Small intestine Amino acids Nucleotidase Small intestine Nucleotides, Nucleosides Nucleosidase Small intestine Purines, Phosphoric acid 4. Large intestines  A complex ecosystem of anaerobic microorganisms further degrades and metabolizes undigested residues entering the large intestines  Microbial fermentation of carbohydrates results in the production of volatile fatty acids (VFAs), carbon dioxide, and methane.   Microbial fermentation of proteins and amino acids, on the other hand, releases ammonia that is absorbed via portal blood and converted to urea in the liver. - Tryptophan is metabolized to indole and skatole, which gives the characteristic odor of the feces.  Hindgut (cecum, colon, and rectum) in simple non-ruminants absorb water, ferments bacteria and form feces.  Bacterial growth in the hindgut results in the synthesis of B-complex vitamins. Species Differences in Specific Aspects of Gastrointestinal Anatomy and Physiology  Strict carnivores (minks, cats) have a short intestinal tract and rapid transit time of ingesta through the GIT because of the higher digestibility if their meat-based diet.  Omnivores (swine) tend to have a long small intestine and an enlarges hindgut with a much more significant microbial population of some fiber digestion. AVIAN DIGESTIVE SYSTEM Functions of each major segment: 1. Mouth  Beaks replace lips and teeth with its shape, size and length varying with type of diet consumed. - Seed crackers have short conical beaks while birds of prey have powerful beaks for tearing fish.  Tongue is also adapted to type of food consumed. - Long and narrow tongue (woodpeckers) functions as a spear to extract insects from the holes they drill. - Short, thick and fleshy tongues (birds of prey) allow manipulation of food. 2. Foregut (crop, proventriculus, gizzard)  Gizzard (muscular part) performs function of mammalian teeth. The churning action of the gizzard and grit (small stones) it contains grind feed into smaller particles.  Proventriculus (glandular part) is where HLC and digestive enzymes are secreted. RUMINANTS  Differs from a simple non-ruminant because it has a large, compartmentalized stomach  Microbes that inhabit the large stomach mostly accomplish digestion, rather than enzyme the animal produces itself. Functions of each segment: 1. Mouth and teeth   Absence of upper incisions, instead dental pads are present. A large gap between the incisors of molars allow cattle to harvest and chew a large amount of fibrous feeds.  Teeth are primarily for grinding and tongue is used to gather and grasp grasses.  Saliva contains sodium bicarbonate to keep rumen at a proper neutral pH for good bacterial growth. 2. Stomach  Rumen - A fermentation vat that contains an immense microbial population of bacteria (protozoa, fungi and yeast) that ferment the ingested feeds. - Major source of energy are the fermentation end products, also known as VFAs (acetic, butyric, and propionic acids). - Also produces large quantities of gasses mainly carbon dioxide and methane, which are removed by a process called ERCUTATION.  Failure to eructate normally causes BLOAT  This condition is caused by rapid change in feed, overeating of grains (gaseous bloat) and grazing on pastures on pure stands of clover of alfalfa (frothy bloat).  Rumination or “chewing the cud” is a characteristic feature of all ruminants. It is where they REGURGITATE a bolus of incompletely chewed feed and REMASTICATING it into smaller particles to facilitate fermentation before REDEGLUTITION.  Reticulum - Traps foreign materials such as nails, wire and stones, which accumulate in this compartment that eventually punctures the small intestine (HARDWARE DISEASE). - As a preventive measures/treatment, a magnet is administered orally to trap and hold metallic objects.  Omasum - Regulates flow of ingested by acting as a sieve or filter thru its membranous leaves. - Ingested feeds must be thoroughly degraded to smaller particles in order to pass thru the omasal leaves thereby promoting maximum fermentation efficiency.  Abomasum - Site of secretion of gastric juices such as HCl and proteolytic enzymes. - Acidity in this compartment allows digestion of microbial protein, which serves as the major source of amino acids for ruminants.   Ruminant stomach has profound implications in animal production and feeding. - Fermentation allows utilization of fibrous feeds because rumen microbes secrete CELLULASE, which degrades cellulose, a major constituent of plant fiber. Because of this, ruminants can utilize roughages, forages and other fibrous feeds, in-contrast to non-ruminants.  Ruminants can be fed poor quality proteins and still be upgraded by rumen fermentation to microbial protein.  Rumen microbes can also synthesize vitamins, specifically the water-soluble vitamins K. Vitamin D is also synthesized when the animal is exposed to sunlight.  During the early months, the compartments of the ruminant stomach not yet fully developed. Milk rots instead of being fermented because of the absence of microbial flora in the rumen. However, in the young animal, the suckling causes a reflex closure of muscular folds preventing milk to be directed to the other compartments but instead directly to the abomasum, thru the reticular/esophageal groove. Once inside the abomasum, rennin curdles milk and is eventually digested enzymatically. Non-Ruminant Herbivores  Include horses, rabbit, guinea pig, zebra, elephant and hippopotamus.  They are intermediate between simple non-ruminants and ruminants in their digestive physiology and nutrition requirements.  Can be further subdivided into three groups: foregut fermenters, colon fermenters and cecal fermenters. Foregut fermenters  No important domesticated species of non-ruminant foregut fermenters. Colon fermenters  Equids are examples  Enlarged colon is the site of microbial fermentation of materials that resists breakdown in small intestine, primarily fibers.  High-quality proteins, starch and lipid are digested in the small intestine, as they are simple non-ruminants.  Compared to rumen fermentation, hindgut fermentation is less efficient as they only get what the animal cannot digest, so there is less favorable nutritional environment for them.  Fiber digestion is less efficient, as can be seen in horse manure where much fibrous material is still present.  The animal inefficiently utilizes bacterial proteins from digestion/metabolism of microbes because there is no significant absorption in the large intestines.  However, they are able to subsist in a low-quality, high fiber diet than do ruminants because there is no OMASUM that restricts particle size of feeds. Therefore, they are able to obtain sufficient nutrients to survive. Cecal fermenters  Example is rabbit  They have low digestibility of fiber so they can adapt by selectively separating and excreting indigestible fiber and retaining the more digestible non-fiber contents for fermentation in the cecum.  Small herbivores produce two types of feces: hard and soft. - Hard feces, or fecal pellets, consist largely of indigestible fiber that is rapidly eliminated in the body. - Soft feces, or cecotropes, are actually cecal contents (non-fiber components and fluids that are fermented in the cecum). The animal utilizes cecotropes because of its nutritional value, in a process called COPROPHAGY. - It provides bacterial proteins and vitamins synthesized in the cecum.