Lesson 2: Digestive Anatomy & Physiology PDF

DIGESTION VS. ABSORPTION  DIGESTION MECHANICAL CHEMICAL MICROBIAL Mastication Enzymes secreted Action of bacteria, Muscular in various protozoa, and contractions of digestive juices fungi the alimentary canal C...

DIGESTION VS. ABSORPTION  DIGESTION MECHANICAL CHEMICAL MICROBIAL Mastication Enzymes secreted Action of bacteria, Muscular in various protozoa, and contractions of digestive juices fungi the alimentary canal CLASSIFICATION OF DIGESTIVE SYSTEM Based on the Anatomy of the Digestive Tract I. Non-Ruminant Monogastric Simple stomach, non- Dogs, cats, minks, functional cecum pigs, catfish Functional Nonruminant Horse and rabbits cecum herbivore Based on the Anatomy of the Digestive Tract II. Ruminant Complex stomach Cattle, goats, sheep, buffaloes, camels Based on the Anatomy of the Digestive Tract III. Avian Chickens, ducks, turkeys According to their Primary Type of Feed and their Feeding Behavior I. Carnivores or the flesh eaters Dogs and cats Short and simple organization of GIT According to their Primary Type of Feed and their Feeding Behavior II. Herbivores or the vegetarians Cattle and horse Eat only plant materials which are not readily digestible Have long and large GIT with complex and modified parts According to their Primary Type of Feed and their Feeding Behavior III. Omnivores or flesh and plant eats Pig and man Have GIT intermediate of those of carnivores and herbivores anatomically and efficiency in utilizing fibrous plants According to their Primary Type of Feed and their Feeding Behavior IV. Granivores or the grain feeders Birds Large proportion of their feeds consist of plant seeds or grains/cereals having very high energy content No teeth, but have unique specialized GIT to digest grains Based on the Location of Sites of Microbial Fermentation I. Pregastric, foregut fermenters Ruminants Predominant site of microbial fermentation preceding the stomach (gastric pouch) and small intestine Hindgut fermentation also occurs Baed on the Location of Sites of Microbial Fermentation II. Hindgut fermenters Non-ruminant herbivores Predominant site of microbial fermentation following the SI Elephant, rhinoceros, horses, and rabbits Baed on the Location of Sites of Microbial Fermentation III. Monogastric, non-functional cecum Carnivores Have no major specialized site of microbial fermentation Some microbial fermentation occurs in colon DIGESTIVE TRACT OF MONOGASTRIC ANIMALS The Alimentary Canal Function: prehension, ingestion, comminution, digestion, absorption, and elimination. DIGESTIVE TRACT OF MONOGASTRIC ANIMALS DIGESTION IN THE MOUTH PREHENSION AND CHEWING Prehension – the act of bringing food into the mouth. Mastication – chewing Carnivorous animals Simple teeth and tear their food but do little grinding Herbivorous animals Hypsodont teeth; upper jaw is wider than the lower jaw; chewing of food is thorough. DIGESTION IN THE MOUTH PREHENSION AND CHEWING Mastication can be controlled voluntarily, but the presence of food in the mouth will stimulate reflex chewing. DIGESTION IN THE MOUTH SALIVA AND SALIVARY GLANDS Saliva consists of water, electrolytes, mucus, and enzymes. Water and mucus soften and lubricate the ingesta to facilitate mastication and swallowing. Lysozyme – salivary enzyme with antibacterial actions. Amylase- starch-digesting enzyme Present in saliva of pigs Limited degree in horses Absent in ruminants and dogs – found in pancreas DIGESTION IN THE MOUTH SALIVA AND SALIVARY GLANDS Saliva consists of water, electrolytes, mucus, and enzymes. Water and mucus soften and lubricate the ingesta to facilitate mastication and swallowing. Lysozyme – salivary enzyme with antibacterial actions. Amylase- starch-digesting enzyme Present in saliva of pigs Limited degree in horses Dogs- pancreatic amylase Absent in ruminants and dogs Ruminant- microbial population in rumen degrade the starch DIGESTION IN THE MOUTH SALIVA AND SALIVARY GLANDS Adult cattle may secrete up to 200L of saliva per day as compared to 1-2L per day for humans. This large volume maintains the fluid consistency of the rumen contents and prevent frothing of the rumen fluid. DIGESTION IN THE MOUTH SALIVA AND SALIVARY GLANDS Parasympathetic nerves control salivary secretion as part of neural reflexes. Saliva production is stimulated by signals like the sight and smell of food, presence of food in the mouth, and conditioned reflexes. DIGESTION IN THE MOUTH SWALLOWING Deglutition- the act of swallowing. Divided into three stages First stage Passage of food or water through the mouth Second stage Passage through the pharynx Third stage Passage through the esophagus into the stomach DIGESTION IN THE MOUTH SWALLOWING First stage: swallowing is under voluntary control. Chewed food mixes with saliva to form a bolus (rounded mass of food) The base of the tongue acts as a plunger, forcing the bolus into the pharynx. DIGESTION IN THE MOUTH SWALLOWING As the bolus enters the pharynx, pressure receptors in the walls are stimulated. This triggers the SECOND STAGE OF SWALLOWING: Bolus passing through the pharynx The pharynx shortens, and a peristaltic action of the pharyngeal muscles pushes the bolus into the esophagus. DIGESTION IN THE MOUTH SWALLOWING THIRD STAGE OF SWALLOWING: involves reflex peristalsis in the esophagus, triggered by the presence of food. Peristalsis is the alternating relaxation and contraction of circular muscle rings, along with regional contraction of longitudinal muscles around the bolus.  VOMITING Vomiting (emesis) is a protective reflex to expel harmful contents from the stomach and upper small intestine. Drugs that activate this center to induce vomiting are called emetics. This reflex is controlled by a vomiting center in the BRAINSTEM. The chemoreceptor trigger zone (CTZ) receives a stimulus that might warrant vomiting. The vomiting center begins a choreographed sequence of actions. MONOGASTRIC DIGESTION IN THE STOMACH (MONOGASTRIC) Simple compartment ~ digestion and storage Esophageal area no glands a-amylase activity may continue Active microbial population Cardia Secretes alkaline, enzyme-free, viscous mucus formed of a gel- forming glycoprotein that protects epithelium from acid attack Gastric glands Secretes glycoprotein and fucolipid mucus and contains oxyntic cells = produce HCL Produces pepsinogen Pyloric Region Has glands like cardia region that secrete protective mucus  Gastric Juice Water Pepsinogens Inorganic salts Mucus Hydrochloric acid Intrinsic factor for absorption of Vit B12 Stimulation of glands to secrete gastric juice The sight, smell, or taste of food triggers the brain to send Cephalic phase signals via the vagus nerve, increasing stomach secretions. Food entering the stomach stimulates proteins to release Gastric phase hormones (gastrin and histamine), boosting HCl production. As food moves to the small intestine, hormones like Intestinal phase cholecystokinin, GIP, and secretin reduce stomach activity and acid production. REGULATION OF HYDROCHLORIC ACID (HCL) SECRETION The production of HCl is carefully regulated to balance digestion and protection: Gastrin and Histamine Acetylcholine Coordination Secreted by cells in the A neurotransmitter from All three signals- stomach lining. the vagus nerve gastrin, histamine, and ACTH Stimulate parietal cells to Promotes HCl Production Must be present for produce HCL. optimal HCl secretion DIGESTION IN THE SMALL INTESTINE Majority of digestion and absorption occurs in the small intestine Duodenal area - site for mixing digesta and secretions Jejunal area - site of absorption  Duodenal (Brunner’s) gland = produce an alkaline secretion that acts as a lubricant and protects duodenal wall from the acidic HCL entering from the stomach.  EMULSIFICATION OF FATS & OILS Bile salts play an important part in digestion by activating pancreatic lipase and emulsifying fats.  Pancreatic lipase is responsible for breaking down fats. Ø Works more efficiently on smaller fat droplets Ø Emulsification facilitated by bile salts aids in fat breakdown. DIGESTION IN THE LARGE INTESTINE Plays an important role in the retrieval of nutrients, electrolytes, and water in the digesta. Mucosal surface does not have villi but has small projections that increase surface area.  In the ascending colon, fluid and fine particles are retained for a longer period, allowing for the absorption of water and electrolytes. In contrast, coarser particles move more quickly toward the descending colon, where they are eventually excreted. This process helps concentrate the waste and form solid stool. DIGESTION IN THE LARGE INTESTINE Large intestine primarily contains mucous glands ~ do not produce enzymes Microbial activity aids in digestion in LI Digestive processes in LI rely on enzymes that have been brought down with food from upper part DIGESTION IN THE LARGE INTESTINE Extensive microbial activity occurs Abundant nutrient sources encourage growth of bacteria Complex population of aerobic and obligate anaerobic bacteria DIGESTION IN THE LARGE INTESTINE Certain substances like cellulose, hemicelluloses, lignin, and raw starches resist enzymatic digestion. Lignin is indigestible and may trap proteins and carbohydrates. DIGESTION IN THE LARGE INTESTINE Vitamin Synthesis: Bacteria synthesize some B vitamins, but these are insufficient to meet daily requirements, necessitating dietary supplementation. Waste Formation: Feces include water, undigested residues, secretions, sloughed epithelial cells, inorganic salts, bacteria, and microbial byproducts.  DIGESTION IN YOUNG PIG From birth until about 5 weeks of age the concentration and activity of digestive secretions are different from adult animals. DIGESTION IN YOUNG PIG During the first few days after birth – intestine is permeable to native proteins Essential for the transfer of y- globulins (antibodies) via mother’s milk DIGESTION IN YOUNG PIG Early on, the stomach produces limited hydrochloric acid and pepsinogen but secretes CHYMOSIN. DIGESTION IN YOUNG PIG Chymosin Clots milk thereby avoiding flooding the small intestine with nutrients. As piglet develops – pepsinogen and HCL secretion increases. DIGESTION IN YOUNG PIG ENZYME CHANGES High at birth, helps digest milk, then Lactase decreases after a few weeks. Maltase and Start working better after about 4 weeks Sucrase Low at birth, improves around 4 weeks to Amylase digest starches DIGESTION IN YOUNG PIG WEANING DIETS Diets need milk-based lactose because piglets Early Weaning (14 days) can’t digest complex carbs well Cooked cereals can be added as digestion Later Weaning (3-4 weeks) improves. This avoids undigested starch causing diarrhea DIGESTION IN YOUNG PIG WEANING DIETS Early weaning diets use lactose- rich milk products, High level of EARLY WEANING avoiding raw lactase BUT low (14 DAYS) starches that amylase piglets can't digest, to prevent fermentation and diarrhea. DIGESTION IN YOUNG PIG WEANING DIETS Cooked cereals can be added to the diet as cooking makes starch easier to digest, preventing issues Maltase and in the large LATE WEANING intestine. amylase increase. (3-4 Weeks) Lactase decline DIGESTION IN DOG AND CAT The saliva of dogs and cats has no -amylase activity, as the natural diet is low in starch. Stomach secretes gastric lipase and pepsin Pepsin is most active when animal ingested collagen ~ important in digestion of meat; more important in cat. DIGESTION IN DOG AND CAT In dogs, gastric juice has antibacterial activity Similar to pig, young dogs and cats are less efficient in digesting solid food than older animals. PRE-CAECAL DIGESTION IN HORSE The horse is a non-ruminant herbivore, having a simple monogastric digestive tract similar to that of the pig but with a much enlarged hind gut (especially the caecum), which contains a microbial population. PRE-CAECAL DIGESTION IN HORSE Small intestine is the main site for digestion of non-fibrous carbohydrate, protein, and fat. Microbes in large intestine ferment fibrous materials. PRE-CAECAL DIGESTION IN HORSE Horses digest food enzymatically first in the stomach and small intestine, breaking down simple carbohydrates, proteins, and fats. PRE-CAECAL DIGESTION IN HORSE Microbial fermentation occurs later in the large intestine (cecum and colon), where fiber is broken down into volatile fatty acids (VFAs). PRE-CAECAL DIGESTION IN HORSE Absence of Gall Bladder Horses lack a gall bladder and cannot store bile HCL acid in the duodenum stimulates bile secretion from the liver. RUMINANT DIGESTION OF RUMINANT Stomach of ruminant is divided into four compartments. 1. Rumen 2. Reticulum 3. Omasum 4. Abomasum DIGESTION OF RUMINANT In young animals, milk bypasses the undeveloped rumen and reticulum via the oesophageal groove to reach the omasum and abomasum. DIGESTION OF RUMINANT RUMEN DEVELOPMENT Fibrous foods (e.g., hay) stimulate rumen growth Volatile fatty acids (VFAs) from fermentation, especially butyric acid = promote papillae development for nutrient absorption. DIGESTION OF RUMINANT VFAs are absorbed through the rumen wall, while microbes and undigested food move to the abomasum and small intestine for enzymatic digestion. DIGESTION OF RUMINANT RUMINATION CYCLE 1. Regurgitation 2. Remastication 3. Reinsalivation 4. Redeglution DIGESTION OF RUMINANT In grazing cattle it is commonly about 8 hours per day, or about equal to the time spent in grazing. DIGESTION OF RUMINANT Each bolus of food regurgitated is chewed 40– 50 times and thus receives a much more thorough mastication than during eating. DIGESTION OF RUMINANT CARBOHYDRATE BREAKDOWN The rumen's digestion of carbohydrates involves two main stages: CONVERSION TO SIMPLE SUGARS FERMENTATION Complex carbohydrates like cellulose are Microbes metabolize these sugars into broken down by microbial enzymes into volatile fatty acids (VFAs) such as acetate, simpler sugars. propionate, and butyrate, along with gases like These sugars are rarely detected in the methane and carbon dioxide. rumen because microbes immediately use VFAs are absorbed as an energy source, while them. gases are released through belching. CARBOHYDRATE BREAKDOWN CARBOHYDRATE BREAKDOWN CARBOHYDRATE BREAKDOWN Acidosis CARBOHYDRATE BREAKDOWN DIGESTION OF RUMINANT CARBOHYDRATE BREAKDOWN Diet Influence: High-fiber diets produce more acetate, while high- concentrate diets increase propionate levels but may risk acidosis due to lactate buildup. Methane Production: Methane is formed during fermentation, representing about 7% of the energy loss in ruminants. DIGESTION OF RUMINANT CARBOHYDRATE BREAKDOWN Gas Issues: If gas builds up, it can cause bloat, a potentially fatal condition managed by antifoaming agents. Cellulose Digestion: Lignin content in plants affects cellulose breakdown, with young plants being more digestible than older ones. A low rumen pH from rapid fermentation can inhibit cellulose digestion. DIGESTION OF RUMINANT PROTEIN BREAKDOWN Rumen Degradable Protein Rumen Undegradable Protein (RDP) (RUP or Bypass) Rumen microbes break down Dietary protein not broken down in protein into small peptides, rumen amino acids, and ammonia. Proteins not formed through microbial Microbes use these products to fermentation produce microbial proteins in Beneficial when animals requirement the rumen. of proteins/amino acids is not being met – dairy cows. PROTEIN BREAKDOWN Protein Ammonia conc. Breakdown of carbohydrate will be retarded Slow growth of rumen organisms  When protein degrades faster than it is synthesized, excess ammonia builds up in the rumen. This ammonia enters the blood, is converted to urea in the liver, and mostly excreted in urine, leading to waste. Some urea may return to the rumen via saliva or the rumen wall. Excess urea intake can rapidly increase rumen ammonia levels, potentially overwhelming the liver's capacity to convert it into urea, leading to toxic ammonia levels in the bloodstream. Urea Poisoning DIGESTION OF RUMINANT LIPID BREAKDOWN Bacterial lipases hydrolyze lipids into free fatty acids. Unsaturated fatty acids undergo biohydrogenation: Convert to monoenoic acids and eventually to stearic acid. Includes intermediate trans fatty acid formation. Rumen microbes synthesize unusual fatty acids (e.g., branched chains). DIGESTION OF RUMINANT LIPID BREAKDOWN Capacity Limitations: Typical ruminant diets contain ~50 g/kg lipids. Lipid levels >100 g/kg: Reduce microbial activity and fiber Fats can coat the feed particles, blocking microbial access and fermentation. interfering with their activity. Decrease food intake. When fermentation slows down, rumen motility and digestion are disrupted. DIGESTION OF RUMINANT LIPID BREAKDOWN Fatty Acid Absorption: Absorbed directly from the rumen into the Short-chain fatty acids bloodstream Cannot be absorbed in the rumen because their size and structure make them unsuitable for this process. Long-chain fatty acids These fatty acids are primarily absorbed in the small intestine. AVIAN DIGESTION OF AVIAN DIGESTION OF AVIAN Lips and cheeks are replaced by beak Teeth being absent Taste buds located on the back half of tongue DIGESTION OF AVIAN Diverticulum of the esophagus Reservoir for holding food Filled and emptied by peristalsis DIGESTION OF AVIAN Microbial activity occurs during storage of food Lactobacilli predominate adhering to the crop wall DIGESTION OF AVIAN Glandular stomach Produces HCL and pepsinogen Minimal inherent motility  DIGESTION OF AVIAN Muscular organ with internal ridges Grinds the food with moisture = smooth paste Wall produces KOILIN – hardens in the presence of HCL.  DIGESTION OF AVIAN Pancreatic juices of fowls contains same enzymes as the mammalian secretion Digestion in SI is similar to that in pig DIGESTION OF AVIAN Intestinal mucosa produces mucin, a- amylase, maltase, sucrase, and proteolytic enzymes. DIGESTION OF AVIAN Chicks have maltase and sucrase activities in their SI – perform well on diets containing uncooked cereals. Satisfactory amylase activity DIGESTION OF AVIAN Function as absorptive organs but are not essential in fowls DIGESTION OF AVIAN Transport of digesta to its termination at the cloaca