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Anatomy & Physiology- CH 11- The Digestive System

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The Digestive System
AKA: Digestive tract, Gastrointestinal (GI) Tract, Alimentary Canal, or Gut. Tube that runs from the mouth to the anus; accessory digestive organs (pancreas, liver). Responsible for breaking down food into nutrients (digestion) and then absorbing those nutrients into the bloodstream for use by cells.
Species Variation
Requirements for digestion and absorption of foodstuffs vary depending on the diet of the animal.
Species Variation: Herbivores
Plant-eating animals like cattle, sheep, goats, and horses
Species Variation: Carnivores
meat-eating animals like cats
Species Variation: Omnivores
Animals that eat plant material and meat like humans.
Species Variation: Monogastric Animals
Simple, single stomachs
Species Variation: Ruminants
Multiple mixing and fermentation compartments in addition to the stomach
Digestive Tract Functions
1. Prehension (grasping) of food with the lips or teeth. 2. Mastication-mechanical grinding and breaking down of food (chewing). 3. Chemical digestion of food. 4. Absorption of nutrients and water. 5. Elimination of wastes.
Digestive Tract Structure: Wall of the GI Tract- Mucosa
Lining of GI tract; epithelium and loose connective tissue.
Digestive Tract Structure: Wall of the GI Tract- Submucosa
Dense connective tissue; may contain glands.
Digestive Tract Structure: Wall of the GI Tract- Muscle Layer
Outside the submucosa
Digestive Tract Structure: Wall of the GI Tract- Serosa
Outermost layer; thin, tough connective tissue.
Digestive Tract Structure: Mesentery
Sheets of connective tissue. Suspend digestive tube in the abdomen from the dorsal body wall. Contains blood and lymph vessels and nerves that supply the GI tract.
Digestive Tract Epithelium: Stratified Squamous Epithelium
Thick and tough. Lines the mouth, pharynx, esophagus, and anus
Digestive Tract Epithelium: Simple Columnar Epithelium
Runs from junction of esophagus and stomach through the intestines to junction of rectum and anus. Nutrients are absorbed through this thin epithelium.
Digestive Tract Musculature: Skeletal Muscle
Voluntary control. Allows the process of chewing, mixing saliva with food, initiation of swallowing and control of defecation. Mouth, pharynx, the cranial part of the esophagus, and the external anal sphincter.
Digestive Tract Musculature: Smooth Muscle
Wall of the majority of the esophagus, the stomach, the small intestine, the large intestine, and the internal anal sphincter. Contraction of circular muscle fibers narrows the tract. Contraction of longitudinal fibers shortens the tract.
Peristalsis
How things move. Circular muscle contractions. Wavelike movement along the tract. Propels digestive tract contents along the tube ahead of them.
Segmental
How things move. Periodic circular muscle contractions. Occur in different adjacent sites. Mixes digestive tract contents and slows their movement through the tract.
Buccal Cavity
AKA the oral cavity and mouth. Consists of the lips, tongue, teeth, salivary glands, hard palate, soft palate, and oropharynx. Lips may play a role in prehension.
Buccal Cavity: Salivary Glands
Produce salvia (contains enzymes and lubricates food); usually three pairs, with ducts that carry the saliva to the oral cavity.
Buccal Cavity: Salivary Glands- Parotid Salivary Glands
Ventral to the ear canals
Buccal Cavity: Salivary Glands- Mandibular Salivary Glands
Ventral to the parotid glands at eh caudal angle of the mandible.
Buccal Cavity: Salivary Glands- Sublingual Salivary Glands
Medial to the shafts of the mandible just under the base of the tongue.
Teeth: Mastication
Chewing; physically break down food into smaller pieces.
Teeth: Upper Arcade
Contained in maxilla and the incisive bones
Teeth: Lower Arcade
Contained in the mandible.
Teeth Shape: Carnivore Teeth
More pointed on their occlusal surface; slightly curved toward back of mouth. Good for holding prey, tearing, cutting, and shredding.
Teeth Shape: Herbivore Teeth
Have flat occlusal surfaces. Good for grinding plant and grain material.
Types of Teeth: Incisors
Grasping teeth. Most rostral teeth of upper and lower arcade.
Types of Teeth: Canines
Tearing teeth. Located at the corners of the incisors. Longer than other teeth. Pointed at the tip.
Types of Teeth: Premolars
Cutting teeth. Rostral cheek teeth. Sharp points and surface in carnivores. The 4th premolar on the upper arcade in dogs= carnassial tooth.
Types of Teeth: Molars
Grinding teeth. Caudal cheek teeth. Larger, flatter occlusal surfaces. Used for grinding. The 1st molar on the lower arcade in dogs= carnassial tooth.
Teeth Terminology: Lingual
Inner surface of the lower arcade of teeth
Teeth Terminology: Palatal
Inner surface of the upper arcade.
Teeth Terminology: Labial
Outer surface of the upper and lower arcade at the front the mouth
Teeth Terminology: Buccal
Outer surface of the teeth more caudal in the mouth
Dental Formula
Represents the typical number of each type of tooth found in the upper and lower arcade. Tooth type designated I for incisor, C for canine, P for premolar, and M for molar. Upper case for adult teeth lower case for deciduous teeth. Ruminants have no upper incisors or upper canine teeth. Tooth type followed by two numbers separated by a slash mark or expressed as a fraction of one number over the other, First number- number of teeth in half of the upper arcade. Second number- number of teeth in half of the lower arcade. Total number determined by summing all the numbers and multiplying by two.
Dental Pad
Flat thick connective tissue structure on the maxilla opposite the lower incisors and canine teeth
Dental Formula: Canine - Puppy
i3/3 c1/1 p3/3 Total 28 teeth
Dental Formula: Canine- Adult
I3/3 C1/1 P4/4 M2/3 Total 42 Teeth
Dental Formula: Feline- Kitten
i3/3 c1/1 p3/2 Total 26 teeth
Dental Formula: Feline- Adult
I3/3 C1/1 P3/2 M1/1 Total 30 teeth
Dental Formula: Equine- Adult
I3/3 C1/1 P3-4/3 M3/3 Total 40 or 42 teeth
Dental Formula: Porcine- Adult
I3/3 C1/1 P4/4 M3/3 Total 44 teeth
Dental Formula: Bovine- Adult
I0/3 C0/1 P3/3 M3/3 Total 32 teeth
Structure of Teeth: Pulp
Center of tooth. Blood and nerve supply enter at the apex of the tooth root.
Structure of Teeth: Dentin
Surrounds and protects the tooth pulp.
Structure of Teeth: Cementum
Hard connective tissue. Covers tooth root. Helps fasten the tooth securely in its bony socket.
Structure of Teeth: Enamel
Covers the crown of the tooth. Hardest, toughest tissue in the body.
Structure of Teeth: Gingiva
Epithelial tissue that composes the gums around the teeth.
Oral Cavity Functions
1. Prehend the food (take hold of food) 2. Initiate mastication-breaks down food into smaller particles that increase the surface area available for exposure to the enzymes involved in chemical digestion. 3. Initiate chemical digestion- Saliva added to food as it is chewed moistens, softens and shapes food into a form that is more readily swallowed. 4. Prepare food for swallowing.
Digestive Enzymes
Proteins that catalyze chemical reactions that split food molecules into smaller, simpler compounds.
Digestive Enzymes: Amylase
Enzyme in saliva of omnivores. Breaks down amylose (sugar component of starch)
Digestive Enzymes: Lipase
Enzyme that digests lipids. May be found in the saliva of some young animals while they are nursing or on a high-milk diet.
Digestive Enzymes: Sodium Bicarbonate and Phosphate Buffers
Found in saliva of cattle. Neutralize acids normally formed in the rumen and helps maintain pH.
Digestive Enzymes: Autonomic Nervous System
Controls most of the glands in the digestive system
Digestive Enzymes: Parasympathetic Stimulation
Increases salivation. Anticipation of eating can cause parasympathetic stimulation of the salivary glands.
Digestive Enzymes: Sympathetic Nervous System
Fear or parasympathetic nervous system inhibitors like atropine produce dry mouth.
Esophagus
Transports swallowed material from pharynx to stomach. Enters the stomach at an angle in cardia region of the stomach, surrounded by cardiac sphincter muscle. As stomach expands the fold of the stomach against esophagus closes the lower end of esophagus, reduces the risk for reflux. In some species this closure is strong enough to prevent reflux or vomiting (horses and rabbits).
Monogastric Stomach: Five different areas
1. Cardia 2. Fundus 3. Body 4. Pyloric Antrum 5. Pylorus
Monogastric Stomach: Cardia
Opening from the esophagus
Monogastric Stomach: Fundus
Distensible blind pouch; expands as more food is swallowed.
Monogastric Stomach: Body
Distensible middle section. Fundus and Body contain numerous glands. Gastric glands contain: Parietal Cells- produce hydrochloric acid. Chief Cells- produce enzyme pepsinogen. Mucous Cells- produce the protective mucus.
Monogastric Stomach: Pyloric Antrum
Grinds up swallowed food; regulates hydrochloric acid. Glands contains G cells- secrete gastrin. Gastrin stimulates production of hydrochloric acid.
Monogastric Stomach: Pylorus
Muscular sphincter; regulates the movement of chime from the stomach into the duodenum. Prevents backflow of duodenal contents into the stomach.
Gastric Motility
Each area of the stomach has different motor functions. Peristalsis also occurs in stomach and intestines. Stomach contains mucosal layer, submucosa, muscle layer (longitudinal and circular muscle fibers), and outer serosal layer. Smooth muscle in stomach wall responds to hormones, peptides, nervous system controls. Parasympathetic stimulation(via vagus nerve) cause Fundus to relax (so it can fill) and increases contractions in the Antrum(for mixing and peristalsis). Sympathetic Stimulation (stress from surgery or illness) can cause a decrease in motility-gastric atony.
Gastric Motility: Fundus and Body
Relax with swallowing of food to allow stomach to fill with food
Gastric Motility: Body
Contracts to help mix food
Gastric Motility: Pyloric Antrum
Contracts in response to swallowing; stimulates mixing, grinding, and propulsive contractions that move food toward the Pylorus.
Gastric Motility: Gastrin
Produced by the G cells in the Antrum of the stomach. Increases production of hydrochloric acid. Inhibits muscle activity of the Fundus allowing the Fundus to relax and allow filling.
Gastric Motility: Enterogastric Reflex
Distension of the intestines or increased acidity in the duodenum inhibits stomach contractions. Delays gastric emptying.
Gastric Motility: Secretin
Hormone released from duodenum in response to excess stomach acid in small intestine. Can inhibit peristalsis of the body and antrum of the stomach to slow gastric emptying.
Gastric Motility: Cholecystokinin (CCK)
Released in response to large amount of fats or proteins in duodenum; decreases contraction of the antrum, body, and fundus to slow gastric emptying.
Gastric Secretions: Pepsinogen
Secreted by chief cells in the body; precursor for the enzyme Pepsin. Breaks down proteins into chains of amino acids (peptides)
Gastric Secretions: Intrinsic Factor
Secreted from glands in submucosa. In some species, intrinsic factor must combine with vitamin B12 in order for B12 to be absorbed from the small intestine.
Mucus
Complex of substances; provides protective coating for the stomach against acidic gastric environment, must be secreted continuously or ulcers will result.
Mucus: Mucins
Produced by goblet cells in gastric glands; main constituent of he mucous coating.
Mucus: Bicarbonate Ion
Alkalinizes the mucus (neutralizes it)
Hydrochloric Acid
Hydrogen (H+) and Chloride (Cl-) ions- are secreted by parietal cells in the gastric glands. Combine in the stomach to produce hydrochloric acid. Receptors on parietal cells for gastrin, histamine and acetylcholine involved in regulation of H+ and Cl- secretion. Selectively blocking one of these receptors decreases the production of stomach acid.
Prostaglandins (PGS)
Inhibit gastrin release (blocking HCl release). Stimulate the gastric glands to produce the bicarbonate ion (neutralizing the HCl when in comes into contact with mucus). Enhance blood flow to the stomach. Stabilize destructive lysosomes within gastric cells. Regulate the activity of macrophages and mast cells.
Ruminants
One true stomach and three forestomachs. Ruminants swallow their food, regurgitate it to chew on it some more before swallowing it again. (rumination, chewing the cud)
Ruminants: Abomasum
True stomach
Ruminants: Forestomachs
Reticulum, Rumen, Omasum
Reticulum
Smallest, most cranial compartment of the forestomach. Separated from the rumen by the ruminoreticular fold. Lining composed of honeycomb arrangement of folds, increase surface area for absorption. Reticulum and rumen-coordinated contractions. Because it is located ventrally, heavily swallowed objects "fall into" the reticulum and if sharp can pierce the wall= Hardware Disease.
Rumen
Fermentation vat that process plant material. Series of muscular sac partially separated from on another by long muscular folds of rumen wall called Pillars. Pillars aid in mixing and stirring of ruminal contents.
Rumen: Reticuloruminal Contractions
Allow partially digested plant food to be regurgitated. Allow build up carbon dioxide or methane gas to be expelled from the rumen (eructation)
Rumen: Rumen Motility
Controlled by parasympathetic nervous system (vagus nerve). Rate and strength of contraction determined by rumen pH, presence of fatty acids, consistency of the ingesta in the rumen, stretch receptors, and feedback from the brain stem and other parts of the GI tract.
Fermentative Digestion
Rumen bacteria use their surface cellulose enzyme to turn cellulose into simple carbohydrates/sugars; use their proteases to breakdown protein. Carbohydrates and peptides absorbed by microbes, converted to Volatile Fatty Acids (VFAs). Some peptides converted to ammonia (NH3+). VFAs absorbed by ruminant into the bloodstream, converted to back to glucose by the liver, adipose tissue, milk fat, and other components.
Omasum
Reticulorumen contractions move ingesta into the omasum. Muscular organ with many muscular folds. Break down food particles down further. Absorbs VFAs. Removes bicarbonate ions and absorbs some water form the ingesta.
Abomasum
True stomach. Functions much the same as Monogastric stomach.
Newborn Ruminant Digestion
Newborn ruminants digestive tract functions primarily as a Monogastric digestive system. Rumen and reticulum are nonfunctional at birth. Rate of development of the rumen and reticulum affected by type of diet(milk versus grain). Reticular groove (esophageal groove) in wall of reticulum conveys liquid from the esophagus directly to the omasum.
Small Intestine
Majority of absorption happens here. Divided into three sections: Duodenum, Jejunum, and ileum. Inner mucosal layer, submucosal layer, muscular layer, outer serosal layer. Relative thickness of the layers differs in each segment of intestine. Parasympathetic stimulation involved in small intestinal motility, secretions, and blood flow. Sympathetic stimulation decreases blood flow, little effect on motility.
Duodenum
First short segment that leaves the stomach.
Jejunum
Longest portion, Majority of SI
Ileum
Separated from colon by ileocecal sphincter; regulates movement of materials from the small intestine into the colon or the cecum
Small Intestine Mucosa: Mucosa
Many folds and villi for absorption Each villus contains thousands of microvilli (brush border).
Small Intestine Mucosa: Microvilli
Digestive enzymes and carrier molecules embedded in cell membranes.
Small Intestine Mucosa: Crypts
Invaginations of mucosa around each villis. Produce cells that are pushed from the bottom of the crypt up the villus to replace older cells shed at the tip of the villus.
Small Intestine Mucosa: Goblet Cells
Produce mucus. Helps protect the intestinal mucosa from passing ingesta.
Small Intestine Peristalsis
Associated with the coordinated contractions of the longitudinal and circular muscle layers. Stimulated by reflexes in response to dilation of a segment of the bowel. Ileus-decreased movement of ingesta due to decreased peristalsis, can be caused by stress(surgery) or drugs. Cholecystokinin (CCK) and Prostaglandin(PGs) may also stimulate intestinal motility.
Small Intestine Motility: Segmental Contractions
Mix intestinal contents; slow movement through intestines. Help mix digestive enzymes with the intestinal contents. Bring digested materials into contact with the surface of the intestinal tract for absorption.
Small Intestine Digestion
Electrolytes, water and vitamins- absorbed intact across the small intestine wall. Carbohydrates, proteins, and fats- chemically digested. Chemical Digestion involves: 1. Enzymes in the lumen of the intestine. 2. Enzymes associated with the microvilli.
Carbohydrate Digestion
Starch converted into disaccharides into lumen of the duodenum by Pancreatic Amylase. Disaccharides further digested by enzymes in microvilli cell membranes. Resulting Monosaccharides transported across the microvilli cell membrane and absorbed into the blood.
Protein Digestion
Gastric pepsin breaks apart some protein chains into small polypeptides. Five Pancreatic Proteases: Trypsin, Chymotrypsin, Elastase, Aminopeptidase, and Carboxypeptidase- partially digest peptides. Partially digested peptides further digested by peptidases in microvilli cell membranes. Amino acids, dipeptides, and some tripeptides are then absorbed across the cell membrane.
Fat Digestion
Agitation of the pyloric antrum in stomach breaks large fat globules (triglycerides) into small droplets. Bile acids coat the fat droplets in duodenum to prevent them from forming back into large globules, also allows the fat to become water-soluble. Pancreatic lipases penetrate bile acid coating- digest fat molecules to produce glycerol, fatty acids and monoglycerides. Droplets fragment into small particles (micelles). Micelles-bile acid-lipid component absorbed through microvilli
Large Intestine: Components
1. Cecum-blind sac at ileocecal junction
2.Colon-Some microbial digestion
3.Rectum
Large Intestine: Primary Functions
Recover fluid and electrolytes. Store feces until they can be eliminated. and Parasympathetic stimulation causes increased motility and mucus secretion.
Large Intestine: Carnivores
Simple, tubular colon; poorly developed cecum
Large Intestine: Nonruminant Herbivores
Very large colon and cecum (hindgut). Fermentation Site.
Hindgut Digestion
Equine, Guinea pigs, rats, rabbits. Modifications of cecum and colon allow fermentative digestion in hindgut similar to rumen. VFAs produced by microbes absorbed from the cecum and colon for energy need just like they are from the rumen in ruminants. Acids buffered by secretion of bicarbonate directly into the colon and cecum.
Recum
Terminal portion of large intestine, acts as reservoir for feces. Nervous system control of motility and secretions is similar to that of the colon. Numerous mucus-secreting glands lubricate and aid the passage of contents. Sensory receptors detect stretching and stimulates the defecation response.
Anus
Composed of internal and external muscular sphincters, internal sphincter is under autonomic control, external sphincter is under voluntary control. As the rectum distends stretch receptors in rectum wall cause partial relaxation of the internal sphincter(parasympathetic stimulation). This allows feces to move into the internal sphincter canal and contact the anal mucosa. Anal mucosal receptors increase the sense or need for defecation.
Digestive System Functions of Liver
Removes/filters toxins, infectious agents and so forth that enter the body through wall of the GI tract. The Portal blood vessels transport blood from the SI capillaries to the hepatic capillaries. Stores or metabolizes nutirents absorbed from the GI tract. Stores glucose in the form of oxygen. When needed it is broken down in a process called glycogenolysis and glucose is then released into the blood.
Digestive System Functions of Liver: Bile
Produces. Contains bile acids, cholesterol, bilirubin. Secreted into bile ducts then on to hepatic ducts then to gallbladder for storage. Horses do not have a gallbladder. Stimulation by CCK causes gallbladder contraction. Forces bile down the common bile duct into the duodenum.
Digestive System Functions of Liver: Albumin
Main source for albumin. Blood protein that maintains proper fluid balance in the blood. Responsible for oncotic pressure which keeps fluid in the blood vessels.
Digestive System Functions of Pancreas
Production of pancreatic amylase, proteases, and lipase(exocrine function). Secretes bicarbonate into the duodenum, helps neutralize acidity of contents and maintains the pH in the duodenum needed for proper enzyme function. Produces insulin and glucagon(endocrine function), help regulate blood glucose levels. Beta cells produce insulin in response to elevated blood glucose. Insulin moves glucose from the blood into cells. Lack of insulin or response to insulin resulting in hyperglycemia = diabetes mellitus.
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