EAT! ENJOY!“We must have pie. Stress cannot

exist in the presence of a pie.”David Mamet

The Digestive SystemChapter 11

Takes complex food and breaks them down into simple nutrient molecules through process of digestion.

After digestion takes place, nutrient molecules are absorbed into the bloodstream for use by the body’s cells.

Consists of a tube that runs from the mouth to the anus and accessory digestive organs that aid in the process of digestion and absorption.◦ Known as digestive tract, Gastrointestinal (GI) tract,

the alimentary canal, gastroenteric tract, or gut.

Introduction to the Gastrointestinal Tract

The lumen is the opening in the middle of the intestinal tract or any hollow organ.

Anything in the lumen is considered outside the body.◦ What must occur for things to enter the

body?

Lumen of the Gastrointestinal Tract

The requirements for digestion and absorption of foodstuffs vary considerably depending on the diet of the animal.

Each type of animal has different mechanisms to handle digestion and absorption.◦ Herbivores

Plant eating animals

◦ Carnivores Meat eating animals

◦ Omnivores Animals that eat both plants and meat.

Animal Diets

Monogastric◦ Animals that have simple, single stomachs.

Complex Stomachs◦ Animals that have fermentation and mixing

compartments in addition to the stomach.( Ex: ruminants)

Classification of the Stomach

1. Prehension (grasping) of food with the lips or teeth

2. Mastication - mechanical grinding and breaking down of food (chewing)

3. Chemical digestion of food4. Absorption of nutrients and water5. Elimination of wastes

If any function fails, malnutrition may result. ◦ Usually failure is followed by clinical signs.

Examples of clinical signs?

Functions of the GI tract

Gastro- refers to the stomach.

Entero- refers to the intestine.

GI Terminology

Made of multiple layers. The mucosa-the lining layer and consists of

the lining epithelium and some loose connective tissue

The submucosa-beneath the mucosa and contains glands and dense connective tissue.

The thick muscle layer is external to the submucosa.◦ What type of muscle is this?

The serosa- is the outermost layer that consists of a thin, tough layer of connective tissue.

GI tract structure

GI Tract Structure

Mesentery - Sheets of connective tissue◦ Suspends digestive tube from dorsal body

wall ◦ Contains blood vessels, lymph vessels, and

nerves that supply GI tract

Digestive Tract Structure

2 types of epithelium are found in the GI tract:◦ Stratified Squamous

Thick and tough. Lines mouth, pharynx, esophagus, and anus.

◦ Simple Columnar Change form stratified squamous occurs where

esophagus enters stomach. Allows for better absorption of nutrients.

Epithelium of the Digestive Tract

SIMPLE COLUMNAR EPITHELIUMW/GOBLET CELLS AND MICROVILLI

STRATIFIEDSQUAMOUS EPITHELIUM

2 types◦ Skeletal Muscle

Under voluntary control Mouth, pharynx, cranial esophagus, external anal

sphincter. Allows process of chewing, mixing saliva with food,

and swallowing to be conscious acts. Controls defecation.

◦ Smooth Muscle Present in remainder portion of digestive tract. Arranged in circular and longitudinal layers.

Circular Layer- narrows segment of tube. Longitudinal Layer- shortens segment of tube.

Muscle of the GI tract

GI Tract Structure

Food is moved and mixed by two types of muscle contractions.◦ Peristaltic contractions

Move contents along digestive tract.

Circular muscle contractions that move food along in waves.

Propels food ahead of contractions.

How food is moved through the GI tract

http://www.youtube.com/watch?v=h0E9ITyRlh0

◦ Segmental contractions Cause back and forth mixing

movements of the digestive tract contents.

Consist of periodic, circular muscle contractions that occur in different adjacent sites.

Aid in digestion and absorption by mixing the digestive tract contents and slowing their movement through the tract.

How food is moved through the GI tract

Also called the buccal cavity. Where food is initially taken in and where

digestion actually begins. Structures include:

◦ Lips Prehensile organ Labia- term for referring to the lips.

◦ Tongue◦ Teeth◦ Salivary glands

Produce saliva, which performs digestive and lubrication functions.

Also involved in evaporative cooling.◦ Hard palate◦ Soft palate◦ Oropharynx

Mouth/Oral Cavity

Most animals have 3 matching pairs of glands.◦ Parotid salivary glands

Located just ventral to ear canals◦ Mandibular salivary glands

Located ventral to parotid glands at the caudal angle of the mandible

◦ Sublingual salivary glands Located medial to the shafts of the mandible just

under the base of the tongue. All have ducts that carry saliva to oral

cavity. Salivary glands are controlled by

autonomic nervous system◦ What happens during “fight or flight”?

Salivary glands

Parotid salivary glandsMandibular salivary

glandsSublingual salivary

glandsBuccal salivary glands

Responsible for breaking down food into smaller pieces by process of mastication.

Increases surface area of the food that is exposed to digestive processes.

Maxilla contains upper arcade

Mandible contains lower arcade.

Teeth

Lingual (tongue)- inner surface of lower arcade

Palatal (hard palate)- inner surface of upper arcade

Labial (lips)- outer surface of upper/lower arcades (rostral)

Buccal (cheek) - outer surface of teeth (caudal)

Occlusal – surface that grinds with other teeth

Terminology of Teeth Surfaces

Carnivore teeth - pointed on occlusal surface; slightly curved toward back of mouth◦ Good for holding prey, tearing, cutting, shredding

Herbivore teeth - flat occlusal surfaces◦ Good for grinding plant and grain material

Shapes of Teeth

Tooth Structure

Incisors ◦ Grasping teeth◦ Most rostral teeth of upper

and lower arcade Canines

◦ Tearing teeth◦ Located at corners of incisors ◦ Longer than other teeth ◦ Pointed at tip

Premolars◦ Cutting teeth ◦ Rostral cheek teeth◦ Sharp points and surfaces

in carnivores Molars

◦ Grinding teeth◦ Caudal cheek teeth ◦ Larger, flatter occlusal surfaces ◦ Used for grinding

Types of Teeth

Typical number of each type of tooth found in upper/lower arcades

Tooth type designated by the following letters:◦ I=incisor ◦ C=canine ◦ P= premolar ◦ M=molar

Upper case: adult teeth Lower case: deciduous teeth

Ruminants have no upper incisors or canine teeth. Instead have a dental pad- a flat, connective tissue structure of the maxilla opposite the lower incisors and canine teeth.

Dental Formula

Species Dental FormulaTotal Canine - puppy i3/3 c1/1 p3/3

28 Canine - adult I3/3 C1/1 P4/4 M2/3 42 Feline - kitten i3/3 c1/1 p3/2 26 Feline - adult I3/3 C1/1 P3/2 M1/1 30 Equine - adult I3/3 C1/1 P3-4/3 M3/3 40 or 42 Porcine - adult I3/3 C1/1 P4/4 M3/3 44 Bovine - adult I0/3 C0/1 P3/3 M3/3 32

Dental Formulas of Common species

The fourth premolar is also referred to as the carnassial tooth.◦ If this tooth abscesses may cause drainage through

the skin under the eye.◦ Because of root system and length of roots, removal

is hard work.

Teeth

CARNASSIAL TOOTH ROOT ABSCESS

Living structures. Have nerve, blood

vessel and lymph supply.

Are susceptible to damage and pain.

Consist of:◦ Apex◦ Pulp◦ Dentin◦ Cementum◦ Enamel◦ Gingiva

Structure of Teeth

Apex◦ the pointed part at the tip of the root◦ Where blood and nerve supply enter the

tooth Pulp

◦ The center of the tooth Dentin

◦ Surrounds the tooth pulp.◦ More dense than bone but less dense than

enamel. Cementum

◦ Hard connective tissue that helps to fasten tooth in bony socket.

Enamel◦ Hardest, toughest tissue in body. ◦ Outer cover of crown of tooth

Gingiva◦ Epithelial tissue that forms the gums

around the teeth

Teeth Structure

The scaling away of tartar from the teeth.

Horses get teeth “floated”-this reduces points on buccal edge of teeth.

Dental Prophylaxis

Prehension of food

Initiate mastication◦Also referred to as mechanical digestion

Initiate chemical digestion◦Saliva has enzymes that aid in breakdown

of food.

Prepare food for swallowing.

Functions of the Oral Cavity

Proteins that promote the chemical reactions that split complex food molecules up into simpler compounds.

Secreted in the digestive system to break down different components of food.

Two common Digestive enzymes:◦ Amylase

Found in saliva of omnivores but absent in carnivores

Breaks down amylose- a sugar component of starch.

◦ Lipase Digests lipids Found in saliva of young animals while nursing

or on high milk diet.

Digestive Enzymes

Cattle use sodium bicarbonate and phosphate buffers in saliva to neutralize acids that form in rumen.

Cattle can produce 25 to 50 gallons of saliva a day.

Digestive Enzymes

Tube that extends from pharynx to the stomach.

Function is to take swallowed material to the stomach.

No digestion takes place here, it serves only for transport.

Enters the cardia of the stomach. The cardiac sphincter surrounds the cardia. Esophagus enters stomach at an angle.

◦ As the stomach fills with food, the folds in the stomach can close off against esophagus decreasing chance of reflux.

Esophagus

Divided into five different areas.◦ 1. Cardia◦ 2. Fundus◦ 3. Body◦ 4. Pyloric antrum◦ 5. Pylorus

Monogastric (Simple) Stomach

Cardia◦ Where esophagus enters stomach.◦ Helps reduce reflux

Fundus & Body◦ Forms a pouch that can distend as more food is

swallowed◦ Rich with glands

Pyloric Antrum◦ Distal part of the stomach that grinds food.◦ Regulates hydrochloric acid

Pylorus◦ Sphincter that regulates movement ofdigested

stomach contents (Chyme) from stomach to duodenum (first part of intestine).

◦ Prevents backflow of duodenal contents into the stomach.

Stomach

Contain 3 key cells:◦ Parietal cells (oxyntic cells)

Produce hydrochloric acid (HCl)

◦ Chief cells Produce pepsinogen, precursor to pepsin

◦ Mucous cells Produce mucus that is protective in nature.

Gastric Glands

G- cells◦ Endocrine cells

◦ Secretes gastrin hormone

◦ Presence of food stimulates G-cells to dump gastrin into the blood.

Pyloric Antrum glands

C-shaped Inside called lesser curvature of the

stomach. Outside called greater curvature of the

stomach. Rugae- long folds in the stomach. Gastritis- inflammation of the stomach. Gastric ulcers- erosions of epithelium of

stomach.

Stomach shape

Each part of the stomach has different muscle functions.

Fundus and body relax with swallowing, allowing stomach to distend and fill with food.

Body of stomach contracts to help mix food within the stomach.

Pyloric Antrum increases contractions with the swallowing of food. ◦ Grinds food and propels food toward pylorus.

Gastric Motility

Empty vs. Full Stomach

Stomach Structure Contains:

◦ Inner mucosal layer

◦ Submucosa◦ Muscular layer

Circular Longitudinal

◦ Outer serosal layer

Pylorus◦Circular muscle fibers◦Maintains constant tone◦Chyme is forced through by

contractions into antrum of stomach. ◦Remains partially open so liquid

contents can move from stomach to duodenum.

Stomach

Enterogastric reflex- prevents stomach from emptying until intestines are able to handle incoming contents.

Stomach

Vagus Nerve. Stimulates parasympathetic activity

Gastric atony- decreased muscle tone in the stomach.

Control of Gastric activity

Gastrin◦Produced by which cells?◦Produced where?◦Helps to increase hydrochloric acid

production.◦Inhibits muscle activity of fundus.◦Causes relaxation and greater filling of

the stomach.

Stomach secretions

Hydrochloric acid◦Produced by parietal cells.◦Produce hydrogen and chloride

separately which combine in stomach.◦Accounts for stomach’s low pH◦When enough acid is produced, gastrin

release is inhibited.

Stomach Secretions

Secretin◦Hormone released by intestines◦Can inhibit peristalsis which slows gastric

emptying.

Cholecystokinin◦Nutrients high in fat or protein stimulate

release of this hormone. ◦Decreases contraction of antrum, body

and fundus.

Stomach secretions

Intrinsic factor- protein that combines with vitamin B12 and aids in absorption of this vitamin.

Pepsinogen- precursor to pepsin which breaks down proteins to chains of amino acids.◦ Peptides (chains of amino acids) stimulates

release of gastrin which increases hydrochloric acid production and pepsinogen release.

◦ Pepsin is inactivated by pH change from stomach to intestine. Which area has a higher pH? Which area has a lower pH?

Stomach secretions

Mucus◦ Produced by gastric glands◦ Complex of many substances

Mucins- produced by goblet cells Main constituent of mucus

Bicarbonate Ion- makes mucous coat more alkaline in nature This helps to neutralize hydrochloric acid.

◦ Help to coat the stomach pH of stomach is generally about 2-3

Stomach secretions…

3 receptors on “blood side” that regulate acid production◦Receptors are for: Acetylcholine- neurotransmitter Gastrin- increases hydrochloric acid

production Histamine- chemical associated with

inflammatory response Inhibiting these receptors decreases

hydrocholoric acid production.◦This is how antacids work.

Parietal cells and receptors

Small molecular structures released by the body that have a wide variety of effects.◦ Involved in inflammation.◦ Some are beneficial to body.

PGE & PGI Reduce hydrocholric acid production by inhibiting gastrin

release from G cells. Directly inhibit Parietal cells Stimulate bicarbonate ion production Enhance blood flow to stomach Help stomach to repair quickly.

Prostaglandins

Gastric Ulcers