digestive system

purpose

• breakdown and absorb food

  • take in food

    • digest food and water

    • break down a reabsorb

    • eliminate solid waste product

  • increase surface area by mechanical breakdown

    • chewing and mixing

  • convert to simple molecules via enzymes

• large surface area of digestive tract with folds, projections

• digestion

  • mechanical and chemical breakdown of food

• resorption

  • absorption from intestinal epithelium to blood stream

• one long muscular tube from mouth to anus

• lumen is outside the body

monogastrics

• simple non-ruminant mammalian

• humans, pigs, mink

avian

• simple non-ruminant

• chickens, pigeons

foregut fermenter

• ruminant

• non-ruminant

  • hippo

hindgut fermenters

• cecal fermenter

  • rabbit

• caeco-colic fermenters

  • horses, elephants

oral cavity

fuction = parts

• prehension

  • acquisition of food

  • lips, tongue, teeth, hands, split lip

teeth

• helps decrease food size by grinding and cutting

• types

  • incisors

    • biting off, cutting, front teeth

  • canines

    • tearing, sharp

  • premolars

    • grinding

  • molars

    • grinding

• ruminants

  • rough dental pad instead of incisors

• horses

  • sharp incisors to grab forage

tooth anatomy

• 3 hard substances

  • enamel

    • outermost, white, very hard, calcium and phosphorus

  • cementum

    • middle, born, acts as the glue, connective tissues, bind tooth root to gum

  • dentin

    • yellow, calcified tissue, odontoblasts

• pulp cavity

  • blood and nerve supply

• crown

  • above gumline

• socket

  • unique joint called gomphosis 

brachydont

• low crown

• all carnivores

• stop growing after eruption

• pulp cavity in middle

hypsodont

• high crown

• equine

• continues to grow

• pulp cavity does not extend above gumline

tongue

• skeletal muscle in bundles

  • moves in three different directions

• keratinized stratified squamous

• papillae on dorsal surface 

  • fungiform

    • taste buds

  • filiform

    • spikes, rough, traction for food

• moist stratified squamous on ventral surface, no papillae

salivary glands

• several pairs located on head and neck, under tongue

• parotid, mandibular/submandibular, sublingual, zygomatic

• contain amylase

  • starts chemical digestion of starches, contains buffer

• mucous

  • thick, serous, or mixed

• ducts empty saliva into oral cavity to lubricate food for chewing and swallowing

mastication

• chewing

• first mechanical breakdown

• jaws, cheeks, tongue

• salivary amylase

  • some chemical digestion of scratch to simple sugars

  • herbivores and omnivores

pharynx

• a space, not an organ

• swallowing

  • closes epiglottis over glottis to keep food and salvia out of larynx and trachea and sending it into esophagus 

  • reflex triggered by food moving into pharynx

  • can be conscious

esophagus

• muscular tube from pharynx to stomach

• skeletal and or smooth muscles depending on species and area

• lays flat, very muscular

• will expand and layers wills trach

• luman enclosed with folds

simple stomach = monogastric

• abomasum of ruminant similar

• enlarged area at end of esophagus just caudal to diaphragm

• glandular types determine regions

• gastric folds

  • rugae

esophageal region

• Only significant in herbivores

• No glands

• Lined with keratinized stratified squamous epithelium

• Mixing, regurgitation

• Ruminant forestomach and other foregut fermenter pouches = esophageal region

cardia region

• Near heart and near esophageal junction, most cranial

• Not glandular, but has some non-complicated mucous glands  protection from acid

fundus region

• body

• deep gastric pits.glands

  • most of secretion

• 3 cells

3 cells

• mucous cells

  • produce mucus

  • protection from acid

• chief cells

  • produce inactive pepsinogen

  • proteolytic enzymes precursor

• parietal cells

  • produce HCl

  • lowers pH of stomach to as low as 1.5

  • kills bacteria

  • drops to proper pH to turn inactive pepsinogen to active pepsin

  • breaks down protein

pylorus region

• Pyloric sphincter joins stomach to small intestine

  • Dictates how much foodstuff enters the duodenum

• Glands produce 

  • mucous, HCl, pepsinogen, and G cells  

    • gastrin 

      • Stimulates HCl release when food present

herbivore specializations

• foregut fermenters

  • Microbes ferment plant material prior to rest of GIT

  • Ruminants have 4 compartments 

  • Camelids: 3 compartments

  • Kangaroo: pouches cranial to rest of stomach

rumen

• large fermentation vat

• left side of animal

• papillae

• symbiotic relationship with bacteria and protozoa

  • cellulase to degrade cellulose

  • microbes’ products absorbed

  • microbes digest in rest of GIT

reticulum

• honeycomb stomach

• Food moves back and forth   

  • forms the bolus  

  • “chewing the cud”

• Most cranial, nearest to the heart

• Esophageal opening dorsally

• Continuation of rumen, liquid contents

• Metal may get stuck here  

  • hardware disease (reticulopericarditis)

omasum

• Layers look like a book

• Increased surface area

• Layers of muscle and mucosa

• Sorting, grinding  filtering

• Reabsorption of water

• Regulates passage on by particle size

abomasum

• True stomach

• Secretes HCl and enzymes, similar to monogastric stomach

• Right side

esophageal groove

• Muscular ridged groove through reticulum, omasum to abomasum

• Closes in young ruminant with action of suckling to become a tube

  • Milk to abomasum for digestion, not fermentation

  • Bypasses the rumen

duodenum

• First part or SI, joins stomach

• Pancreas lies in 1st loop and secretion enters along with bile from gallbladder and liver

• Pancreatic and common bile ducts

• Enzymatic digestion occurs here

• Simple columnar epithelium  

  • 1 layer thick

• Many submucosal and mucosal glands

  • digestive enzyme production

jejunum

• Middle part

• Largest and longest part

• Longest villi

• Simple columnar epithelium

• Most final digestion and absorption of nutrients occurs here

• Many mucosal and submucosal glands 

ileum

• Final part, joins to large intestine

• Smallest part

• Absorption of nutrients, little digestion

• Shorter villi

• Lymphatic nodules in mucosa and submucosa

  • Peyer’s patches

• Increased goblet cells  

  • mucous

surface area

• Digestive and absorptive function made possible by the large surface area

• Villi – small projections of the small intestinal mucosa

  • Each villus is lines with single layer of cells  enterocytes

  • Absorption of nutrients take places across the surface of villi

Goblet cells

• produce and secrete mucus

• Protects the intestinal lining  

• provides lubrication

Microvilli

• projections from each villi

Glycocalyx

• filamentous fuzzy projections off of microvilli

• Trap nutrients

large intestines

• No true villi

• Large folds and projections/pockets

  • Haustra

• Fermentation by microbes

• Absorbs water, water soluble things

  • Microbial products such as vitamins, VFAs

• Parts: 

  • cecum, colon, rectum, anus

cecum

• Blind pouch where ileum and colon meet

• Fermentation

• Presence, size depends on species

  • None in mink, very large in horse and rabbits

• Human appendix is extension off short cecum (vestigial) 

• Birds have 2

colon

• Length depends on species

  • Carnivores: Short colon

  • Spiral colon in pigs, ruminants, camelids

  • Dorsal and ventral colons in horses

  • Ascending, transverse and descending in humans, some other mammals

  • Very short in mink

Rectum

• Final part of large intestine

• Primarily a holding area for undigested “waste” as feces

• Some water absorption

Anus

• 2 layers: external and internal 

  • Smooth and skeletal muscle sphincter

• Changes to skeletal muscle for control of sphincter

• Opening is reflex triggered by feces in the rectum, can also be by conscious control

herbivore specializations

• hindgut fermenters

  • Microbes ferment plant material near end of GIT in large intestine (colon and cecum)

  • All animals do some hindgut fermentation, even carnivores

• Cecal fermenters (rabbit)

• Caeco-colonic fermenters (Horses, elephant)

tubular digestive tract

• tunica serosa

• tunica muscularis

• tunica submucosa

• tunica mucosa

accessory organs

• liver

• gallbladder

• pancreas

liver

• just caudal to diaphragm

• organized into lobes grossly, lobules microscopically

• Produces bile (from hepatocytes)

• Stores glucose as glycogen

• Filters blood

• Organized into lobes (lobules microscopically)

Lobules

• hexagonal with 3 structures: portal triad

  • Hepatic portal vein, hepatic artery, bile duct

Canaliculi

• very small bile ducts

blood flow in the liver

• Portal system: bypasses general circulation

• Blood comes to the liver from hepatic portal vein and hepatic artery

• Hepatic portal vein: 

  • from stomach and intestines, contains nutrients

• Hepatic artery: 

  • coming from celiac artery from aorta, contains oxygen

• Hepatocytes process nutrients, “detoxify” blood as it runs through sinusoids

• Bile is produced in liver by hepatocytes to aid in fat digestion

  • It runs through canaliculi to bile ducts in portal areas which join larger bile ducts until they run into the common bile duct to the duodenum

gallbladder

• animals without

  • rats, camelids, horses

• stores and concentrate bile from liver

• cystic duct from gallbladder

  • joins common bile duct from liver to gallbladder and duodenum

• lined with simple columnar

pancreas

2 roles

• Endocrine

  • Islets of Langerhans

  • Lighter colored circular areas of cells

  • Produce insulin and glucagon for blood glucose utilization

• Exocrine

  • Rest of pancreas (darker)

  • Produces digestive enzymes, mucus and bicarbonate through pancreatic duct into duodenum