Basic GI overview and comparative GI Flashcards
List the major components of the basic GI tract in order from head to tail
- Mouth
- Oesophagus
- Stomach
- Liver
- Small intestine
- Caecum
- Colon
- Rectum
What is the basic function of the headgut (oral cavity)?
- Receives ingested material and breaks it down
What is the basic function of the foregut (oesophagus, stomach)?
- Conducts, stored and digests
- May also ferment
What is the basic function of the midgut (small intestine?)
- Digests and absorbed nutrients
What is the basic function of the hindgut (large intestine)?
- Absorbs water
- Vitamin production
- Ion balance and storage of faeces
- usually fermentation in herbivores
What organs are responsible for the breakdown of food?
- Prehension, mastication and dentition
- Lips, teeth, head
What organs are responsible for the swallowing and transport of a food bolus?
- Pharynx
- Oesophagus
What organs are responsible for secretion of digestive juices?
- Stomach
What organs are responsible for digestion of enzymes and absorption of nutrients?
- Small intestine
What organs are responsible for the absorption of water, ions and microbial digestion of remaining CHOs and proteins?
Large intestine
What organs are responsible for the excretion of waste products?
- Rectum
- Anus
What are some of the accessory organs of the GI tract?
- Salivary glands
- Liver
- Pancreas
- Gall bladder
Define carnivore
- Eats exclusively meat
Define herbivore
Eats exclusively plant material
Define omnivore
Eats both meat and plant material
Briefly outline the main features of a carnivore GI tract
- Large stomachs (infrequent but large meals)
- Normal sized small intestine (most digestion occurs here)
- Protein easily digestible
- Large intestine has minimal function, smaller
- Relies on enzyme digestion
Brielfy outline the main features of a ruminant GI tract
- Breaks down cellulose (fermentation in forestomach)
- Uses bacteria as protein
- Waste energy as CH4
- Forestomach non-secretory, no enzymes churned, products plus bacteria go into stomach
- Chew cudd
- Large fore-stomach, small stomach, long small intestine, large large intestine
Briefly outline the main features of a simple stomached herbivore GI tract
- Use hind-gut fermentation (caecum in rodents, colon in horse)
- Less efficient but less bulky
- Faster, small stomach
- Normal small intestine
- Very large large intestine
- Poorly supported, torsion common
Briefly outline the main features of the bird GI tract
- No teeth
- Can’t chew, proventriculus and gizzard used to break down food
- Proventriculus is true stomach
- Gizzard crushes food using grit
Briefly describe the ruminant forestomach
- Bovids, vervids and antelopes have rumen, reticulum and omasum
- Camels and llamas lack omasum
- In forestomach enzymes present from microflora
- Slow digestion of fibre
- Fermentation process (no O2, anaerobic)
- Forestomach lined by stratified squamous epithelium, keratinised
- No secretion of digestive enzymes
- Abomasum makes up true stomach
- Contents of rumen layered
- Ruminate and regurgitate
Describe the structure of the stomach
- Differs between species
- 4 main mucosal zones
- Oesophageal, cardiac, fundic, pyloric
- Columnar epithelium produces protective mucus
- Cardia small (except in pig)
- Fundus receives and stores (glandular area)
- Corpus contains food, saliva and gastric juice
- Pylorus is muscular to mix
Outline the functions of the stomach
- Storage, mixing, digestion
- Produces chyme (mixture of fluid and gastric secretions)
- Acidic secretions (HCl) kill bacteria
- Protein digestion - proteases initiate proteolysis
- Starch partially degraded
- Water absorption
Describe the locations of the stomach glands
- Cardia is only muscle cell (no glands)
- Fundus and corpus contrain chief and parietal cells in main secretion area
- Pylorus secretes small amount of pepsinogen from chief cells
- Mucin producing at neck of dlang, less viscous than stomach epithelium, prevents self-digestion
Describe the function of the parietal cells
- Secrete HCl
- Secrete intrinsic factor glycoprotein
- Involved in vit B12 absorption
Describe the function of chief cells
- Produce pepsinogen
- undergoes conversion to proteolytic enzyme pepsin
- (Variation in amount produced depending on area)
What is the function of the endocrine cells in the corpus of the stomach?
- Produce histamine (ECL cells)
- Acts as paracrine hormone, stimulates HCl secretion by binding to receptors on adjacent parietal cells
What is the function of endocrine glands in the pylorus of the stomach?
- Produce gastrin (G cells)
- Increases HCl secretion and gastric motility
What is produced by the mucous cells of the stomach?
- Mucin
- Protection against Hcl
What is produced by the parietal cells?
- HCl and intrinsic factor
What is produced by the chief cells?
Pepsinogen (converted to pepsin)
What is produced by the ECL cells?
Histamine
What is produced by the G cells?
- Gastrin (to the blood)
Briefly outline the structure of the small intestines
- AKA small bowel
- Divided into duodenum, jejunum, ileum
- Pancreas sits in U-bend of pancreas
- Jejunum makes up most of SI
- Ileum has antemesenteric blood supply as well as normal mesenteric supply
- Intestinal folds, villi then microvilli (forms brush border)
- In crypts have rapidly dividing cells which migrate up villi then shed
- Lining constantly recycled
- mature enterocytes on villi tips digest adn absorb nutrients
- cryptes of Lieberkuhn produce immature enterocytes and other gut cells from stem cells
- Each villus has arterial supply and venous drainage
Briefly outline the functions of the small intestine
- Secretions to neutralise stomach acid
- Huge surface area
- Degradation and rapid absorption of proteins, carbohydrates and fats into hexoses, peptides and amino acids
- Overspills into large intestine to complete digestion by microbes
- mature enterocytes on villi tips digest and absorb nutrients (CHO, amino acids, lipids etc)
- absorption of minerasl (Fe, Ca, Cu, Zn), ions (Na+, K+, Cl-, HCO3-)
Briefly describe the structure and function of the SI brush border
- Thick mucus layer
- Microvilli
- Mature enterocyte difestion enzymes (CHO, amino acids, lipids, vitamins, minerals)
Briefly describe the secretions and function of the pancreas
- Secretes pancreatic juices (HCO3-, alkaline)
- Neutralises stomach acid, protects duodenum
- Optimal pH for pancreatic enzymes
Briefly describe the secretions and function of the liver and gall bladder
- Bile production (liver)
- Bile storage (gall bladder)
- Breakdown and absorb fats
- important in species with fat in diet
Explain why horses lack a gall bladder
- Storage of bile in gall bladder important in intermittent feeding
- Horses have continuous digestion so bile not stored but secreted continuously by liver
List the cell types found in the crypts of Lieberkuhn
- Enterocytes
- Entero-endocrine cells
- Goblet cells
- Paneth cells
Briefly describe the structure of the large intestine
- Large diameter tube
- made up of colon, caecum, rectum and anus
- Entrance from SI lies between caecum adn colon (except horse)
- taenia of longitudinal muscle contract to form haustra
- Caecum -> ascending colon -> transverse colon -> descending colon -> rectum
- No villi, only crypts
What is different about the large intestine of the horse compared to other species?
- Very large caecum
- Si opens directly into the caecum via the ileo-caecal valve
Briefly outline the function of the large intestine
- Microbial enzymes digest remaining CHO and proteins from SI (fermentation)
- Major site of water absorption and ion balance
- Faeces produced
What is unusal about the colon in cows and pigs?
- Spiral colon
Briefly describe the structure and function of teh caecum
- Appendix in species where redundant
- Blind ending sac, 2 muscular valves controlling entry and exit of ingesta
- Capacity for 25-35 litres of food material in horses
- Not as bulky as rumen
- major site for microbial digestion of cellulose
- Absorption of water and electrolytes
What is unusual about the caecum in birds?
Have paired caeca
Outline the microbial flora of the GI tract
- Flora along entire tract = open system
- Few major bacterial groups
- Individual has personal commensals
- Established at birth
- Stable flora - resistant to change
- In rumen stick in patches, not smooth carpet, cannot be cultivated outside rumen
What are the two types of digestion that can be employed by herbivores?
- Foregut fermentation
- HIndgut fermentation
Briefly describe foregut fermentation
- Complex stomach pouches (sacs or rumen)
- large structures
- Gas by-product readily released
- Saliva buffers fermentation
- Very coarse feed remixed, repeated digestion
- Fermentation may detoxify diet components
- Products more easily available to intestinal absorption
- Digestion largely complete before LI
Briefly describe hindugt fermentation
- Simple stomach
- Long sacculated hindgut
- Majority of fermentation in large LI
- Slow gut passage time
- horses, rodents, large caecum
- taenia, haustra slow digestion further (increase chance for microbial digestion of fibre)
Describe some of the evolutionary adaptations of the herbivores in digestion
- Large volume of food intake
- main energy source carbohydrates
- Slow mixing adn digestion
- Symbiotic microbial digestion of cellulose in fermentation process
- Need high water intake
- large fermentation chanbers
- Produces gas by-products (methane)
What are the advantages of grass fermentation?
- Rough, coarse feed can be eaten
- Microbial fermentation delivers valuable nutrients (VFA and B vits)
- microbial action produces valuable proteins for digestion
- Microbial digestion produces water soluble B and K vits
What are the disadvantages of grass fermentation?
- Low energy diet
- Silicates wear tooth enamel quickly
- Vertebrates do not have innate cellulases
- Microbial fermentation to digest essential
- no diet alternatives - starvation if no grass
- neophobic - fear of new diet so evolutionary disadvantage
Describe some of the adaptations of omnivores in terms of their GI system
- Wide range of food items
- Self select for 21% crude protein intake
- Basic GI pattern - SI, LI (intermediate size)
- Neophilic behvaiour (adapt diet easily)
- includes humans, pigs, bears
- May lack cellulase positive bacteria
Describe some of the adaptations of insectivores in terms of their GI system
- very high protein levels
- Low fat diet
Reduced teeth format - Need to locate diet sources
- Short intestines
- No obvious demarcation SI vs LI
- No caecum
Describe some of the adaptations of arbivores in terms of their GI system
- Leaf eating animals
- Very poor nutritive levels
- Slow metabolism
- very long GI tract
- long transit time
- slow microbial digestion in foregut and/or hindgut
- may have lower body temp to lower metabolic demand
Briefly describe the GI structure of birds
- Beak - no lips/teeth
- NO soft palate - beak and oesophagus is combined cavity
- Crop is expansion of ventral wall of oesophagus at thoracic inlet
- Stores food, allows bulky items to be eaten
- Proventriculus (stomach)
- Gizzard - food grinder (grit)
- Hindgut - 2 blind ending caeca, common digestive, urinary and genital systems
- Opens at single vent (cloaca)
Define prehension
The action of grasping or seizing food
Define dysphagia
Difficulty or discomfort swallowing food
Define regurgitation
The casting up of completely undigested food
Define vomiting
Ejection of matter from the stomach through the mouth
Define diarrhoea
A condition in which faeces are expelled from the body frequently and in a liquid form
Define constipation
A condition in which there is difficulty removing faeces from the bowels, often associated with hardened faeces
What is meant by the mesentery?
A fold of the peritoneum that attaches the stomach, small intestine, pancreas, spleen and other organs to the posterior wall of the abdomen
What is different about the stomach of the horse?
- Large fundus
- Stepped edge inside (margo plicatus)
- Oblique entrance of the oesophagus
What histological features are present in all of the GI tract?
- Mucosa made up of epithelium, lamina propria and muscularis mucosa
- Submucosa
- Muscular layers
- Serosa
What are the 2 large vessels found at teh duodenal flexure?
- Dorsally: caudal vena cava
- Ventrally: hepatic portal vein
Describe the hepatic portal vein
- Blood from GIT to spleen and liver
- Blood rich in nutrients
- Many branches
What are the branches of the hepatic portal vein?
- Splenic, left gastric, gastroduodenal (enter at same level)
- Cranial and caudal mesenteric and ileocolic
Describe the cranial mesenteric artery
- At root of mesentery, very obvious
- Near mesenteric lymph nodes
- Plexus of nerves meshed across artery
Describe the cranial mesenteric ganglion
- Close to plexus of nerves across cranial mesenteric artery
- Close to coleic ganglion
Describe the coeliaco-mesenteric plexus and ganglion
- Solar plexus
- Made up of cranial mesenteric and coeliac plexuses and ganglia
- Contain both parasympathetic fibres of vagal origin and sympathetic fibres
- Derived from sympathetic chain via splanchnic nerves and sympathetic synapses
- Afferent and efferent fibre present
- Innervate viscera in cranial abdomen
Describe the autonomic plexuses
- Several of these
- Hepatic, splenic, left gastric, phrenicoabdominal, adrenal, renal and pair associated with reproductive organs
- Caudal mesenteric plexus and ganglion around caudal mesenteric artery (esp left colic)
- Supply digestive tract caudal to descending colon and pelvic plexus via paired hypogastric nerves
Describe the blood supply to the intestine
- Most supplied by vessels on mesenteric border
- Ileum exception
- Supplied by ileal branch of ileocolic artery on antemesenteric side
- Position of blood and nerve supply affects choice of sites of entry into intestine
- Most receive unilateral supply
Describe the position of the caudal mesenteric artery
- Level of caudal duodenal flexure (crossing from right to left of abdomen)
- 5th or 6th lumbar vertebrae
What are the two types of faeces excreted by rabbits?
- Caecotrophs
- Hard pellets
What is the fate of large particles in rabbit digestion?
- Made up of indigestible fibre
- Down colon, quick elimination, hard dry pellets
What is the fate of smaller particle and fluids in rabbit digestion?
- Retrograde movement
- To caecum
- Microbial fermentation takes place here
- Produces caecotrophs which are ingested for redigestion
When do the hard faeces phase and soft faeces phase take place in the rabbit?
- Hard during feeding
- Soft when at rest
Describe the hard faeces phase in rabbit digestion
- During feeding
- Small particles to haustra then caecum
- Water to proximal colon
- Caecal contractility greatest
- Haustral activity moves small particles to caecum
- Fusus coli squeezes water out
- In distal colon water, K, Na and VFAs absorbed
- Dry indigestible matter expelled
Describe the soft faeces phase in rabbit digestion
- Caecotroph production
- Increased distal colon motility, decreased caeca and proximal colon motility
- Caecal material moved to large colon
- Fusus coli forms pellets and adds mucus
- Rapid excretion takes place
- 4+ hours after feeding
- No separation or absorption of water
Describe the caecotrophs produced by rabbits
- Outer greenish membrane (mucus)
- 2x protein, 1/2 fibre compared to hard faeces
- Absorb undigested nutrients
- Add essential nutrients by microbial fermentation
- Contains vits B and K
- Eaten directly from anus
- Contain bacteria (protein), essential AAs, vits and minerals
What is the importance of coprophagy in rabbits?
- Caecotrophs contain 2x protein of hard pellets
- Allows absorption of undigested nutrients
- Contain protein, essential AAs, vits and minerals
How are caecotrophs digested in order to gain protein?
Lysozyme in distal colon digests bacterial cell walls to access microbial protein
What are the types of contraction in hindgut motility of the rabbit?
- Haustral activity (3sec), oral direction
- Segmental activity (14sec), aboral
- Peristaltic contractions (5 sec hard faeces, 1.5sec soft faeces), progressive
Describe the role of the fusus coli in the digestion in rabbits
- Is pacemaker
- Initiates peristaltic waves
- Highly innervated
- Hormonal influence
- Aldosterone high = hard faeces
- Prostaglandings high= caecotrophs
What is the effect of fibre on rabbit digestion?
- Stimulates hindgut motility
- Increases caecotroph production
What is the role of fat in rabbit digestion?
- Increases motility
- Is also energy source
What is the effect of protein on rabbit digestion?
Decreases production of caecotrophs
What is the effect of carbohydrates on rabbi digestion?
- High glucose, aids growht of C. spiroforme and E. coli
- Excess VFAs produced
- Can lead to enterotoxaemia
- Prevents growth of normal microflora, affects digestion
Describe the appearance fo the caecum in rabbits
- Green
- Proximal colon leaves base of caecum
Describe the process of caecal fermentation in rabbits
- Bacteroides, protozoa and yeasts change depending on time of day, age, diet and pH
- Urea in blood stream is nitrogen source
- fermentation of undigested food, excretion products, mucopolysaccharides, desquamated cells
- VFAs, AAs, water soluble vits produced
- VFAs into blood stream (A, B and P)
Describe the caecal pH of rabbits
- pH has diurnal rhythm
- Alkaline AM, acid mid afternoon
- Ammonia and VFAs from fermentation cahnge pH
- Bicarbonate ions from appenix buffer
- Fibre also buffer
Describe the anatomy of rabbit large intestines
- GALT present
- Sacculus rotundus (caecal tonsil)
- Ampulla caecalis coli
- Caucum coild spiral 3 folds
- Appendix has GALT and bicarbonate ions
Describe the colon anatomy of the rabbit
- Proximal and distal colon different
- Proximal proximal colon has 3 haustra and 3 taenia and warzen
- Distal proximal has 1 haustra and 1 taenia and warzen
- Proximal analogous to ascending colon in other animals
- Proximal ends in fusus coli
- Distal colon analogous to transverse and descending colon in other species
Describe the dentition of guinea pigs and chinchillas?
2(I1/1, C0/0, PM1/1, M3/3) = 20
- Both open rooted
- Have palatal ostium
Describe the GIT of guinea pigs
- Long caecum
- Produce caecotrophs
- ## Cannot make vit C, dietary source essential
What is meant by palatal ostium?
- Soft palate continuous with tongue
- Membranous covering posterior pharynx
- Palatal ostium is the hole in that membrane
Describe the GIT of chinchillas
- Long tract
- large coiled caecum
- Colon highly sacculated
- Produce caecotrophs
Describe the GIT of rodents
- Dental formula 2(I1/1, C0/0, PM0/0, M3/3)
- Incisors open rooted, lower incisors longer
- Hindgut fermenters
- Caecotrophs
Describe the GIT of hamster
- Thin walled, highly distensible pouches
- Pregastric fermentation in pregastric pouch
- Pouches do not have lymphatic drainage
- Stomach has constriction between forestomach and glandular stomach
- More cranial pregastric pouch is where fermentation is taking place
Describe the diet of ferrets
- Strict carnivores
- Fat main energy source (never carbohydrates)
- High quality meat based diet needed
- 30-40% crude protein of animal origin
- Several small meals - rapid GI transit time
Describe the GIT of ferrets
- Dental formula 2(I3/3, C1/1, PM3/3, M1/2) = 34
- Extremely short GIT
- Simple expandable stomach
- Short SI (poor nutrient absorption)
- No caecum, no appendix
- Simple GI flora
- LI 10cm
