Comparative GI anatomy Flashcards
What are the sections of the GIT and their functions?
Headgut - oral cavity
Foregut - oesophagus, stomach
Midgut - SI
Hindgut - LI
What is the function of the hindgut?
Hindgut - LI - absorbs water, vitamin production, ion balance and storage of faeces, usually fermentation in herbivores
What is the function of the midgut?
Midgut - SI - digests and absrobs nutrients
What is the function of the foregut?
Foregut - oesophagus, stomach) - conducts, stores, digests (may ferment)
What is the function of the headgut?
Headgut - oral cavity - receives ingested material and breaks it down
What is the function of the GIT?
breakdown food
swallowing and transport
secretion of digestive juices
digestion
absorption
excretion
Label this simplified diagram of the GIT
Which GIT structure shows the least variation across species?
Small intestine
Its role (nutrient absorption and digestion) is a conserved function across all species
What is an autochthonous flora?
A stable flora - resistant to change
Personal to each individual
Why is it important to gradually change an animals diet and make new diets similar to old ones?
to reduce restricted intake and prevent dramatic change in microflora
Describe the diet and GIT of carnivores
High-energy (fats) and protein
Vomit indigestible bones and cartilage
Large stomach
Short GIT (most digestion occurs before end of SI)
Relies on enzyme digestion
Smaller LI
What are the 2 types of herbivores with examples?
Foregut fermenters - complex stomach pouches - sacs or rumens e.g., cows,sheep
Hindgut fermenters - fermentation with simple stomach e.g., horses, rabbits, rodents
What are the advantages of grass fermentation?
- rough, coarse feed can be eaten
- microbial fermentation delivers valuable nutrients (VFAs and Vit B)
- microbial action produces valuable proteins for digestion
- microbial digestion produces Vit K and the Vit B complex
What are the disadvantages of grass fermentation?
- Grass is low energy diet
- grass contains silicates which wear teeth enamel quickly
- vertebrates do not have innate cellulases
- microbial fermentation to digest is essential
- no diet alternatives - starve if no grass
- “neophobic” - fear of new diet
Describe the evolutionary GIT adaptations of herbivores
Large volume of food intake
Main energy source is carbs (starch and cellulose from plant fibre)
Slow mixing and digestion
Symbiotic microbial digestion of cellulose in fermentation
Needs a high water intake
Large fermentation chambers
Produces gas by-products
Describe the GIT of insectivores
Short intestines
No obvious demarcation SI vs LI
No caecum
Describe the diet and GIT of arbivores
Leaf eating animals
Poor nutritive levels
slow metabolism
long GIT
long transit time
slow microbial digestion in foregut/hindgut
may have lower body temp to reduce metabolic demand
Why do piscivores (fish diet) have simple GITs?
Fish are easy to digest due to softer tissues and fewer complex carbs
Less detoxification requirements
Minimal fermentation
Describe the general anatomy of smooth muscle in the GIT
Unitary/syncytial
Lots of fibres contract as a single unit
Arranged in sheets or bundles
Cells joined by gap junctions to enable free movement of ions and APs to transmit the force
Nerves branch over many fibres
How does contraction and relaxation of the GIT smooth muscle occur?
Due to transmission of acetylcholine and noradrenaline
Stretch leads to contraction (stimulus for peristalsis)
What is the oesophagus?
continuation of laryngopharyx connecting the pharynx to the stomach
What are the sections of the oesophagus and where do they travel?
Cervical - passes to the left side as approaches thoracic inlet
Thoracic - begins at thoracic inlet, passes through inlet on the left but then moves back dorsal to trachea, runs in mediastinum
Adbominal - follows passage trough the oesophageal hiatus of the diaphragm, very short, terminates at cardia of stomach
What are some structures related to the cervical portion of the oesophagus?
common carotid artery
internal jugular vein
tracheal duct
cervical lymph nodes
vagosympathetic trunk
recurrent laryngeal nerve
Describe the basic structure of the oesphageal wal
3 layers:
connective tissue adventitia (outer layer)
muscular layer
mucous membrane/ tunica mucosa
Label these structures associated with the oesophagus
Clinical relevance - surgery and the oesophagus
Heal very poorly, likely to from a fistula (abnormal connection between 2 hollow spaces)
Label this diagram of the oesophageal wall
Label this diagram of the oesophagus
Describe the structure of the tunica mucosa/mucous membrane of the oesophagus
Stratified squamous epithelium
Cornified in herbivores
No glands in lamina propria
Well developed submucosa:
- mucous glands
- loosely binds the mucosa and muscularis layers
Explain the species variations in the submucosa of the oesophagus
Mucous glands over entire length in dog, cranially in pig and restricted to pharyngo-oesophageal junction in cat and horse
Label the layers of the mucosa of the oesophagus
Describe the species differences in the muscularis layer of the oesophagus
Canine and bovine muscularis entirely striated
Pig - short part with smooth muscle near cardia
Cat and horse - distal 1/3 is smooth muscle
Describe the structure of the muscularis layer of the oesophagus
Inner and outer layers
Continues cranially with pharyngeal muscles
Mid region - muscle layers are arranged in intercrossing spirals
Caudally - outer fibres run longitudinal, inner fibres more circular and thicker
What is chock and where is it most common?
Obstruction of the oesophagus
Most commonly occurs in cows close to the pharynx and thoracic inlet
Explain the species differences in the epithelium of the oesophagus
- high degree of keratinisation in ruminants and horses
- non-keratinised in carnivores and pigs
Label this oesophagus
Describe the arterial supply to the oesophagus
Cervical portion: thyroid and subclavian (branches from L and R common carotids)
Thoracic portion: broncho-oesophageal
Abdominal portion: left gastric and splenic
Describe the venous drainage of the oesophagus
Cervical drainage enters external jugular veins
Thoracic drainage enter the oesophageal vein
Describe the lymphatic drainage of the oesophagus
Cranial aspect into the cervical, middle and caudal deep cervical nodes
Thoracic lymph drains into cranial and caudal mediastinal lymph nodes
Describe the innervation of the oesophagus
Sympathetic from cervico-thoracic ganglion
Parasympathetic from branches of vagus nerve
What is megaoesophagus?
abnormally large oesophagus
can cause loss of motility so bolus’ can accumulate in oesophagus
What are the components of the rumen/forestomach?
rumen, reticulum, omasum
Describe how the forestomachs function
Enzymes from microorganisms cause the slow digestion of fibre via anaerobic fermentation
What kind of epithelium can be found in the forestomachs?
stratified squamous
keratinised
What are the 4 mucosal zones of the stomach?
oesophageal
cardia
fundus/corpus
pylorus
What is chyme?
mixture of fluid and gastric secretions
What processes occur in the stomach?
Acid secretions (HCl) kill bacteria
Protein digestion via proteases
Starch is partially degraded
Water absorption
Why is it difficult for horses to vomit?
When there is gas or excess food in the stomach, the oesophageal sphincter closes, preventing gastric reflux so vomiting cannot happen easily
Clinical relevance: gastric torsion
Occurs in large and medium dogs with wide chest
Twists around long axis -> selas of oesophageal and pyloric sphincters so stomach fills with gas and dilates
Leads to tissue damage
Can interfere with vena cava return to heart -> decreased CO, diaphragm displaced cranially, inspiration inhibited and hypoxia worsened -> circulatory shock
What is special about the muscular layers of the equine stomach?
Have the normal longitudinal (outer) and circular layers but have an additional layer - the inner oblique muscle
What is special about ruminant digestion in the stomach?
Ruminants don’t chew efficiently so larger fragments swallowed lay on low density stomach acid on top
They regurgitate the larger fragments, chew the cud and reswallow -> small particles that sink lower into higher density acid
Name the structures in the foregut of the chicken
Crop
Proventriculus
Gizzard
what is the function of the stomach cardia
Mucus producing, storage
What is the function of the stomach fundus
storage, HCl and pepsin production, main secretion area
What is the function of the stomach corpus?
mixing region, HCl and pepsin production, production of histamine
What is the function of the stomach pylorus?
mixing and propulsion onwards, mucus production and small amounts of pepsin, production of gastrin
What are the section of the SI
duodenum (descending and ascending limb)
Jejunum (largest section)
Ileum
Where is the pancreas found?
attached between the U-bend of the duodenum
How do you distinguish ileum from jejunem
Ileum has antimesenteric blood supply as well as usual mesenteric blood supply
What are the functions and adaptations of the SI?
- basic secretions to neutralise stomach HCl
- huge surface area for absorption
- degradation and rapid absorption of proteins, carbs and fats into hexoses, peptides and amino acids
- overspills into LI to complete digestion by microbes
- fats are exclusively degraded in SI
What is the function of the pancreas
Secrete pancreatic juices (alkaline)
- neutralises stomach acid
- protects duodenum lining
- optimal pH for pancreatic enzymes
What is the function of the liver?
- Secrete bile
- breakdown and absorb fats
What is different about horse bile production?
Have no gall bladder so cannot store bile - but it is secreted continuously by liver
Where is bile stored?
Gall bladder
Where does most venous drainage of the GIT go?
Collected by the liver via the hepatic portal vein -> cranial vena cava
What cells are found in the mucosa of the SI and what are their functions?
Enterocyte - absorption
Entero-endocrine - hormone secretion
Goblet - mucous producing
Paneth - lysozyme inhibits bacteria
Microfold cells - antigen sampling
What are the nutrients primarily absorbed in the SI?
CHO, amino acids, lipids, vitamins, minerals
Outline the structure of a villus unit
- central lacteal and blood capillaries surrounded by enterocytes with microvilli
- forms the brush border on the apical surface, facing the intestinal lumen
What are taenia and what is their function?
Concentrations of longitudinal muscles
On contraction, haustra/sacculation are formed with slow transit and increase opportunity for microbial digestion of fibre by cellulases
What is the caecum and what is its function?
Blind ending sac with 2 muscular valves controlling entry and exit of ingesta
Major site for microbial digestion of cellulose and absorption of water and electrolytes
Can be used to store water and electrolytes
What are the components of the LI?
caecum -> ascending colon -> transverse colon -> descending colon -> rectum
What is different about anatomy of the LI of horses?
The SI opens directly into caecum via ileo-caecal valve
Where does the SI enter the LI?
Between the caecum and colon
what is the function of the LI?
Microbial enzymes digest remaining CHO and proteins from SI (fermentation)
Major site of water absorption and ion balance
Faeces production
What is different about the histology between the SI and LI?
LI has no villi or microvilli
Label this avian GIT
What is the function of the crop in birds?
Stores food
What is the structure and function of the proventriculus in birds?
Columnar epithelium, prominent papillae
Contains HCl and starts digestion (equivalent to glandular region of stomach)
What is the structure and function of the gizzard in birds?
Keratinised “tooth brush” lining
grinds food by muscular action - plus grit in some species
Where is the cardia found in the horse?
Very narrow
Lies between margo plicatus and fundus
What is the margo plicatus?
The boundary between the keratinized and glandular regions of the stomach in horses and pigs
In horses, which areas of the stomach are keratinised and which are glandular?
The oesophageal part is keratinised - acts as fermentation chamber
The fundic and pyloric regions are glandular - usual acid digestion
Label this avian GIT
What is the function of the spiral valve in fish intestines?
Lenthens the path the ingesta has to travel
More time for absorption
