GIT

Question 1

digestive system parts

Answer 1

oral cavity + organs
esophagus
forestomachs (ruminants)
true stomach ‘monogastric’ (‘abomasum’ in ruminants)
small intestine
liver
peancreas
large intestine
rectum
anus

Question 2

major functions

Answer 2

ORAL CAVITY
- prehension (taking hold of food)
- mastication (chewing to break down + mix w/saliva)

STOMACH
- sterilization (bacteria) + holding chamber

SMALL I
- digestion (chemical breakdown of food)
- absorption (of food and water)

LARGE I
- evacuation of waste

Question 3

food

Answer 3

actually remains OUTSIDE the body (tube), must be broken down to be absorbed
- digestion
- fermentation
- microbial break down

Question 4

3 diet types

Answer 4

1) carnivore
- meat, not fiber
- little fermentation, no real fermentation chamber in GIT
- protein doesn’t need to be fermented to be absorbed

2) herbivore
- lots of fiber
- lots of fermentation, large cecum/rumen

3) omnivore
- some fermentation, enlarged colon

*overall parts of GIT will be the same, but size and function varies….
- monogastrics: single true stomach
- ruminants: large rumen (fermentation chamber)
- cecal/hind-gut fermenters: large cecum and/or large i for fermenting

Question 5

mouth/buccal cavity structures

Answer 5

lips (labia)
tongue
teeth
salivary glands
hard + soft palate
oropharynx

Question 6

salivary glands

Answer 6

produce saliva for lubrication/digestion
- controlled by ANS, primarily parasympathetic
- sympathetic stimulation usually inhibits (dry mouth when scared)

3 pairs, all w/ducts to carry saliva to oral cavity:
- parotid: below ear canal; caudal to mandible
- mandibular: medial to bones of mandible
- sublingual: under base of tongue
*know 2 types, read text

Question 7

saliva

Answer 7

ENZYMES:
1) amylase
- in omnivores (pigs), not dogs, cats, ruminants
- breaks down amylose (starch AKA simple sugars)

2) lipase
- present in young animals while nursing
- breaks down lipids

3) lysozyme
- not really digestive, more for antibacterial

OTHER FUNCTIONS:
- dog: evaporative cooling

• cattle: sodium bicarb + phosphate buffers to neutralize rumen acids (prevents rumen acidosis)
• buffers recycled from GIT
• 200L saliva/day

Question 8

teeth

Answer 8

mechanical breakdown, defense/offense

row of teeth = ‘arcade’
- upper arcade + lower arcade

STRUCTURE
- embedded in socket (‘alveolus’) in bone (mandible in lower, maxilla/pre-maxilla in upper)
- crown = above gums
- root = in alveolus
- enamel = outer surface (hardest part of body!)
- dentin = bulk of tooth (hard as bone)
- pulp cavity = central hollow, vascular, innervated

Question 9

tongue

Answer 9

mass of muscle covered by mucus membrane + papillae
- functions to prehend + move food for mastication and swallowing
- taste buds on dorsal surface

Question 10

prehension differences

Answer 10

equine:
- lips retract, use incisors

bovine:
- tongue + dental pad

sheep/goat:
- split upper lip, graze closer than horse/cow
- lips more mobile than tongue

pig:
- root + jerk into mouth w/pointed lower lip

canine/feline:
- hold w/forelimbs rip and tear w/head

DRINKING
-cat/dog: tongue ladles water in
- other: tongue creates a vacuum

Question 11

palate

Answer 11

hard is roof of mouth, becomes soft caudally
- soft separates oropharnyx from nasopharynx

Question 12

esophagus

Answer 12

muscular tube w/ 2 orientations (longitude + circular)
- peristalsis: moves food from oral cavity to stomach

4 LAYERS (inside to outside)
1) mucosa: epithelium lining organ walls (exposed to food/external environment)
2) submucosa: CT supporting mucosa
3) muscularis: smooth muscle layer allowing distensibility + propulsion of lumen contents
4) serosa: serous membrane layer on outside

Question 13

swallowing/’deglutition’

Answer 13

starts voluntarily (not present if anaesthetized)
- food pushed back into pharynx

continues involuntarily
- reflex contraction of pharynx -> epiglottis moves to cover glottis -> esophagus relaxes to let food bolus enter -> peristalsis (wave of muscular contractions along tubular organ) -> contraction of longitudinal/relaxation of circular opens lumen on aboral [away from mouth] side of bolus -> contraction of circular muscles on oral side

once in stomach
- natural fold and tone in cardiac sphincter preventing reflux
- in horses sphincter also prevents vomiting (emesis)

Question 14

emesis

Answer 14

• reflex controlled by a center in brainstem
• drugs stimulating: emetics

relaxation of pyloric sphincter -> reverse peristalsis moves ingesta into stomach -> relaxation of cardiac spincter -> inspiratory movement against a closed glottis + forceful contraction of abs = vomiting

• closed glottis prevents aspiration
• soft palate directs food out of mouth

Question 15

monogastric stomach/ruminant abomasum

Answer 15

5 parts:
1) cardia: area surrounding opening of esophagus into stomach (small part at very top)
2) fundus: distensible blind pouch, expands as more food swallowed (top bulgy part)
3) body: distensible, middle of stomach
4) antrum: distal portion, grinds food, regulates acid production, produces mucus (where stomach narrows)
5) pylorus: muscular sphincter that regulates movement of chyme (semi-digested) into small i
- constant tone: allows fluid through, large particles stay
- prevents backflow from small i into stomach

shaped like a C
- inner curve = lesser curvature of stomach
- outer curve = greater curvature of stomach

• mucosal lining of stomach = longitudinal folds (rugae)
• stomach of carnivore empties quickly (few hours), horse + pig takes 24hrs

Question 16

ruminants

Answer 16

• cattle, sheep, deer, goats, llamas, alpacas
• allow food gathering while watching for predators, then mastication later when safe + resting
• very efficient fiber digestion (repeatedly mixed w/saliva + ground to increase surface area exposed to microbes in rumen)

ruminants ruminate their food
- masticate, swallow, mix + ferment food in the rumen
- regurgitate, chew again (‘rumination’)
- one cycles takes about a minute

Question 17

ruminant stomach

Answer 17

4 compartments:
1) reticulum
- small, cranial
- honeycomb looking mucosa
- coordinated contraction w/rumen (‘reticulorumen contractions’)

~rumenoreticular fold separates~

2) rumen
- largest, left half of abdominal cavity
- mucosa has pile rug appearance
- fermentation chamber (along w/reticulum)
- separated into compartments (muscular sacs) by ‘rumenal pillars’

RUMENAL PILLARS:
- helps mix rumen contents during contractions
- synchronized contractions
- controlled by vagus nerve of PSN
- affected by pH, VFAs, consistency of contents, degree of distension, feedback from other GIT areas

3) omasum
- lots of surface area due to mucosal folds (look like pages in a book)
- strong muscles in omasal wall for further mechanical breakdown
- absorbs VFAs that were made in rumen, water, + bicarbonate (prevents neutralization of abomasal acid)

4) abomasum
- true stomach
- where HCl breaks it down further before going to SI

Question 18

eructation

Answer 18

expulsion of CO2 + methane from fermentation
- accumulates in top of rumen ‘gas cap’
- coordinated contraction of rumen + relaxation of esophagus

Question 19

fermentation process (in rumen)

Answer 19

1) enzymes from microbes (bacteria, fungi, protozoa) in rumen breakdown
- cellulose, pectin from plants can’t be digested in monogastrics but can by ruminants
- cellulase converts cellulose -> monosaccharides + polysaccharides
- starch (multiple sugars stuck together) -> monosaccharides

2a) microbes convert saccharides into volatile fatty acids, methane, CO2
- VFAs = propionic, butyric, acetic acids
- VFAs absorbed -> go to liver -> converted to glucose (propionic acid) to make adipose, milk fats in dairy cows, OR burnt for energy
- (cows get glucose from VFAs metabolized in rumen)

2b) protein broken down by microbial proteases to amino acids
- AAs used by microbes to make proteins OR converted to VFAs + ammonia (NH3)
- ammonia can be picked up by other microbes to synthesize new AAs…… ruminants have no essential AA required from diet
- vitamins B + K also made in rumen

3) ammonia that goes back to liver converted to urea -> secreted BACK into rumen or saliva to provide nitrogen to microbes (increases ability to make protein in rumen)
- urea sometimes added to feed to increase nitrogen (often recycled from chicken waste)

4) microbes move w/ingesta through GIT -> digested in SI as protein
- protein source

• very important for microbes, feed, pH, and gas production to be balanced OR bloat, acidosis, death
• be careful of sudden diet change (esp more grain)

Question 20

young ruminant GIT

Answer 20

milk doesn’t need fermentation
- rumen + reticulum of nursing calves underdeveloped, lack microbial population necessary to ferment
- ‘reticular groove’ directs milk into omasum directly when swallowed (bypasses rumen + reticulum)
- reticular groove functions until calf is introduced to grain + hay (then muscles atrophy, no longer active although doesn’t disappear completely)

Question 21

gastric secretions

Answer 21

stomach submucosa contains glands
- produce HCl, enzymes, mucus, intrinsic factor

PEPSINOGEN
- secreted by chief cells -> stimulated by gastrin
- precursor for pepsin (made by cleaving pepsinogen w/ HCl)
- pepsin catabolizes proteins to smaller AA chains = ‘peptides’

MUCUS
- mucin (from goblet cells) + water + bicarb ions (secreted on surface to neutralize stomach acid)
- protects stomach wall from autodigestion due to pH 2-3
- broken down by HCl so must be continuously secreted to prevent gastritis + ulcers
- prevents us from digesting our own stomach wall!

HYDROGEN + CHLORIDE IONS (HCl)
- secreted by parietal (oxyntic) cells as separate ions (H+ and Cl-), mix in the stomach and become acidic (HCl)
- acid sterilizes food, protects from infections, starts denaturing proteins

Question 22

gastric glands

Answer 22

controlled by receptors on parietal cells for gastrin (hormone), acetylcholine, and histamine (hormone)
- stimulation of all 3 needed for optimal HCl release
- gastric pH below 3 -> gastrin release inhibited -> HCl inhibited
- gastrin inhibits parietal cell, signals to stop excess production of acid
- histamine stimulates parietal cell, increases acid production

drugs:
- can inhibit HCl release by blocking histamine receptor (H2) eg. cimetidine, ranitidine
- block acetylcholine receptors eg. atropine
- can directly block H excretion (proton pump inhibitors) eg. omeprazole

Question 23

gastric motility

Answer 23

stomach + intestine contain circular and longitudinal muscles responsible for food propulsion, hunger contractions (stomach growling), + assisting in grinding
- motility depends on stage of ingestion/digestion and area of stomach

• initiated by pacemaker nervous cells that rhythmically depolarize using Ca channels -> stimulate smooth muscle to contract
• contraction is modified by ANS (increased by para [except fundus, which relaxes to prepare space for food], inhibited by symp [can cause atony - no tone in muscle])

OVERALL AFFECTS:
- fundus + body tend to relax w/ingestion (allowing distension and food storage)
- relaxes when antrum is distended w/food
- antrum contracts w/ingestion, inhibited by excess acidity, fat, and proteins in SI (propels food towards pylorus, liquid enter SI, solids stay for more mixing and grinding)

Question 24

motility and hormones

Answer 24

GASTRIN
- produced by G cells in antrum in response to filling + presence of peptides
- increases HCl production in parietal cells + pepsinogen release from chief cells of fundus and body
- causes muscular relaxation in fundus for greater filling

SECRETIN
- produced in duodenum in response to excess stomach acid in SI (acid signals for secretin to secret in duodenum)
- causes relaxation of fundus
- inhibits peristaltic movement in antrum + body to delay further movement of ingesta into duodenum
- says ‘don’t put any more food into the SI, focus on stomach’

CHOLECYSTOKININ (CCK)
- produced in response to fats + proteins in duodenum
- inhibits gastric contractions and slows gastric emptying
- stimulates gallbladder to contract (we’ll need bile to breakdown all this fat)

Question 25

small intestine

Answer 25

digests proteins, fats, carbs
- does most of the nutrient absorption (sugars, AAs, fats, water, minerals)

PARTS
1) duodenum:
- extends from pylorus -> jejunum
- receives ingesta from stomach, bile from liver, secretions from pancreas
2) jejunum
- longest part of SI
3) ileum
- short terminal part
- connects to LI via ileocecal spincter

LAYERS (in all parts)
1) luminal mucosa (simple columnar epithelium w/goblet cells)
2) submucosa (CT)
3) muscular layer (smooth muscle)
4) outer serosa (visceral peritoneum)

MECHANISMS to increase mucosal surface:
- large surface area exposed to lumen = more absorption
- protected by mucus, just like stomach
1) long…. many loops and coils
2) mucosa folded into plications (villi above and intestinal crypts below surface). crypts are site of cell regeneration for SI. secrete fluid into intestine to dilute chyme.
3) villi between crypts
4) microvilli on each cell (tiny, on apical surface of epithelial cells on villi). brush border. cells have enzymes for digestion and carrier molecules for absorption in membrane

MOTILITY
2 types of contractions:
1) peristalsis
- progressive propulsive contractions to move ingesta
2) segmental
- mixing, churning contractions
- slow movements of ingesta
- increase exposure of chyme to intestine wall

ENZYMES
- needed to break down carbs, fats, protein
- produced in proportion to diet
- sudden diet changes result in incomplete digestion -> diarrhea. must be gradual
- 2 sources:
1) mainly from pancreas
2) enzymes on microvilli

• carb digestion:
  1) amylase
• sometimes in omnivore saliva, always in pancreas
• starch -> disacc
  2) sucrase, maltase, isomaltase, lactase
• in microvilli, amount dependent on diet
• disacc -> monosacc -> absorbed through epithelium via ‘second active transport’ (low sodium in cells thanks to Na-K ATPase, sodium + glucose co-transported into cell down sodium concentration gradient, glucose absorbed efficiently) -> glucose leaves cells for blood by diffusion through basement membrane
• protein digestion:
  1) pepsin
• breaks proteins into polypeptides in stomach
  2) pancreatic proteases
• incl. trypsin, chymotrypsin, elastase, carboxypeptidase
• in SI
• originally secreted as inactive substances -> activated in lumen of GIT
• luminal digestion yields free AAs + small peptides -> peptidases in microvilli final stage -> produces AAs -> absorbed on carrier molecules in microvilli by secondary active transport (like glucose) -> leave for blood by diffusion through basement membrane
• fat digestion:
• fat clumps in droplets, emulsification breaks them up -> warmed and mixed in stomach -> bile acids added (secreted by liver through bile duct into duodenum, bipolar w/hydrophobic end for fat and hydrophilic end for water) -> droplets now smaller w/more surface area -> fat soluble vitamins (A D E K) can attach -> pancreatic lipases digest (dietary triglycerides hydrolyzed, free fatty acids and monoglycerides produced) -> monoglycerides, fatty acids, bile salts form water soluble micelles that transport the lipid to villi -> bile salt stays in lumen (only absorbed in ileum) -> monoglycerides and fatty acids diffuse into cell -> reassembled into triglycerides -> packaged w/protein + excreted from cell as chylomicrons (too big to enter capillaries) -> travel in lymph to vena cava
• exception = medium chain triglycerides enter blood directly (shorter chain)

Question 26

large intestine

Answer 26

• recovers water + electrolytes (principle function of LI in cats and dogs, in small colon of horses)
• stores feces
• ferments fiber (to variable extent by species)

PARTS:
1) cecum
- blind diverticulum (sac) at beginning of colon (at ileocecal junction)
- poor in carnivores, large in ruminants, very developed in horses/rats/guinea pigs/rabbits

2) colon
- has numerous mucus secreting glands + uses peristaltic and segmental contractions for movement
a. ascending
- proximal, courses cranially
b. transverse
- courses R to L
c. descending
- connects transverse to rectum

LAYERS
- same as in SI but muscosa is not at folded and contains more mucous cells

HORSE
- cecum on right side of peritoneal cavity
- ascending colon known as large colon (4 large banks traveling in pairs (one dorsal, one ventral))
- cecum + large colon divided into sacs called ‘haustra’
- longitudinal muscle forms bands called ‘taenia’
- descending colon known as small colon (longer than in carnivores)

RUMINANT
- ascending colon long + doubled into a spiral that supports the SI

HIND GUT FERMENTORS
- equids, rabbits, guinea pigs
- fiber doesn’t digest in SI
- cecum and large colon ferment fiber
- microbes + end products here identical to those in ruminants, VFAs absorbed across cecal and LI wall
- differs from ruminants: much starch and protein digested and absorbed in SI, not in ruminants, and microbial protein can’t be absorbed
- bicarbonate directly secreted across cecal + large colon wall into lumen
- microbes that ferment are sensitive to antibiotics!

Question 27

rectum

Answer 27

stores feces
- terminal portion of colon, continues into pelvic cavity
- many mucus glands for lubrication
- sensory receptors for stretch (stimulates defecation)

Question 28

anus

Answer 28

internal + external muscular sphincters to allow controlled passage of feces

1) internal spincter
- ANS controlled
- para relaxes (usually along w/increase in colon motility), symp constricts

2) external sphincter
- voluntary control
- internal relaxes -> fecal contents contact anal mucosa -> stimulates conscious need to defecate
- voluntary relaxation of external sphincter causes defecation

• rectum + anus muscle and nerves easily damaged by perianal surgery, tumors, and trauma. can cause incontinence

Question 29

pancreas

Answer 29

exocrine glands for digestion, secretions travel through pancreatic duct to duodenum

secretes:
- proteases, amylase, lipase from acinar cells
- bicarbonate to neutralize acid

Question 30

liver

Answer 30

second largest organ!
- caudal to diaphragm
- divided into lobes -> subdivide into microscopic lobules

• hepatic portal system drains intestine and delivers blood directly to liver
• assists the phagocytic and nutrient processing functions of liver

FUNCTIONS
1) filter + detoxify substances absorbed from GIT into blood (via phagocytic cells in hepatic sinusoids)
2) absorb + store vitamins, minerals, glucose from GIT
3) produces bile acids, cholesterol, bilirubin (‘yellow’)
- bile collects in canaliculi -> drains into bile ducts -> merge to form hepatic duct -> connects to gallbladder (where bile is stored, horses don’t have one) -> common bile duct drains gallbladder + liver -> merges w/pancreatic duct just before opening into duodenum
4) makes blood proteins
- albumin: important for maintaining blood oncotic pressure (keeps water in our circulatory system, not leaking into our body!)
5) glucose metabolism
- glycogenesis: making glycogen from sugars absorbed from gut
- glycogenolysis: breaking glycogen down b/w meals to maintain blood glucose
- gluconeogenesis: make glucose during starvation (all), constantly in ruminants and true carnivores, from AAs (cats), from propionate (herbivores)

GALLBLADDER
- contracts in response to cholecystokinin (released in response to fats + proteins in intestine, provides bile for fat emulsification when needed)
- bile salts reabsorbed w/fats through hepatic portal system -> removed from blood by liver to make more