module 8 asci 229

functions of the digestive tract

prehension
-grasping or ripping or tearing the food with the lips and teeth
mastication
-chewing and grinding with the teeth
digestion
-enzymatic
-fermentation
absorption
elimination

what does gastro- or gastric refer to anatomy wise?

what does gastro- or gastric refer to anatomy wise?

what does enteric or entero- refer to anatomy wise?

the intestines
-often used to refer just to the small intestine
-colonic = referring to large intestine

what are the layers/walls of the stomach and intestine (innermost to outermost)?

mucosa
-composed of 3 layers itself
submucosa
muscularis layer
-composed of 2 layers of smooth muscle
serosa

what are the 3 layers that make up the mucosa?

epithelium (nearest the lumen)
lamina propria
muscularis mucosae

t/f: most of the intestinal tract is suspended from the dorsal aspect of the abdominal cavity by mesentery

true
-it attaches the GI tract to the dorsal aspect of the abdominal cavity

what does mesentery contain?

blood vessels, lymph vessels, and nerves supplying the tract (arteries, veins)

what happens if mesenteric blood supply is compromised?

the intestines will twist on themselves

what types of muscle tissue in present in the digestive tube?

skeletal muscle and smooth muscle

is skeletal muscle voluntary or involuntary?

voluntary to facilitate initiation of eating and control of elimination

where is skeletal muscle found within the digestive tract?

found in the mouth, tongue, pharynx, parts of the esophagus, outer layer of anal sphincter

is smooth muscle voluntary or involuntary?

involuntary which coats the rest of the tube

where is smooth muscle found in the digestive tract?

lower esophagus, stomach, SI, LI, internal layer of anal sphincter

where are noncornified stratified squamous epithelial cells found?

oral cavity and rectum

what type of epithelial cells make up most of the GI tract?

simple columnar epithelial cells
-line the external surface of each villus and each have microvilli (hairlike projections) which greatly increase the absorptive surface area

what are goblet cells?

mucus-secreting cells interspersed among the columnar cells

how is motility in the GI tract regulated?

by a system of the CNS in combination with the endocrine (hormonal) system AND the enteric or intrinsic system

what are the 2 nerve plexuses involved with motility in the GI tract?

the submucosal and myenteric plexuses
-the nerve plexuses themselves are a layer of nerve fibers that run the length of the GIT wall itself

how is smooth muscle arranged along the digestive tube?

it is arranged in circular and longitudal layers

circular muscle contraction in the GI tract

circular muscle contraction narrows the tube in that segment
-circular contractions close behind a food bolus and travel along the length of the tube, pushing the bolus forward

longitudinal muscle contraction in the GI tract

longitudinal muscle contraction shortens the length of that segment
-this also helps move a food bolus forward along the tube

what are the 2 kinds of intestinal contractions?

peristalsis
segmental

peristalsis contraction

propels ingesta forward
alternating waves of contraction

segmental contraction

retains and slows passage of ingest for mixing and absorption
moving materials back and forth to aid in mixing

what is the crown in animal dentition?

the portion above the gum line

what is the root in animal dentition?

the portion below the gum line

what is the neck in animal dentition?

the area between the crown and the root

hyposodant teeth

large and grow continuously throughout most of an animal's life

what is the difference between radicular hyposodont teeth and aradicular hyposodont teeth?

radicular: have apex of the tooth root open throughout life, enabling continual growth
-found in horses
aradicular: lack a true root but still grow continuously
-found in lagomorphs and rodents

what is floating?

a term for using a rasp to file off sharp points and edges of teeth in horses

what is the order of canine dentition (front of mouth to back)?

incisors
canine teeth
premolars
molars

what is the intestinal mucosal barrier?

the tightly selective impermeability of the lining epithelium cells

t/f: the digestive tract begins at the oral cavity

true

what is the role of saliva in the digestive process?

begins enzymatic digestion
buffering rumen and gastric acids

what digestive enzymes contained in saliva begin digestion?

amylase (in some species)
-begins starch breakdown
lysozome
-controls bacterial populations
lipase (in some species)
-begins fat breakdown

t/f: saliva is rich in bicarbonate and phosphate

true
-this buffers the VFAs in the rumen and Hal in the stomach (of monogastrics)

how are salivary glands controlled?

controlled autonomically
-PSNS: increases saliva production
-SNS: causes dry mouth when we are nervous or frightened

what are the diff types of salivary glands?

parotid salivary glands
mandibular salivary glands
sublingual salivary glands

t/f: the salivary glands are exocrine glands

true
-determined by the fact that they all empty saliva into ducts that empty into the oral cavity

esophagus

point of entry into stomach = cardiac sphincter or cardia

what is choke?

it is esophageal obstruction caused by dry feed, non dissolvable food items (carrots, apples, potatoes), or even masses of grass
-occurs most commonly in horses and rabbits as they are unable to spontaneously vomit

what regions is the mono gastric stomach divided into?

cardia
fundus
body
pyloric antrum
pylorus
lesser curvature
greater curvature

what is secreted within the fundus and body?

parietal cells: HCl and intrinsic factor
-intrinsic factor is a protein necessary for vit B12 absorption in the SI
chief cells: pepsinogen
mucous cells: mucus
enterochromaffin-like cells: histamine

what is secreted within the pyloric antrum?

G cells (endocrine): gastrin

what is the purpose rugae?

transient folds of gastric mucosa that allow the stomach to expand and increase surface area

where are the gastric glands found?

inside of the deep invaginations of the mucosal surface of the stomach (gastric pits)

how do parietal cells secrete HCl into the gastric lumen?

H+ ions and Cl- ions are secreted separately and then combine within the lumen to form HCl
secreted by ions pumps (therefore energy-req) on apex of parietal cells

what is the source of intrinsic factor in felids?

the pancreas

how is pepsinogen converted to pepsin and how is it inactivated?

pepsinogen = precursor form of an enzyme
converted to pepsin in presence of HCl
inactivated by the higher pH encountered as ingesta enters duodenum

what is the purpose of pepsin in regard to digestion?

it breaks down proteins into polypeptides and peptides

what is the pylorus and its purpose?

a concentrated area of circular muscle fibers, functions as a sphincter

what does the rate of gastric emptying depend on?

it depends on how active the pyloric antral contractions are versus how tight the pyloric sphincter tone is

what increases the rate of gastric emptying?

stretching of the stomach by food
-releases ACh from PSNS nerve endings that increase the rate and strength of antral contractions
presence of gastrin
-increases muscle contraction while dilating pyloric sphincter

what decreases the rate of gastric emptying?

all inhibit antral contraction and tightens pyloric sphincter
-presence of chyme in duodenum
-high fat or high protein meal
-hypo- or hyperosmotic chyme
-acidic chyme

what is mechanical digestion?

the breaking down of food into small particle sizes by chewing with the teeth and then pulverizing, churning action of the stomach and intestines

what is chemical digestion?

a series of hydrolysis reactions in which macromolecules are reduced into their composite monomers
-carbs to monosaccharides
-proteins to amino acids
-lipids to glycerol + free fatty acids

description of the ruminant stomach

one stomach, 4 compartments
-first 3 are the 'forestomachs'; non glandular (non secretory cells in pit glands)
-4th compartment is the true acid (glandular) stomach

t/f: ruminants do most of their digesting before the intestinal tract

true

what are the 3 fore stomachs of the ruminant?

reticulum
rumen
omasum

what is the acid stomach of the ruminant?

abomasum

reticulum

most cranial compartment - lies just ventral to the esophageal entrance
honeycomb appearance
-mechanism for increasing surface area and trapping foreign bodies

rumen

largest fore stomach compartment
fermentation vat: plant material fermented into VFAs (no enzymatic digestion)

what are the rumen pillars?

muscular folds that divide the rumen into sacs

why is the rumen + reticulum referred to as the reticulorumen?

because they communicate with each other

how are the enzymes made for the fermentation within the rumen?

they are made by the microbes (microflora) that populate the rumen
-bacteria, protozoa, and fungi (ruminant host won't be healthy if these resident microflora aren't healthy)

how do the microflora aid in the digestion of cellulose?

microflora cellulose enzymes change cellulose into simple sugars and then into VFAs
-ruminants use VFAs like other animals use glucose
-of the 3 VFAs, propionic acid is used in the liver to make glucose

how do the microflora aid in the digestion of proteins?

microflora render proteins into peptides and amino acids
-peptides and HAs either incorporated into the microbe's own system or converted to ammonia (NH4+) and VFAs

what is hardware disease (aka TRP - traumatic reticulopericarditis)?

a syndrome of heart failure arising from pericarditis secondary to piercing by sharp objects caught in the reticulum

what do reticuloruminal contractions enable?

enables material from the rumen and reticulum to travel back up the esophagus to the oral cavity

how do reticuloruminal contractions benefit the animal?

regurgitation of small portions of rumen contents enables further chewing
-rumination = chewing the cud
enables removal of fermentative gases (CO2 and methane)
-eructation = releasing a bolus of ruminal gas up the esophagus into the mouth

omasum

very muscular
inner surface had numerous parallel leaf-like folds
-muscular action and surface area further break down particulate matter
-also absorbs VFAs and H2O

abomasum

true acid stomach in ruminants; function same as in monogastrics
-have same types of gastric glands
has mucosal folds to increase surface area

t/f: neonatal ruminants' digestive tracts function as a mono gastric system

true

how is milk directed to the omasum rather than the reticulum in neonatal ruminants?

via the esophageal groove (aka reticular groove)

parts of the small intestine

duodenum
jejunum
ileum

parts of the large intestine

cecum
large colon
-ascending
-transverse
-descending
rectum

what part of the digestive tract varies among species?

length/size of the large intestine

t/f: horse digestion is all about the cecum and large colon = the 'hindgut'

true

order of hindgut in horses

cecum
ascending colon
-right ventral colon
-sternal flexure
-left ventral colon
-pelvic flexure
-left dorsal colon
-diaphragmatic flexure
-right dorsal colon
transverse colon
small colon
rectum

what is colic and what can it be caused by?

means 'abdominal pain'
-clinical sign or symptom, not a diagnosis
can be caused by volvulus or twist, impaction, inflammation, other

what makes up most of the SI length?

the jejunum

t/f: the small intestine is the site for enzymatic/chemical digestion

true

what are the 3 magnitudes of mucosal folding to increase the surface area for nutrient exposure to digestive enzymes and for absorption?

inner circular folds (plicae)
the mucosa on the plicae is thrown up into fingerlike projections called villi
the villi themselves are lined by columnar epithelial cells that have tiny surface projections called microvilli

what is the SI's major function?

to absorb nutrients

what digestive enzymes does the duodenum secrete?

cholecystokinin (CCK)
secretin
enteropeptidase

what is CCK secretion increased by?

high amino acid content, high fatty acid content, low chyme pH

what are the functions of CCK?

inhibits gastric emptying, so chyme can exit in controlled matter
increases release of pacreatic digestive paraenzymes
triggers gallbladder contraction and ejection of bile into duodenum
stimulates release of enteropeptidase

pancreas

contains both endocrine and exocrine glandular tissue
all the pancreas' outputs are involved with digestion and metabolism

what does the exocrine part of the pancreas do?

it secretes the digestive enzymes in the duodenum

what does the endocrine part of the pancreas do?

it secretes hormones into the bloodstream that will regulate metabolism of the nutrients that will be absorbed by the action of the exocrine enzymes

what are the endocrine products of the pancreas?

insulin: synthesized by beta cells in the islets of langerhaans
-decreases blood glucose by stimulating cell to take it up
glucagon: synthesized by alpha cells
-increases blood glucose by stimulating liver to make and release glucose

what are the exocrine products of the pancreas?

amylase, lipase, nuclease, and proteases: secreted directly into SI lumen in an inactive precursor form
HCO3- is also secreted into the duodenum to alkalinize chyme

what are the proenzymes (proteases) secreted into the duodenum?

trypsinogen, chymotrypsinogen, proelastase, and procarboxypeptidase

how are the proenzymes activated?

by CCK stimulating duodenal mucosal cells to secrete enteropeptidase (EP)
-EP activated trypsinogen into trypsin
-trypsin activates all the other proenzymes

why is the liver well named?

because it is vital to so many necessary functions that it is what makes animals able to 'live'

what are some of the functions of the liver?

glucose metabolism and keeping blood glucose levels stable
synthesizes and secretes bile which is necessary in fat digestion
makes nearly all the other plasma proteins (except immunoglobulins)

what are the 6 lobes that make up the liver in carnivores?

right medial
right lateral
left medial
left lateral
caudate
quadrate

liver histology

liver tissue is arranged into tiny lobules
-in the center of each lobule is the central vein
-at the periphery of the lobules are the hepatic triads (hepatic artery, portal vein, bile duct)

t/f: bile is necessary for the digestion of fats

true

what is emulsification?

the breaking down of fat globules into smaller droplets
-results in vastly increased surface area for the next steps in digestion