Digestive system Flashcards
What does the GI tract consist of?
Mouth, esophagus, stomach, small intestine, large intestine, caecum, rectum
what components are added to the GI tract to create the digestive system?
- salivary glands, gall-bladder, liver, pancreas
general function of the mouth in digestion
- mechanical breakdown of food
general function of the esophagus in digestion
- unidirectional tube that connects the mouth to the stomach
general function of the stomach in digestion
- holds food
- distinguishes the 4 systems from each other
- can be monogastric, avian, or 4 compartment
general function of the small intestine in digestion
- takes up nutrients into the body
- “nutrient gateway”
general function of the large intestine in digestion
- flows into caecum
- majority of gut bacteria resides here
general function of the caecum in digestion
- abundance of gut bacteria
- roll in breaking down indigestible carbs
general function of the rectum in digestion
- things not digested are excreted from the body
what are different ways you can classify a dietary CHO by for digestion?
- solubility
- digestability
- fermentability
solubility of CHOs
is a CHO soluble in the aqueous environment of the digestive tract?
yes = soluble
no = insoluble
- determined by physicochemical properties of CHO
digestability of CHOs
does the host organism have the enzymes necessary to digest a CHO?
yes = digestible
no = not digestible (fibre)
- determined by the host digestive enzymes
fermentability of CHOs
do gut bacteria have the enzymes necessary to break down a CHO?
yes = fermentable
no = non-fermentable
- determines by gut bacteria
4 types of digestive systems
- simple system (w/o caecum)
- simple system (w/ functional caecum)
- ruminant system
4.avian system
simple system w/o caecum: key features
- human, cat, pig, dog
- monogastric (single stomach)
- non-functional caecum (still there but does not contribute to fermentation of digestible CHO)
- hindgut fermenter
- suited for a nutrient-dense, low fibre diet
how does the oral cavity work in a simple system?
- food is chewed and mixed with saliva
- 2 enzymes get the ball rolling…
1. a-amylase: starts to break down CHO
2. lingual lipase: starts to break down dietary fats
How does the stomach work in a simple system?
- includes the cardia, fundus, body and antrum
- empty = 50mL, filled = 1-1.5L (remarkable ability to expand when food is consumed)
- gastric emptying takes 2-6 hours
- pH is about 2 (acidic)
- food becomes chyme (food + acid)
- gastric glands secrete gastric juice (includes H2O, electrolytes, HCl, enzymes)
how does the small intestine work in a simple system?
- includes the duodenum, jejunum and ileum
- main site for nutrient digestion and absorption
- surface area = 30m^2
- intestinal motility is controlled by longitudinal and circular muscles
- chyme acidity neutralized by pancreatic juice
- food is digested by pancreatic juice and bile acids
- soluble fibre = slows down digestion, insoluble = speed up
how does the large intestine (aka colon) work in a simple system?
- by the time food reached the LI most nutrients have already been taken up
- site of fermentation - break down indigestible carbs
- production of SCFA aka volatile fatty acids
- site for water absorption
- bile acids are taken back up and recycled
What gives the small intestine such a large surface area
- kercking folds
- villi (and crypts) - pertrusions
- microvilli
What are the different nutrient transport mechanisms for molecules to pass through the intestinal lumen?
- Diffusion
- molecules follow a concentration gradient
- small molecules with the right chemical properties can pass through with no energy needed - Facilitated diffusion
- like diffusion but involves a transporter - Active transport
- requires energy (ATP) to move molecules against a concentration gradient
how can we decide which transport mechanism to use to get molecules from the intestinal lumen to the cytoplams
- solubility of the molecule
- concentration gradient
- molecular size
Gut bacteria
- region specific - not evenly distributed along the tract
- 1000:1 anaerobic to aerobic bacteria
- very important for the fermentation of non-digestible CHO
- CHO fermentation produces many compounds (e.g lactate and SCFA)
- associated with disease
simple system w/ functional caecum: key features
- horse, rabbit and hamster
- pseudo-ruminant
- hindgut fermenter
- functional caecum
- all other regions of the gut function similar to the monogastric system
- suited for a diet with large amounts of fodder and foraging
what is the purpose of a functional caecum in a simple system?
- enormous hindgut (20-30L capacity) filled with bacteria
- SCFA provide 70% of total energy needs for host
- site for the production of vitamins
what are signs of an energy or nutrient deficiency for animals with a functional caecum?
- Coprophagy (eating dung or feces)
- young animals eating feces colonize their guts with bacteria
where are nutrients absorbed in a simple system w/ a functional caecum?
stomach: nothing
small intestine: glucose, amino acids, fatty acids
large intestine/caecum: lactic acid, amino acids, SCFA
- indigestible fibre does not get absorbed
- few of the fat, proteins and CHO are not digested and are excreted from the body
lactic acid in horses
- lactic acid is produced in a diet high in carbs for horses
- horses cannot vomit
- colic will develop if there is too much lactic acid produced
Ruminant (multiple) system: key features
- cattle, sheep, goats
- stomach is divided into 4 regions: reticulum, rumen, omasum, abomasum
- system highly suited for animals that eat a high quantity of fodder and forage plant materials
- all other regions of the gut function similar to the monogastric system
ruminant digestion: reticulum
- honeycomb appearance serves as a filter: can capture and trap foreign materials that are accidentally swallowed
- can cause hardware diease
- rich in bacteria (fermentation vat)
ruminant digestion: rumen
- the largest section of the stomach
- rich in bacteria (fermentation vat)
- rumen papillae: similar to vili, increase surface area for absorption
- food is mixed and partially broken down and stored temporarily
- 60-80% of total energy produced here as SCFA
ruminant digestion: omasum
- resorption of water and some electrolytes
- filters large particles
ruminant digestion: Abomasum
digestive enzymes secreted from gastric glasnds (HCl, mucin, pepsinogen, lipase, etc)
fermentation in the ruminant system
- fermentation takes place before entering the intestine (foregut digestion)
- nutrients produced by bacteria are available for digestion and absorption by the ruminant
2 ways to get rid of products of fermentation…
1. rumination: partially digested food called cud is regurgitated and chewed further then re-swallowed
2. eructation: periodic belching where methane gas is released from the stomach - can be bad because energy is lost
pros and cons of the ruminant system
pros
- vitamin synthesis (B and K)
- non-protein nitrogen used for making protein
cons
- carbohydrates degraded into gases and lost through eructation
- heat production (large methane released)
avian system: key features
- chickens, turkeys
- beaks and claws break up foods into smaller pieces that birds can swallow (don’t have teeth)
- rapid digestion (can starve if deprived of food for even a few hours)
unique components of the avian system
- crop
- two chambered stomach
- small intestine
- ceca
- large intestine (slight different function)
- cloaca
avian system: crop
- enlarged area of the esophagus
- well developed in most species but not in all
- temporary storage location for food
- food is softened here and often regurgitated to feed offspring
avian system: two-chambered stomach
Glandular portion: proventriculus
- chemical digestion, gastric enzymes and HCl secreted
- most comparable with monogastric stomach
Muscular portion: Gizzard
- has stone-like objects used for mechanical breakdown
- grind and digest tough food
avian system: ceca
- minor site of bacterial fermentation
- fermentation is not critical for avians but happens because bacteria are here
avian system: large intestine
- very short and serves predominantly to connect the small intestine and cloaca
- a bit of storage of undigested material
- water absorption
avian system: cloaca
- very unique
- where the digestive, urinary and reproductive system meets
- makes it difficult to understand how much is actually absorbed in birds
What is digestibility?
- measure of the fraction of a specific nutrient (or of energy) that is extracted by the GI tract
-how much nutrient actually gets absorbed by the body - calculated from the amount of nutrient consumed and the amount appearing in feces
- represents a combination of nutrient release from the food matrix, microbial fermentation and absorption
why is determining digestibility important?
- can prevent deficiency and ensure essential nutrients are available to the organism ( to support their lifestyle)
measuring digestibility: total collection method
- measures everything that goes in (food) and comes out (feces)
- allow the animal to adapt to a diet over 7-21 days
- isolate the animal for quantitative analyses
- measure intake over a 3-10 day period
- collect and weight all feces
- analyze for nutrient
how do you calculate the apparent digestibility coefficient?
apparent digestibility coefficient = (total intake - total feces) / total intake
the closer to 100%, = almost complete absorption of the nutrient
limitations of the total collection method
- accuracy in measuring food intake (e.g. crumbs)
- metabolic cages create anxiety in animals, causing them to behave abnormally
- labour intensive
- animals confined in costly equipment
- not feasible for captive wild animals (not much control)
what do metabolic cages do
collect and analyze urine and feces
measuring digestibility: Indicator method
- incorporates a marker into the food and tracks it
- internal marker: a natural component of the feed
- external marker: a component added to the feed
- adapt the animal to test diet containing the marker
- collect a feed and fecal sample
- analyze each for the marker and nutrient of interest relative to your indicator
what is the difference between the total collection method and the indicator method
- indicator method just requires a sample since the marker will appear in both the feed and feces
what are characteristics of a marker for the total collection method?
- non-absorbable
- must not affect / be affected by the GI tract (not upset stomach)
- must mix easily with the food
- easily and accurately measured in samples
apparent digestibility coefficient for indicator method
(A - B) / A
A = ratio of nutrient/marker in the feed
B = ratio of nutrient/marker in feces
advantages of indicator method
- less labour intensive
- ideal for wild animals
What is apparent digestibility?
- under-estimates the true digestibility
doesn’t take into account…
1. endogenous secretions - epithelial cells (e.g. fatty actids released from dying intestinal cells)
- secreted into lumen and appear in feces
2. Bacterial growth in gut - nutrient synthesis takes place (e.g. biotin produced by gut bacteria)
3. Digestive enzymes - protein secretion (e.g. digestive enzymes released by cells)
What is true digestibility?
- perform a digestibility study using a test diet
- switch to diet containing none of the nutrient of interest
- analyze the feces after test diet is cleared
- subtract the level of nutrient in feces of animals fed the zero nutrient diet from a test diet
how can you calculate the true digestibility coefficient?
use indicator marker method
(A - (B -C)) / A
A = ratio of nutrient/marker in TEST DIET
B = ratio of nutrient/marker in feces
C = ratio of nutrient/marker in feces after ZERO NUTRIENT DIET
what are some factors that affect digestibility?
- feed intake (what are they eating?)
- particle size (smaller particles have faster transit time)
- chemical composition (what nutrients?)
- climate (hotter temps have different digestibility than colder)
- age (younger can have different efficiency as older)
