1.1 Comparative Digestive Anatomy Flashcards
What is the definition of digestion? What does it involve?
- Process that breaks feed down into simple substances that can be absorbed
- Involves mechanical and chemical action of enzymes
What is the definition of absorption?
Process of taking nutrients from the digested feed into the blood and lymph systems
- nutrients are absorbed into the blood stream, directly or indirectly
In monogastrics, where does most absorption occur?
Small intestine! Less amount in hindgut.
Oral cavity of a chicken vs a pig
Chicken
- birds use the beak (structure of keratin) to obtain bite-sized pieces of food
- the tongue is controlled by the hyoid apparatus (several bones) that has limited mobility; helps instead with obtaining and swallowing, but not with moving bolus around
Pig
- teeth, tongue, and salivary glands
- chewing action (mechanical part of digestion) wherein food is cut and torn, mixed with saliva
What are the salivary glands in birds well developed for?
Eating dry diets
Is amylase secretion important in the oral cavity of chickens?
Not active in oral cavity bc feed spends a limited amount of time there
- active in crop and proventriculus (if present)
Salivary glands of a pig
- 3 paired sets
- located under lower jaw and ear
Why is there little time for digestive activity in the oral cavity of chickens?
Residence time in oral cavity is minimal bc no chewing is taking place
- if enzymes are present, activity takes place in the crop
How does digestive activity take place in the oral cavity of the pig?
- Saliva contains water, mucin, bicarbonate salts and enzymes
- amylase and maltase
- stimulates taste nerves
- bicarbonate salts buffer pH in stomach - Water moistens feed for chewing and swallowing
- Mucin lubricates feed for swallowing
Importance of feed taste?
Important for weaning piglets; taste is less important for chickens
What does the esophagus connect?
The oral cavity with the stomach
Chicken esophagus
Larger diameter relative to mammals
- expandable (food being swallowed whole)
- mucus, immune & antimicrobial secretions
Many bird species have a crop = modification of the esophagus for storage
Pig esophagus
Simple tube
What does a true crop have?
A controlling sphincter which regulates entrance and exit of food
- in the wild, the bird can forage for a large amount of food at once, then digest in safety
What does the crop do?
Stores food (briefly) for anticipated need
- laying hens will increase calcium intake in anticipation of lights off (bc most of the egg shell is deposited at night)
- galliformes, hummingbirds, others; dusk feeding can supply a large proportion of energy needs during the night
What happens if the gizzard is empty vs if it’s full?
Empty = food goes directly into gizzard
Full = food stored in crop
Beyond storage, what are 3 fxns of the crop?
- Secretion of mucus
- lubrication of bolus
- warm, moist enviro softens food - Enzyme activity
- little/no endogenous enzyme secretion, INSTEAD:
- retrograde peristalsis
- enzymes present in feed
- microbial fermentation - Immune fxn
Because there is little/no endogenous enzyme secretion in the crop, where do enzymes come from to fulfill enzyme activity?
- Retrograde peristalsis
- Feed
- Microbial fermentation
Chicken stomach vs Pig stomach
Chicken: 2 organs
1. Proventriculus: glandular stomach (secretions)
2. Gizzard: muscular stomach (mixing)
Pig: 1 organ
- 4 distinct parts
How is the proventriculus/gizzard affected by a whole wheat diet vs a ground wheat diet?
Whole wheat diet:
- better gizzard fxn = more muscular and a v big distinction btw proventriculus and gizzard
Ground wheat diet:
- flabby gizzard, no big distinction
Chicken proventriculus
- glandular stomach
- papillae on surface; gastric glands open at apex
- HCl and pepsinogen secreted by gastric glands converted to pepsin
4 regions of a pig stomach
*1 organ, many fxns
- Esophageal region
- extension of esophagus - Cardiac gland region
- mucous cells: mucus, proteases, lipase - Fundic (gastric gland) region
- mucous cells
- parietal cells: HCl
- chief cells: proteases - Pyloric gland region
- mucous cells
- chief cells
Gastric juices are secreted by glands in the stomach and proventriculus walls, what does this do?
Contains HCl = drops pH
How does starch digestion in the pig’s stomach differ from starch digestion in the chicken’s proventriculus/gizzard?
Not a lot of starch digestion takes place in the proventriculus/gizzard, unlike the pig where there is starch digestion taking place in the stomach.
- Salivary amylases from oral cavity of pigs are still active to a certain extent in the stomach = continued starch digestion
- Little starch digestion in birds; some attributed to retrograde peristalsis but those amylases are not suited for being active at the low pH
How does protein digestion start in the stomach/ proventriculus & gizzard?
Acidic enviro denatures protein = more points of attack for proteolytic enzymes
Why is low pH important in the stomach/ proventriculus & gizzard?
- Protein digestion
- conversion of inactive protease (pepsinogen) into more active protease (pepsin) for protein digestion
- protein denaturation - Chemical barrier for microbial passage into the SI
- acidic enviro kills a lot of pathogenic bacteria
What happens in the proventriculus?
Enzymes and acids are added
Where is an important point of retrograde peristalsis in chickens?
Between the gizzard and proventriculus
- material goes through the pro to the giz where it is ground and mixed, then goes BACK to the pro where more enzymes etc. are added
- continual mixing; some material goes from giz to SI or from giz to pro (retrograde peristalsis)
Chicken gizzard
Muscular stomach
- grinds food to reduce particle size and increase SA
- most peptic digestion occurs here
- larger/harder particle size = more important the gizzard; size and thickness of gizzard dependent on food type
What is the size of the gizzard and thickness of muscle dependent on?
Food type
Pig stomach
Stomach wall lined with muscle
- churns and squeezes the feed; particle size reduction fxn is achieved largely in the mouth
- gastric emptying forces liquid and feed into SI
What is the cuticle (or koilin membrane) of the gizzard?
Covers the muscle of the gizzard and protects muscle from being degraded by proteolytic activity
- made up of protein and dead cells
- grinding, abrasive surface
- protects tissue from enzymes and acid
- thickness of cuticle dependent on type of diet
What are “gizzard teeth”?
Dentate processes
- rods projecting from tubular glands to koilin surface
- act like sandpaper to grind feed
What is the retention time of food in the gizzard dependent on?
*gizzard is the rate limiting step
- minutes to hours, depending on particle size
- ridges retain larger particles; smaller particles move along the bottom of the furrow
What happens in the SI of both pigs and chickens?
Absorption of nutrients
- secretion of enzymes and hormones
What are the 3 segments of the SI in both chickens and pigs?
- duodenum
- jejunum
- ileum
What are the 3 arbitrary distinctions between segments of the chicken SI?
- Duodenum
- first loop of SI, surrounds pancreas (attached) - Jejunum
- from duodenum to Meckel’s diverticulum
- attached to residual yolk sac
- attachment point of yolk during embryo development - Ileum
- from Meckel’s diverticulum to cecal jxn
What are the 3 arbitrary distinctions between segments of the pig SI?
*based on relative length, not anatomical landmarks like the chicken
- Duodenum
- 4-5%, first part - Jejunum
- 88-91%, middle part - Ileum
- 4-5%, last part
What are 3 sources of secretions into the SI?
- Pancreas
- enzymes, bicarbonate buffer - Liver
- bile acids/salts - Intestinal walls
- enzymes, mucus (rich in threonine, proline)
Where does active digestion take place?
SI
Where does most nutrient absorption occur in the SI?
Jejunum and ileum
Enzyme activity in small intestine
Luminal = pancreatic enzymes
Mucosal = brush border enzymes
Starch
starch = maltose = glucose
Proteins
proteins = poly-, oligo-, di-peptides and AA = simple AA
Fats
fats = fatty acids, glycerol and monoglycerides
Musculature of the Si
Inner circular layer and outer longitudinal layer
- reverse peristalsis in birds leads to a more efficient digestive tract
What do mammals have, that chickens do not in terms of the SI?
Central lacteals
- part of the lymphatic system in mammals that absorb fat from the SI
- in chickens, fat is absorbed into portal blood
What are villi? What do they contain?
- Small cone-shaped projections on the wall of the SI
- Contain a network of blood capillaries through which nutrients enter the blood stream
What are microvilli (brush border)? What are their 3 functions?
Tiny projections from the epithelial cells (enterocytes on the villus) of the intestine (especially SI)
- further increase SA
- digestive enzymes
- nutrient absorption
The SI can broadly be described as being…
Multi-cellular and multi-functional
Multi-cellular and multi-functional roles of the SI
Enterocytes
- nutrient assimilation
Goblet cells, lymphocytes, macrophages
- immune surveillance and host defense
Enteric neurons
- autonomous nervous system
Entero-endocrine cells
- major endocrine fxn
Microbes
- commensal microflora
Why is there a high metabolic cost of a functional intestine?
- Cell turnover
- Nutrient absorption involves active transport
- Several dietary AA used as fuel
What are 3 endogenous nutrients?
Enzymes, mucus, cells
- subject to digestion as dietary (exogenous) nutrients
Chickens - endogenous nutrients
- Endogenous nutrients high, but efficiently recovered
- Retrograde movement allows secretions to be moved up intestine to site of absorption
- Some absorption of microbial nutrients such as vitamins
Pigs - endogenous nutrients
- Unidirectional flow of digesta
- Poorer recovery of endogenous nutrients
What are the 2 main functions of the SI?
- Vasculature
- supply nutrients to tissues
- absorbed nutrients taken to liver - Immune system (lymphocytes)
- diffusely in lamina propria
- aggregated in Peyer’s patches
- more numerous in lower SI
What are the 2 main functions of the hindgut?
- Microbial fermentation of undigested feed
- Pigs and poultry need bacteria to degrade fiber
- Main contribution of microbial fermentation to the overall nutrition of the animal are VFA - Absorb water and electrolytes
Chicken hindgut
- Colon (rectum)
- intestine btw the ileocecal jxn and the cloaca
- short, small in diameter - Ceca
- paired organs at the jxn of the small intestine and colon
- ileocecal valves control entry of digesta
- fermentation chambers —> VFA
- water/nitrogen absorption
- immunosurveillance (a lot of immune cells present) - Cloaca
- Vent
Pig hindgut
- Large intestine
- Cecum: location for fermentation
- Colon: largest part of the LI
- Rectum - Anus
- external opening at the end of the DT
What does the rectum in birds open into?
The cloaca
Bird colon
- rectum opens into the cloaca
- larger diameter than the rectum; storage area for excreta
- receives ureters and ducts of the reproductive tract
Chickens excrete…
Excreta = feces + urine (primarily uric acid)
*feces are material that is not digested/absorbed
