GI System

Monogastric

• simple stomach

• pig, poultry

Ruminant

• 4 compartments

• anaerobic fermentation of forages prior to absorption

• cattle, sheep, goats

hindgut fermenter

• simple stomach (very complex large intestine)

• anaerobic fermentation of forages AFTER absorption

• horses, rabbits

functions of the digestive tract

1. Prehension: obtain a food source

2. Digestion: breakdown a food source

3. Absorption: absorb the food source into the body

4. Elimination: eliminate solid wastes that could not be absorbed

prehension

• grasping feed and bring it into the mouth

• involves coordinated use of lips, tongue, teeth, or beak

• precedes mastication

prehension (cattle)

tongue, lower incisors, dental pad

prehension (sheep & goats)

mobile lips, lower incisors, dental pad

horses (prehension)

mobile lips, upper & lower incisors

pigs (prehension)

snout, lips, & teeth

poultry (prehension)

beak

mastication

the act or process of chewing to break down food particles

digestion

• the physiological process of breaking feedstuff into their smallest pieces that can be absorbed by the body

• occurs starting in the mouth and ending in the anus

• nutrients must be absorbed to have effects on the body

mechanical digestion

physically breaking down food into smaller particles

chemical digestion

use of chemicals to break down molecules (pH)

enzymatic digestion

enzymes break down large molecules into their smaller parts

microbial digestion

microorganisms digest nutrients consumed by animal

mouth

• holding, grinding, and mixing food with saliva

• mastication

• physical digestion with some enzymatic

dentation

teeth

incisors

• cutting teeth

• grab and hold food

molars

• flat

• grind fibrous material

diastema

• gap between incisors and premolars

carnivores

• canines

• sharp for tearing, seizing, and piercing

salivary glands

• serous acinar cells

• mucous acinar cells

• pigs: 4 gallons

• horses: 10 gallons; produced during chewing

• ruminants (cows): 25-38 gallons per day; serve as buffer and lubrication

serous acinar cells

watery, protein rich secretion

mucous acinar cells

secrete mucins

esophagus

• transports bolus of food from mouth to stomach

• epiglottis covers trachea during swallowing

• smooth muscle

  • contracts via peristalsis

  • wave-like movements

peristalsis

• involuntary, wave-like muscle contractions that moves food, liquid, and waste products through the bodies digetive tract

esophageal groove

• young ruminant animals

• redirects milk from the reticulorumen into the abomasum

• avoids microbial fermentation of milk

reticulorumen

• work together as the fermentation chamber

reticulum

• smaller cranial compartment

• honeycomb

rumen

• stores and mixes feed while microbes ferment it

• papillae increases SA for absorption of VFAs

omasum

• smaller, denser particles pass through to the omasum

• contains leaf-like folds called laminae

• absorbs water, electrolytes, and remaining VFAs

found in all stomachs

• chief cells

• parietal cells

• g-cells

• goblet cells

chief cells

• produce pepsinogen to break down proteins

• gastric lipase to breakdown fats

parietal cells

• produce gastric acid (hydrochloric acid) to break down organic matter

G-cells

• produce gastrin to stimulate acid secretion

goblet cells

produce mucus

stomach sphincter

• muscular gates that regulate movement between regions of the GI tract

• cardiac sphincter

• pyloric sphincter

cardiac sphincter

• lower esophageal sphincter

• functional zone of increased muscle tone

pyloric sphincter

• thickened smooth muscle

• regulates chyme entering small intestine

small intestine

• long winding tube structure that connects the stomach to the large intestine

• duodenum

• jejunum

• ilium

duodenum

• Latin for “12 fingers”

• pancreatic bile duct

• chemical digestion

• plicae circulares: folds within the lumen to increase SA

• duodenal papilla: enzymes flow into duodenum

• Brunner’s glands: secrete bicarbonate-rich mucus to neutralize acidic chyme

jejunum

major site of nutrient absorption

ileum

• bile salts absorption

• immune tissue- Peyer’s patches

liver

produces bile

gall bladder

stores bile

pancreas

releases enzymes into common bile duct

• exocrine function

  • acinar cells: secrete digestive enzymes

• endocrine function

  • alpha cells: glucagon

  • beta cells: insulin

  • delta cells: somatostatin

  • F cells: pancreatic polypeptide

    • regulates pancreatic secretion

pancreatic enzymes

• carbohydrates: amylase

• protein: trypsinogen, chymotrypsinogen, and carboxypeptidase

• fats: lipase

• travel to duodenum via pancreatic duct into the common bile duct via sphincter of Oddi

protein breakdown

1. Food enters the duodenum, and enteropeptidase is activated

2. Trypsinogen and chymotrypsinogen are secreted from the pancreas into the duodenum

3. Enteropeptidase activates trypsinogen into trypsin

4. Trypsin activates chymotrypsinogen into chymotrypsin

5. Trypsin & Chymotrypsin break down proteins in the lumen but not in the body

jejunum & ileum

• main sites of absorption

• enterocytes

• goblet cells

• Villi contain

  • artery

  • vein

  • lacteal

enterocytes

absorption of nutrients

cecum

• blind pouch immediately after SI

• digestion of cellulose

• absorption of VFAs

• hindgut fermenters

  • primary fermentation vat

  • nutrients remaining after absorption

• ruminants

  • secondary microbial fermentation vat

large intestine

• water reabsorption

• fecal formation

• coiling fits large mass into smaller area

mesentary

• keeps the intestines anchored to prevent them from being tangled or displaced

• contains blood vessels that supply nutrients and oxygen to the intestines

• plays a role in lymphatic drainage (immune system)

• contains some fat that protects the abdominal organs

crop (avian anatomy)

• stores feed temporarily

• moistens & softens

proventriculus (avian anatomy)

• true stomach (HCl/pepsinogen)

gizzard (avian anatomy)

• muscular stomach

• mechanical breakdown

caeca

microbial fermentation of undigested material

cloaca

common chamber for digestive, urinary, and repro tracts

liver anatomy

• largest discrete organ

• large reddish-brown, multi-lobed organ

• cranial in the abdominal cavity

• multi-functional:

  • nutrient processing

  • neutralizing waste

  • bile production

hepatocytes

• large polygonal cells arranged in interconnected plates

• positioned along hepatic sinusoids to facilitate nutrient exchange

• Space of Disse

  • narrow space between the sinusoidal endothelium and hepatocytes

  • contains hepatic stellate cells that store vitamin A

• Kupffer cells

  • specialized macrophages located in sinusoidal lining

  • remove bacteria, cellular debris, and damaged blood cells from circulation

“liver layover”

• hepatic portal veins empty nutrient rich blood into the liver

• liver manages nutrient availability with body’s metabolic demands

• major metabolic functions:

  • processing nutrients

  • storing energy

  • redistributing nutrients to other tissues

  • maintains blood glucose

  • manages metabolic waste

carbohydrates in the liver

• absorbed by enterocytes within the small intestine

• Enter enterocyte via…

  • glucose = SGLT1

  • galactose = SGLT1

  • fructose = GLUT5

• Exit enterocyte via GLUT2

  • venous circulation

  • hepatic portal system to liver

glycogenesis

store glucose as glycogen

glycogenolysis

mobilize stored glucose

gluconeogenesis

produce glucose from non-carbohydrates precursors

glycolysis

metabolize glucose for energy

lipogenesis

convert excess carbohydrate-derived carbon into fatty acids

fed animal glucose use

glycolysis-

• hepatocytes use nutrients for cellular metabolism

• ATP production

glycogenesis-

• store as glycogen for later glucose release

released to body-

• returns in venous circulation

• picks up oxygen, travels to tissues

• inc blood glucose = pancreatic insulin

lipogenesis-

• energy high, store excess as fat

energy storage options

glycogen

• short term energy

• primarily stored in the liver and skeletal muscle

• cytoplasm of hepatocytes

• increase release during low blood glucose (glycogenolysis)

fat

• long term energy storage

• acetyl Co-A from Krebs

• accumulation in hepatocytes leads to hepatic lipidosis (fatty liver)

mobilizing energy reserves (fasted animals)

glycogenolysis

• breakdown of hepatic glycogen to release glucose into circulation

• helps maintain blood glucose during fasting

lipolysis

• breakdown of stored triglycerides in adipose tissue

• fatty acids provide energy; glycerol travels to the liver

gluconeogenesis

• liver produces new glucose from glycerol, lactate, and glucogenic amino acids

• requires energy, often supplied by fatty acid oxidation

glucose after the liver

• used for cellular metabolism-ATP production

• excess stored as fat in adipose tissue

ruminant glucose differences

• microbes ferment dietary carbohydrates into volatile fatty acods

• limited glucose absorbed directly

• hepatic gluconeogenesis

  • conversion of propionate to glucose

• ruminants rely heavily on gluconeogenesis, even in fed state, to provide glucose for cellular metabolism

• released in blood stream

proteins

• digested to amino acids and small peptides

• enter enterocytes via

  • amino acids = AA transporters (often Na dependent)

  • Di- and tri peptides = PEPT1

• in the enterocyte

  • small peptides broken down to amino acids

• exit enterocyte via AA transporters

  • venous circulation

  • hepatic portal system to liver

liver protein metabolism

build proteins- albumin, clotting factors, transport proteins

remove nitrogen- amino groups → ammonia → urea

make glucose- glucogenic amino acids carbon skeletons

produce energy- carbon skeletons → ATP production

nitrogen breakdown

• AA that are not used for protein synthesis are broken down

• carbon skeleton ia an energy source

• amino nitrogen group must be transferred or removed

  • ammonia (NH_3)

• liver converts ammonia into urea

  • kidneys for excretion

urea cycle - monogastric

• detoxify ammonia and eliminate excess nitrogen

• primarily excreted through kidneys

urea cycle - ruminants

• detoxify ammonia and regulate nitrogen availability

• excreted through kidneys

• recycled to saliva or rumen

• extensive nitrogen recycling

proteins after the liver

• amino acids enter systemic circulation

• cells use AA to synthesize proteins for growth, repair, or maintenance

• skeletal muscle uses AA to build contractile proteins

• some ATP production

  • cannot be stored- nitrogen must be removed

lipids

• dietary triglycerides are emulsified by bile salts

• pancreatic lipase breaks triglycerides into fatty acids and monoglycerides

• lipids enter enterocytes

• reassembled intro triglycerides and packaged into chylomicrons

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