Digestive GI Tract Anatomy & Physiology

Digestion

Reduction/breakdown of feed particles for absorption to support body functions

Digestive system is composed of 2 categories of organs

1. Digestive organs: form tube that runs from mouth to anus (GIT)

2. Accessory digestive organs

Digestible

Able to be broken down & absorbed

Palatable

Acceptable/satisfactory to eat

Structure and function of GIT varies depending on

diet of the animal: herbivores, carnivores, omnivores

Monogastric Animals

1 simple stomach

Foregut Fermenters/Ruminants

3 mixing & fermentations chambers (microbial populations eat plant material and ferment it into nutritious form) before 1 true stomach chamber

Ex. cattle, goats, sheep

Typically fills rumen rapidly, taking little or no time to chew its meal, and finds a place to rest and chew

Microbial fermentation in forestomach

Rumination/Ruminating

Chewing cud

Hindgut Fermenters

Fermentation occurs in large intestines & cecum (ex. rabbits and horses)

Microbial fermentation of roughage occurs after stomach in large intestines (cecum & colon)

General GI Tract

Mouth (salivary glands) → esophagus → stomach → small intestine (pancreas/liver) → large intestines = cecum + colon → rectum

Mouth

1. Prehend food

2. Initiate mechanical digestion: break food into smaller particles that increase surface area for enzymes involved in chemical digestion

3. Initiate chemical digestion: saliva is produced by paired glands & added to food as it’s chewed to provide moisture (soften/shape food into swallow-able form), enzymes (breakdown food), ions (alkaline buffer), evaporative cooling (dogs)

4. Move food toward pharynx for swallowing

Esophagus

• Muscular tube transports food

• As stomach expands, fold of stomach closes against lower end of esophagus (sphincter)

Stomach (Carnivores & Omnivores)

• Monogastric: 1 stomach compartment (acidic)

• Location: left, upper quadrant of abdomen

• Functions: mechanical & chemical digestion (gastric glands line stomach), limited absorption, temporary storage

Stomach (Herbivores)

• Monogastric: 1 stomach compartment

• Foregut fermenter/ruminant: 4 stomach compartments

  • Cows stomachs ≥ 50 gallons

  • Sheep stomachs ≥ 12 gallons

Small Intestine Structure

Structure (DJI)

1. Duodenum (1st short segment, receives secretions from pancreas, gallbladder, liver)

2. Jejunum

3. Ileum

Functions: chemical & mechanical digestion, transport food, absorb nutrients

Pancreas

• Location: “L” shaped gland sits in abdomen along greater curvature of stomach next to duodenum

• Functions: exocrine or digestive, endocrine

• Releases 2 types of hormones: insulin and glucagon

Pancreas Exocrine Functions

• Secrete substances through duodenum (duct to intestines)

• Enzymes: digest food

• Bicarbonate: buffer food coming from stomach into intestines

Pancreas Endocrine Functions

• Secrete substances of hormones into blood for systematic

• Insulin: stimulate glucose uptake by cells

• Glucagon: stimulate glucose production & release from liver

Liver

• Location: immediately below diaphragm, over stomach

• Functions: digestive, vascular, metabolic

  • Digestive: produce bile (bile salts digest lipids and eliminate waste/feces)

  • Vascular

  • Metabolic: make, store, breakdown carbohydrates, lipids & proteins

Bile

• Green-yellow fluid

• Composition: WATER, bile salts (acids), cholesterol, bilirubin (pigment made during breakdown of red blood cells), bicarbonate, phospholipids, detoxification products, drugs, hormones, antibiotics, toxins, copper, zinc, mercury

• Functions: break fat into small droplets to assist in chemical digestion

• Get rid of waste products

Gallbladder

• Location: tucked between liver and small intestines

• Function: storage site for bile between meals

• Horses have no gallbladder (they still make bile) because they’re constantly eating so they release a little bile every time they graze

Large Intestine

Functions

• Absorb water (compact fecal matter)

• Absorb vitamins

• Secrete mucus

• Transport/store fecal matter

• Monogastric herbivores: absorb nutrients and perform fermentation

Structure

• Carnivores: simple, tubular colon, poorly developed cecum

• Nonruminant herbivores: extensive colon & cecum (horses, guinea pigs, rats, rabbits) modifications of cecum and colon allow fermentative digestion in hindgut

CCR

1. Cecum: blind-ended pouch, beginning of colon,

2. Colon: longest part

3. Rectum: short, end segment of GIT

Rectum

• Last portion of large intestine

• Mucus-secreting glands lubricate and aid passage of contents

Anus

• Terminal opening of GIT

  • Internal sphincter: unconscious control

  • External sphincter: voluntary control

Avian Digestive Tract

Beak/mouth → esophagus → crop → proventriculus → gizzard (ventriculus) → small intestines → cecum → large intestines → rectum → cloaca/vent

Beak/mouth

• No teeth

• Glands secrete saliva

• Tongue

Crop

• Not all birds have crop (waterfowls don’t)

• Storage site until rest of digestive tract is ready to receive more feed

• When early empty, crop sends signals to brain so more food in consumed

Proventriculus

• Glandular stomach of acidic and enzymatic digestion

Gizzard (Ventriculus)

• Mechanical stomach

• Pebbles or grit

• Grinds food down

Small Intestine

• Duodenum: contributions pancreas & liver/gall bladder

• Jejunum: Meckel’s Diverticulum (at junction of jejunum and ileum, remnants of yok sac after yok has been absorbed that supplied nutrients first days after hatching)

• Ileum

Cecum (Ceca)

• Blind pouches at junction of small & large intestines

• Re-absorb water; ferment coarse feed, produce 8 B vitamins

• Evacuate contents 2-3 times per day

Coarse Feed

A feeding stuff containing a relatively large percentage of crude fiber or water

Large Intestine

Re-absorb water

Cloaca or Vent

• Common orifice (opening) for waste elimination, breeding, egg laying in females

What is the function of ruminant systems?

To allow the animal to use roughage (cellulose) as a source of energy

Fermentation

Enzymatic breakdown of energy-rich compounds in an anaerobic environment

Oral Cavity of Ruminants

• Dental pad

• Teeth

• Salivary glands: lubricate feed, provide liquid for bacteria & protozoa, buffer rumen (pH 6.2-6.8), recirculate nitrogen & minerals

  • Cows produce 100-150 liters of saliva/day

Ruminant Digestive Tract

Mouth → esophagus → rumen → reticulum → omasum → abomasum → small intestines (duodenum, jejunum, ileum) → cecum → large intestines → rectum

Esophagus

• Ruminants chew food, swallow food, regurgitate it, re-chew it, re-swallow again (constant 2-way street)

Eructating/Eructation

• Burping

• Fermentation in stomach chambers produces gases which must be expelled by burping (1-2 times per minute - carbon dioxide and methane)

Rumen (Fore Stomach)

• Largest stomach chamber

• Structure: muscular sacs lined by papillae (finger-like projections increase surface area to absorb more nutrients) separated by muscular folds

• Warm, anaerobic, pH 5.8 to 6.4 = ideal environment for bacteria, protozoa, fungi

• Function: fermentation

• Location: entire left side of abdomen from diaphragm to pelvis

• Never empties

Why does the rumen/reticulum have always regular, rhythmic contractions?

• Expel/burp built-up carbon dioxide and methane gases

• Regurgitate partially digested plants for further breakdown (rumination or cud chewing)

• Move feed particles that have been reduced in size by cud chewing and microbial action into 3rd stomach chamber

Reticulum (Forestomach)

• Smallest stomach chamber

• Structure: muscular sac lines by honeycomb ridges

• Function: fermentation

• Location: just below entrance of esophagus on left side of cranial (front) end of abdomen

• If a cow eats something heavy like metal, the weight could make it drop into reticulum first

Cow Rumination Rate

3 ruminations every 2 minutes

Sheep & Goat Rumination Rate

2 ruminations every 1 minute

What is the benefit of fermentation for the ruminant?

• Carbs in feed are broken down through microbial fermentation & digestion into volatile fatty acids (VFAs)

Volatile Fatty Acids (VFAs)

• Acetic acid

• Butyric acid

• Propionic acid

• Lactate

• VFAs absorbed through rumen wall into bloodstream OR pass through rumen for absorption in omasum/abomasum

• Converted to glucose, adipose tissue, milk fat, other

• Provide 50-80% of ruminant’s total energy needs

What makes up a ruminant diet?

Protein + Non-Protein Nitrogen

Protein

Microbial enzymes break down protein into amino acids

Rumen Undegradable Protein

Some dietary protein escapes rumen & is absorbed in small intestines

Non-Protein Nitrogen

• NH3 (ammonia)

  • Absorbed through rumen wall & transported to liver to be converted to urea (another non-protein nitrogen compound)

Why is okay to feed non-protein nitrogen (such as urea) to ruminants?

Due to presence of microbes in the rumen, ruminants can use NPN to make protein

Rumen Degradable Protein

• Protein that is easily degraded in the rumen making the nitrogen available for the microbes in rumen

• Microbes are flushed into small intestines to be digested as a source of protein

How are vitamins produced?

• Microbes in rumen produce B-complex vitamins and vitamin K for ruminant

Omasum (Forestomach)

• Reticulum/rumen contractions move feed into omasum

• 3rd chamber

• Structure: globe with many internal parallel muscular folds

• Function: absorb water, bicarbonate ions, remaining VFAs from feed, sometimes breaks food particles down further

• Location: right side of abdomen

Abomasum (True Stomach)

• 4th chamber

• Structure: true stomach chamber lines with glands

• Function: acidic and enzymatic digestion of feed

• Location: right side of abdomen below and behind omasum

Small Intestine

• Measure 20X length of animal

• Nutrients are digested and absorbed

Cecum

• 10% of cellulose escapes fermentation in rumen but is caught and digested in cecum

Large Intestine

• Absorb water

• Some bacterial digestion

Calf/Lamb Digestion

• Nursing stimulates reflex contraction of esophageal groove

Esophageal Groove

Channel which allows milk to bypass reticulum and rumen and pass directly into abomasum

Horse Digestive Tract

Mouth → esophagus → stomach → small intestines → cecum → large colon (right ventral colon → left ventral colon → left dorsal colon → right dorsal colon → transverse colon) → small colon → rectum → anus

What is the difference in fermentation between forestomach and hindgut fermenters?

Forestomach fermenters (cows)

• Microbial fermentation in forestomach

• Carbs in feed → volatile fatty acids in RUMEN → VFAs absorbed into blood = liver = VFA conversion to glucose (glucose transported in blood to tissues)

Hindgut fermenters (horses)

• Microbial fermentation in cecum & colon

• Carbs in feed → volatile fatty acids in LARGE INTESTINES (cecum + colon) → VFAs absorbed into blood = liver = VFA conversion to glucose (glucose transported in blood to tissues)