Digestive Systems

Chris Ellason

Digestion and Absorption:

The process of digestion includes:– The prehension of food or feed– The mechanical chewing and grinding– Mixing with digestive acids and enzymes to

chemically break down feedstuffs

The process of absorption includes:– Transport of the digested foods across the

intestinal mucosa to the blood or lymph system

General Terms

Prehension: to take into the mouth Mastication: chewing Deglutition: swallowing Regurgitation: the backward flow of food

through the esophagus

Three Major Categories

Carnivore: consume flesh of other animals, examples are dogs and cats

Omnivore: consume both plants and flesh, examples are primates

Herbivore: consume plant material, examples are horses and cattle

Carnivore

Very Well developed stomach Uncomplicated intestine Limited fiber digestion

Omnivore

Combination of carnivores/herbivores More complicated GIT than carnivores Colonic digesters

– Pigs, humans

Cecal digesters– rat

Herbivore

Cow, horse, rabbit– Each has a different type of GIT

Cow- ruminant Horse- simple stomach, large cecum, large

sacculated LI Rabbit- larger stomach, very large

sacculated cecum, unsacculated LI

Salivary Glands

3 pair– Parotid– Mandibular– Sublingual

Water - moistens food aids in swallowing Mucin - lubrication for swallowing Bicarbonate salts Enzymes

Salivary Glands

Salivary glands

Different Digestive Tracts

Farm animals have a variety of digestive systems– Ruminants: have 4 different compartments to

the stomach• Examples include cattle, sheep, goats

– Nonruminants (also known as monogastrics)• Hogs, dogs, and cats have a single, simple stomach• Poultry have a two part stomach• Horses have a large, functional cecum

Pregastric vs Postgastric

Pregastric: Fermentation that occurs in the rumen of ruminant animals. It occurs before food passes into the portion of the digestive tract in which digestion actually occurs.

Postgastric: The fermentation of feed occurs in the cecum, behind the area where digestion has occurred.

Pregastric vs Postgastric

Ruminants– More efficient – Less intake

Non- ruminant herbivores– Only postgastric– Less efficient– Greater intake

Monogastric Systems

Mouth: prehension and chewing of food; some carbohydrate enzyme activity

Esophagus Stomach

– Storage– Muscular movements (break down food)– Secretes Digestive Juices (hydrochloric acid)

• pH about 2

Monogastric Systems

Small Intestine

Duodenum– Active Digestion Site– Produce enzymes

• Pancreas– Helps to neutralize ingesta entering the SI

• Liver– Produces bile; breaks down fats

• Intestinal Walls

Small Intestine

Jejunum– Active in nutrient absorption

Ileum– Active in nutrient absorption

Villi pH 6 to 7

Large Intestine

3 Sections– cecum– colon– rectum

Active in water resorption Secretion of some minerals *Bacterial Fermentation*

Horses are Different

Saliva– contains no enzymes– may secrete up to 10 gallons/day– stimulated by scratching

Esophagus– only one way peristaltic movement

• Impossible for regurgitation

Horses are Different

Stomach– much smaller in comparison to other species– not very extensive muscular contraction– So how should we feed differently?

Small Intestine– same as pig but no gall bladder

• Can’t handle a high fat diet

Horses are Different

Large Intestine– over 60% of GIT– 4 parts

• cecum

• large colon

• small colon

• rectum

Cecum and Large Colon

Similar to Rumen– bacterial cellulose breakdown– bacterial protein breakdown– VFA production– Water Soluble Vitamin production

Small Colon and Rectum

Primary site for water resorption Can become impacted with feed

Horse GI Tract

Avian Species

Beak– no teeth– can be used to reduce particle size

Esophagus– ingesta holding and moistening– Salivary Amylase– Fermentation in some species

Avian Species

Proventriculus– Gastric juice production– pH 4– Rapid pass through of food

Avian Species

Gizzard (ventriculus)– thick muscular wall– particle size reduction (similar to mastication)– nonglandular– normally contains grit– no enzymatic secretion

Avian Small Intestine

Functions in digestion and absorption of feed and nutrients just as in other monogastrics

pH is slightly acidic Most enzymes found in mammals except?

Avian Large Intestine

Contains 2 blind pouches instead of ? Mostly water absorption Some bacterial activity but less than in most

mammals very short in comparison

Avian Species

Ruminant Digestive System

Mouth– what is unique about the teeth?– Can only chew on one side of mouth at a time

Saliva production is continuous Production about 12 gallons/d

Rumen Stomach

4 PARTS– reticulum– rumen– omasum– abomasum

Reticulum

Honeycomb most cranial not truly separated from rumen no enzymatic secretion walls are tough, tend to catch heavy objects

Rumen

Large compartment extends from diaphragm to pelvis

papillae Fermentation chamber Majority of absorption of byproducts and

conversion to volatile fatty acids

Rumen Digestion

Reticulorumen provides a favorable environment for bacterial fermentation

Continuous turnover of digesta and removal of fermented digesta

Anaerobic fermentation

Omasum

Manyplies (Stockmans Bible) short blunt papillae very muscular no enzymatic secretion reduction of particle size water resorption

Abomasum

True Stomach First Glandular portion of the tract Very similar in structure and function to

nonruminant stomach

Other Unique Points

Esophageal Groove– cardia to omasum

– milk bypass

Rumination: The process where rumen contents are regurgitated, remasticated, and reswallowed for further digestion

Eructation: expulsion of accumulated fermentation gases from rumen via esophagus

Ruminant GI Tract

Rumination

Regurgitation of ingesta with mastication– reticular contraction that concentrates ingesta at

the cardia– increased inhalation of air at same time– contraction of diaphragm– ballooning of esophageal walls– ingesta sucked into esophagus – returned to mouth by reverse peristalsis

Rumination

Excess liquids are swallowed Mastication commences More time spent masticating here than

initial intake Amount of time ruminating is a function of

diet composition

Rumen Contents and Motility

Rumen contents are not uniform Occur in stratified layers Change from ventral to dorsal Rumen mat

– high concentrate diets eliminate mat– more viscous fluid in high grain diets– lowest dry matter in high forage diets

Rumen Contents and Motility

Motility of rumen mixes contents Divided into

– primary contractions– secondary contractions

Contractions require up to 50 seconds to complete

Rumen Contents and Motility

Contractions begin with reticulum Progress dorsally Finish with ventral blind sac and ventral

pillars

Rumen and Acute Acidosis

Optimal rumen pH is 6.7 Variation in pH is normally +/- 0.5 Introduction of high grains result in

– breakdown of rumen mat– proliferation of facultative anaerobes– these produce high levels of lactic acid

Rumen and Acute Acidosis

Lactate is a much stronger acid that other VFAs

In severe cases lactate can make-up well over 50% of total rumen acids

Succinate and Formate can also appear in high quantities

Rumen and Acute Acidosis

Increased acids can reduce pH as far as 4.0 Severe rumenitis occurs at these pH levels Absorption of lactic acid results in systemic

acidosis All chronic acidosis results in rumen

parakeratosis

Factors Affecting Digestibility

Rate of passage: increased rate of passage of digesta through the tract reduces digestibility– Factors increasing rate of passage include:

• Increased level of feeding/intake (ruminants)• Finer processing (such as grinding) of feed

Note: grinding grain usually increases digestibility but grinding hay decreases digestibility