Anatomy and physiology (diploma).

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ANATOMY AND PHYSIOLOGY OF THE DIGESTIVE SYSTEM

BY

DR. ABDULRAHMAN BELLO (DVM , MSc)

Department of Veterinary Anatomy, Faculty of Veterinary Medicine, Usmanu Danfodiyo University,

Sokoto, Nigeria.

E-mail:[email protected]. +234(0)8039687589

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I. Digestion

1. The process by which food is converted into substances that can be absorbed and assimilated by the body.

2. It is accomplished in the alimentary canal by the mechanical and enzymatic breakdown of foods into simpler chemical compounds.

II. The Digestive Tract

1. GeneralA. Animals classified by their diet in a natural state

a. Carnivoresi. Meat-eatersii. Dog, Cat

b. Herbivoresi. Plant-eatersii. Cattle, Horses, Sheep

c. Omnivoresi. Meat- and Plant-eatersii. Pigs, Humans

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• B. Because of diversity in diet, digestive systems • have developed in different ways• --Cecum, Rumen, Colon, etc.• 2. Mouth• A. Oral Cavity• B. Food is received and reduced in size• C. Mixed with saliva and swallowed as bolus• D. Teeth and Tongue assist with function• 3. Teeth• A. Functions• a. Reduction in size of food by grinding• --Increase surface area for chemical and/or• microbial degradation• b. Cutting of foodstuffs to be presented to mouth• c. Protective function• --Inflict wounds• d. Kill Prey• --Food gathering for carnivores

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• B. Four types of teeth• a. Incisors (I)• i. Most forward• ii. Used for cutting• b. Canines ©• i. Also fangs, eye teeth, tusks• ii. Used for tearing and separation of

food mass• c. Premolars (P)• i. Just caudal to canines• ii. Used for grinding and chewing• d. Molars (M)• i. Larger than premolars• ii. Used for grinding and chewing

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• C. Dental Formulas• a. Represent one side of mouth• b. Numerator = top teeth• c. Denominator = bottom teeth• d. Species:• • Cow & Sheep I 0/4 C 0/0 P 3/3 M 3/3 = 32 • Pig & Horse I 3/3 C 1/1 P 4/4 M 3/3 = 44 • Humans I 2/2 C 1/1 P 2/2 M 2/2 = 32 • D. Animals can be aged by teeth eruption and

wear12-Aug-14

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• 4. Tongue• A. Muscular organ used to maneuver the

food mass• B. Muscle fiber orientated in three

directions• C. Functions:• a. Seizes and brings food to mouth• b. Brings food to molars• c. Assists in swallowing• D. Papillae• a. Projections on tongue• b. Traction for moving food• c. Used in grooming• d. Contain taste buds(extra credit)

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• 5. Pharynx• A. Common Pathway for air and food• B. Food is prevented from entering larynx • and nasal cavity by reflexes and the mechanics • of swallowing.

• 6. Esophagus• A. Muscular tube that extends from Pharynx to Stomach• a. Combination of Skeletal and Smooth Muscle• B. Food and water moved via contraction waves

of the muscular wall.

• D. Opening just above glottis (opening to larynx)• E. Usually lies on left side of trachea

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• 7. Stomach• A. Functions• a. Storage of Food• b. Early digestion• B. Subdivided into parts (continuous) as

viewed from outside• a. Cardia• --Entrance• b. Fundus• c. Corpus• d. Antrum• --Enters small intestine (duodenum) through • pylorus12-Aug-14

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C. Inner aspects based on cell typea. Esophageali. Esophageal-like Epithelium--Stratified Squamousb. Cardiac glandi. Secrete mucousii. Columnar Epitheliumc. Fundic glandi. Contain Gastric Glandsii. Two types1. Parietal Cells --Hydrochloric Acid (HCl)2. Neck Chief Cells--Pepsinogeniii. Also secrete mucousd. Pyloric glandi. Secrete mucousii. Secret gastrin

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8. Small IntestineA. Three sections

a. Duodenumb. Jejunumc. Ileum

B. Receives secretions from Pancreas and Liver (Bile)C. Most digestion and absorption occurs in

small intestinesD. Histology

a. Mucosai. Epithelial Lining

b. Submucosai. Connective Tissueii. Also contains:

-- blood vessels-- lymph vessels--nerve fibers (Meissner's Plexus)--Smooth muscle (Muscularis Mucosae)

iii. Lamina propria--Submucosa inside of Muscularis

Mucosae

c. Muscularis Externa--Smooth Muscle i. Circularii. Longitudinal

d. Serosai. Outer epithelial and C.T. layer

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E. Small Intestine contains large surface area for digestion and absorption.a. Considerable length--Looping and Coiling

Horse 73 feetSheep 85 feetPig 60 feetCow 151 feet

b. Folding of intestinal surfacei. Villiii. Microvilli--Brush Borderiii. Provides 600X more surface area than smooth cylinderiv. Crypts of Lieberkühn--Lie deep between Villi--Only cells that undergo cell division--Cells migrate out and replace older cells--Replacement time:2-4 d young animals7-10 d older animals

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• F. Nerve Networks• a. Meissner's Plexus• i. Control Secretions• ii. Controls Blood Flow• iii. Sensory: Reflexes• b. Auerbach's Plexus• i. Between Circular and Longitudinal Muscularis• ii. Control Movement• c. Two combine to form Enteric Nervous System• i. Contains Pacemakers and Conduction • fibers• ii. Under Autonomic Nervous System • Control• G. Blood and Lymph Supply• a. Capillaries supply each villi• i. Veins go to liver• --Hepatic Portal System• b. Central Lacteals• i. Lymph vessels• ii. Transport large molecules• iii. Bypasses liver to thoracic duct

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• 9. Large Intestine• A. Enter from ileum• B. Site of Fermentation• C. Two Parts• a. Cecum• b. Colon• D. Cecum• a. Different degrees of development• b. Horse is very well developed• --Lies on right side

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• E. Colon• a. Three parts• i. Ascending• ii. Transverse• iii. Descending• b. All animals have Transverse and Descending• c. Pig, Ruminants, and Horses have counterpart of Ascending• i. Pig and Ruminants• -- Ansa Spiralis (Coiled Colon)• ii. Horse• --Large colon• --Divided into Dorsal and Ventral• --Double Horse Shoe• d. Cecum and colon sacculated in pigs and horses• i. Due to bands of muscle• ii. Sacculations called Haustra• iii. Allow more time for microbial digestion• e. Descending Colon• i. Part within Pelvis is called rectum• ii. Anus is the terminal opening• --Sphincter with smooth and skeletal muscle

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• III. Digestive Accessory Organs• 1. Salivary Glands• A. Three Primary Glands• a. Parotid• b. Mandibular• c. Sublingual• d. Also some less defined, scattered salivary • tissue• B. Secretions• a. Saliva • b. Functions• i. Lubricates chewed food• ii. Moistens the oral walls• iii Digestive enzymes• --Ptyalin (amylase)• c. Form from glands• i. Serous• ii. Mucous• iii. Mixed

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2. PancreasA. Exocrine and Endocrine Glanda. Located along duodenumB. Exocrine Functiona. Digestive Secretionsb. Primary duct enter duodenum near common bile duct--In sheep and goats the duct enters the bile duct before entering the intestinec. Accessory duct is also present a short distance downstreamC. Endocrine Functiona. Hormonesb. No ductsc. Endocrine cells located in Islets of Langerhansi. Alpha Cells--Secrete Glucagonii. Beta Cells--Secrete Insuliniii. Delta Cells--Secrete Somatostatin

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3. LiverA. Multipurpose organa. Over 500 functionsb. Production of bile and bile saltsc. Protein Synthesisi. Enzymesii. Hormonesd. Storagee. Metabolic Conversionf. DetoxificationB. Hepatic Portal Systema. Blood is received from stomach, spleen, pancreas, and intestine from portal systemb. Circulated through sinusoids (second capillary bed of portal system)c. Detoxified before entering central vein (second venous drainage of portal system)i. Kupffer cells--Phagocytose Foreign particles and old RBCsii. Other material is removed by membrane

transport through the liver epithelial cells to be metabolized in a usable form or to be excreted.

d. Hepatic artery, portal vein and bile duct form a triad in the liver lobules--Bile flows opposite direction of blood flow

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IV. Foodstuffs1. Carbohydrates2. Proteins3. Fats4. Water5. Inorganic Salts6. Vitamins

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V. Physical and Mechanical Factors1. Prehension

A. The seizing and conveying of food into the mouth.B. Prehensile Structures

a. Lipsb. Teethc. Tongue

C. Horsea. Highly mobile upper lip (Eating grain)b. Incisor teeth (Grazing)

D. Cow & Sheepa. Immobile upper lipb. Tongue is primary prehensile organc. Brings grass to incisors and

upper dental pad--Sheared by upper movement of head

d. Sheep cleft upper lip allows for close to the ground grazing.

D. Piga. Heavy snout and pointed lower jaw adapted

for rooting12-Aug-14

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2. MasticationA. Mechanical breakdown of food in the mouth

a. ChewingB. Varies among species

a. Herbivores need to chew more than CarnivoresC. A bolus is formed from chewing and mixed with

saliva for swallowing.

3. DeglutitionA. Act of swallowingB. Three stages

a. Through the mouth (voluntary)b. Through the pharynx (reflex)c. Through the esophagus (reflex)

C. Sequence of Swallowing Reflexesa. Respiration is inhibitedb. Glottis (opening of larynx) is closedc. Larynx pulled upward and forwardd. Base of tongue fold the epiglottis (forward projection of glottis) over the glottis as the tongue plunges the bolus from the mouth into the pharynxe. Soft palate is elevated, closing the nasal passagef. Pharynx contracts to direct food into the esophagusg. Reflex peristaltic wave in the esophagus is initiated transporting bolus to stomach

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4. Smooth Muscle ActivityA. Food is moved from stomach through intestine

by smooth muscleB. Muscle Activity is Spontaneous

a. Modulated by ANSC. Very excitable tissue

a. Membrane potential is a slow waveb. At threshold, spike potential

(action potential) is observedc. Increase frequency of spike potentials

= the longer the sustained muscle contraction

d. Duration longer than nerve impulses because Ca++ channels also involved in Depolarizationi. Influx slower than Na+

-- Ca++ Channels slower to open and close

ii. Ca++ also involved in myosin/actin interaction

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D. Stimulators (cause depolarization)a. Stretchb. Acetylcholinec. Parasympathetics

E. Inhibitors (cause hyperpolarization)a. Norepinephrineb. Sympathetics

5. Segmentation and PeristalsisA. Segmentation

a. Contractile waves that travel short distancesB. Peristalsis

a. Contractile waves that travel longer distancesC. Both waves conducted toward anus (aboral)D. Pacemakers

a. Stomach --Located at greater curvatureb. Intestine --Located in longitudinal muscle near entrance of bile duct

E. Peristaltic Reflex--Intrinsic Reflexa. Initiated by distention (stretch) of the bowelb. Cranial contraction, Caudal relaxationc. Moves food aborallyd. Also stimulated by gastrin and cholecystokinine. Inhibited by secretin

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F. Extrinsic reflexa. Peritoneal Irritation

6. Physical Functions of the Stomach A. Functions

a. Storage of ingested foodi. Fundusii. Adapts volume so excessive pressure doesn't occur

b. Mixing of the food with secretionsi. Corpusii. Mixes Saliva, Food, and Gastric Secretions

c. Control the emptying of its contentsi. Antrumii. Pump acting with pyloric sphincteriii. Helps with Corpus mixing when sphincter is closedd. Stretch receptors cause increase tone and contractions

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B. Delay of Gastric Emptyinga. Enterogastric Reflex (Neural Mechanisms)i. Osmoreceptors in duodenum respond to hypertonic content

--From electrolytes, or products of CHO and Protein Digestion--Inhibits rapid loss of water from bloodii. H+ receptors respond to high H+ concentration--pH balanceiii. Allows more time for equilibration (homeostasis)b. Entergastrone Reflex (Endocrine Mechanisms)i. Cholecystokinin released from duodenal mucosa in response to lipidsii. Gastric inhibitory polypeptide (GIP) released from jejunal mucosa in response to lipids and CHOsiii. Allows more time for digestion (homeostasis)

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• C. Emesis (Vomiting)• a. Emptying of cranial duodenum and stomach • toward mouth (orad direction)• b. Protective mechanism• i. Prevents absorption of Noxious • chemicals• c. Reflex • i. Antiperistalsis• ii. Closure of glottis and nasal cavity• iii. Controlled by vomiting center in brain• d. Species differences• i. Pigs, Dogs, & Cats• --Vomit easily• ii. Ruminants• --Ejection from abomasum to forestomachs• --No ejection from mouth• --Not the same as rumination• iii. Horses• --Vomiting is rare• --Cardia doesn't open in reverse direction

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7. Physical Functions of Small IntestineA. Functions

a. Mix contentsb. Propel contents

B. Flow must be controlleda. To provide proper mixing of luminal contents

with pancreatic enzymes and bileb. To provide time for digestion of CHOs, Fat,

and ProteinC. Mechanisms

a. Segmentation of the Ileum (Delays Transport)i. Segmentation is a mixing mechanism

where Peristalsis is more of a propelling mechanism

b. Autonomic Nervous Systemi. Parasympathetic increases transportii. Sympathetic decreases transport

c. Hormonesi. Secretin inhibitsii. Cholecystokinin stimulates

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8. Physical Functions of Large IntestineA. Functions

a. Microbial Digestionb. Reabsorption of Electrolytes and Water

B. Motor activity of cecum and colon is directed toward delay in transit and filling of parts (reservoir function)a. Retrograde flow plays a big roleb. Increased colonic activity associated with

constipationc. Decrease colonic activity associated with

diarrhea9. Intestinal Transport of Electrolytes and Water

A. Electrolytes and Water must be reabsorbed to maintain water balance

B. Diarrhea greatly decreases reabsorptiona. If uncontrolled or if water/electrolytes

replenished animal could die--Blood volume loss and circulatory collapse

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10. DefecationA. Evacuation of feces from the terminal

colon and rectumB. Complex Reflex Act

a. Also voluntary controlC. Frequency

a. Horses 5-10 times/dayb. Cattle 10-20 times/dayc. Carnivores 2-3 times/day

D. Passage time: Mouth to Anusa. Pigs 48 hrsb. Horses 24-48 hrsc. Cattle 12-24 hrs

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VI. Digestive Secretions and their Functions1. SalivaA. Functionsa. Lubricates chewed foodb. Moistens the oral wallsc. Amylase--Most abundant in pigd. Ruminants: Buffer for microbial fermentatione. Antifoam agent2. Gastric SecretionsA. Mucousa. Secreted throughout digestive tractb. Function: LubricationB. HCla. Secreted by Fundic Parietal Cellsb. Initiate Protein DigestionC. Pepsinogena. Secreted by Fundic Neck Chief Cellsb. Converted to Pepsin by HClc. Initiates Protein Digestion

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D. Gastrina. Secreted by Pyloric G Cellsb. Stimulate HCl and Pepsinogen secretionc. Stimulate digestive tract motility

E. Intrinsic Factora. Secreted by mucous cellsb. Facilitates Vit. B12 reabsorption in ileum

F. Rennina. Secreted by young ruminantsb. Milk coagulating enzyme

--allows for more digestion in stomachG. Factors regulating gastric secretion

a. Stimulationi. Acetylcholine (Parasympathetic)ii. Gastriniii. Histamineiv. Secretin (Pepsinogen only)

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b. Inhibitioni. Within stomach

-- Decrease in pH to 2ii. From Duodenum--presence of acidic, fatty, and hypertonic

solutionsiii. Mechanisms

1. Neuronal--Inhibition of parasympathetic neurons that stimulate G cells

2. Hormonal--Secretin--Cholecystokinin--Gastric inhibitory peptide (GIP)

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3. Pancreatic Digestive SecretionsA. Bicarbonatea. Neutralizes HClb. Buffer microbes in cecum and colonB. Proteasesa. Secreted as proenzymes--Activated in intestineb. Typesi. Trypsinogen--Activated by enterokinase to trypsin--Trypsin then activates othersii. Chymotrypsinogeniii. Elastaseiv. Carboxypeptidases A & Bc. Breakdown proteinsC. Pancreatic Lipasea. Hydrolyzes Dietary Triglyceridesb. Activated by Bile SaltsD. Pancreatic Amylasea. Hydrolyzes starch to maltose

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E. Control of Secretiona. Autonomic Nervous System

i. Parasympathetic stimulatesii. Sympathetic inhibits

b. Hormonesi. Gastrin

--Stimulates secretion of all enzymes

ii. Secretin--Stimulates Bicarbonate Secretioniii. Cholecystokinin

--Stimulates protease and lipase secretion12-Aug-14

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4. Biliary Secretions

A. Bile is a greenish-yellow solutiona. Bile Saltsb. Bilirubinc. Cholesterold. Lecithine. Electrolytes

B. Continuously synthesized by liverC. Bile salts are recycled

a. Quantity needed exceeds synthesisb. Absorbed by intestine after being usedc. Resecreted by hepatic cells

(enterohepatic circulation)D. Bile salts are formed from cholesterol

a. Combine with lecithin and cholesterol to form micelles

b. Formation of micelles prevent cholesterol precipitation and therefore gall stones--Also depends on alkalinity

(bicarbonate buffers)E. Bile is stored in the gall bladder

a. Can be concentrated depending on storage time

b. Infrequent eaters are more concentratedc. Frequent eaters are dilute

--Farm animals except horsed. Horse has no gall bladder

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F. Sphincter of Oddi controls release into duodenuma. Controlled by cholecystokinin (CCK)

i. Released in response to lipids and amino acids

ii. Causes gall bladder contraction and sphincter relaxation

G. HCO3- secretiona. Stimulated by CCK, Gastrin, and Secretin

b. More secreted from liver than pancreasF. Functions of Bile

a. Bufferb. Fat emulsificationc. Removal of products of lipid digestion

(micelle)12-Aug-14

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5. Breakdown and Absorption of Carbohydrates, Proteins, and FatsA. Most of digestion and absorption of CHOs, Proteins

and Fats occurs in the small intestine

B. Read Book for more details

6. Microbial Digestion in the Large IntestinesA. No enzymatic digestion occurs in large intestineB. Digestion that occurs results from microbial

digestiona. Nonruminant herbivores and omnivores

C. End products of microbial digestion are Volatile Fatty Acidsa. Important energy source

--75% of Horses Energy Requirementb. Also assist in water reabsorption

--Water follows absorbed VFAs12-Aug-14

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V. The Ruminant Stomach1. Ruminants are animals that regurgitate and remasticate

their food (Rumination)A. Two Suborders

a. Ruminantia--Deer, Moose, Elk, Reindeer, Caribou, Antelope, Giraffe, Musk Ox, Bison,

Cow, Sheep, and Goatb. Tylopoda

--Camel, Llama, and Alpacac. Differences

i. Tylopoda don't have a Omasumii. Tylapoda have sacculated surface in Reticulum and Rumen that

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B. Ruminants stomach has adapted for fermentation of ingested food by bacteria and protozoaa. Energy is obtained from plant celluloseb. Fermentation requires controlled conditions

i. Secretionsii. Motilityiii. Temperature

c. Rumination assist fermentation by grinding foodstuffs up more finely and increasing surface area for microbes to attack

2. Four Compartments of Ruminant StomachA. RumenB. ReticulumC. OmasumD. AbomasumE. Rumen, Reticulum, Omasum are considered to

Forestomacha. Lined with Stratified Squamous Epithelium

(Esophageal region of stomach)F. Rumen is on left side, Abomasum on Right

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3. Functions of the CompartmentsA. Rumen

a. Allows for soaking, fermentation, and mixing of bulk fibrous feedstuffs

B. Reticuluma. Pump that causes liquid to flow into and out of rumenb. Directs ingesta to Rumen or from Rumen to Omasumc. Floods cardia prior to regurgitationd. Reticular groove active in young ruminants

--Conveys milk towards omasum and abomasum--Reflex loses responsiveness with age

C. Omasuma. Grinds and triturates (squeeze out fluid)

rumen contents that is deposited within foldsD. Abomasum

a. True Stomach--Functions same as non-ruminant

b. Starts digestion of degraded concentrates and roughagesc. Starts digestion of microbes of fermentation

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4. RuminationA. The process of bringing food material back from the ruminant stomach

a. Occurs in a cycleB. Four Phases of cycle

a. Regurgitationb. Remasticationc. Reinsalivationd. Redeglutition

C. Regurgitationa. Breath taken with closed glottisb. Intrapleural and mediastinal pressure

decrease without lungs inflating

c. Esophagus relaxes and cardia opensd. Reticulum contractse. Rumen contents are aspirated into esophagus

because of negative pressuref. Reverse peristalsis carries bolus to mouth

D. Remasticationa. Liquid is squeezed out of bolus

and swallowedb. Number of chews varies depending on diet

--100 chews for all roughage diet12-Aug-14

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E. Reinsalivationa. Occurs simultaneous to remasticationb. Saliva swallowed 2 or 3 times before

reglutition--Used for microbial fermentation buffering

F. Redeglutitiona. Reswallowingb. Next cycle occurs 5 seconds after

swallowingG.Time spent Ruminating

a. Diet dependentb. Cows on hay diet = 8 h/dc. 14 period/dayd. Happens during times of quiescence

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5. Gas Production and EructationA. Gases produced are products of fermentationa. Mainly CO2 and Methaneb. Trace amounts of N2, O2, and H2c. Produced at rate of 0.5-1 L/minB. CO2 a. Fermentation of CHOs b. Deamination of amino acidsc. Salivary bicarbonate from lipid fermentationd. 60-70% of gas productionC. Methanea. Reduction of CO2 by methane-producing bacteriab. 30-40% of gas productionE. Eructationa. Process by which gas from the forestomach is removed by way of the esophagus to

the pharynx (Burp, Belch, etc.)b. Occurs about once a minute--Doesn't make a soundc. Mechanoreceptors in dorsal sac of rumen and cardia sense gas pressure (Controlled by reflex center in medulla)i. Rumen contracts to move gas bubble forwardii. Reticulum relaxesiii. Cardia opens

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d. Much of gases eructated are inspired by lungsi. May provide carbon source for metabolismii. Also cause of off-flavors

e. Bloat (Tympanism)i. Eructation mechanism fails

ii. Mechanoreceptors covered with foam (tiny bubbles)

--Don't recognize gas build-upiii. Types: Feedlot or grain bloat

--High concentrate diet Legume bloat--Lush green alfalfa or clover

iv. Animals die if not treated from Asphyxia --Pressure against diaphragm

prevents thoracic enlargement12-Aug-14

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6.Chemistry and Microbiology of the RumenA. Bacteria

a. 80% of microbesB. Protozoa

a. 20% or microbesC. Both are anaerobicD. Produce VFAs, gases, and B-complex

vitamins--From CHOs

E. Hydrolyze Proteins and Fats

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THANKS FOR LISTENING

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