Download Digestive System

Digestion and Metabolism
Digestive tract
• one long, continuous tube starting at the mouth and
ending at the anus
Functions
• Ingestion
• Grinding
• Digestion/absorption of food
• Elimination of solid wastes
Classifications of animals:
ruminant vs non-ruminant
carnivore: meat-eating
herbivore: plant-eating
omnivore: both meat and plant-eating
General Arrangement of the Digestive Tract
Long, continuous tube composed of 4 basic layers
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Serosa (visceral peritoneum)
Muscle (mostly smooth muscle)
Submucosa (connective tissue)
Epithelial lining of the tube (mucous membrane)
Peritoneum
A serous membrane that lines the entire abdominal cavity
and covers all visceral organs within the abdomen
lining of the abdominal cavity
is the parietal peritoneum
covering organs is the serosa
or visceral peritoneum
Mesentery
double fold of peritoneum that
supports the intestine and
attaches it to dorsal abdominal
wall
Omentum
refers to peritoneum connecting
the stomach with other structures
Greater omentum
attaches to the greater curvature of the stomach
Lesser omentum
extends from lesser curvature of the stomach to the liver
Ligaments
other folds of peritoneum which connect abdominal organs
with each other or with the parietal peritoneum
Greater Omentum, Mesentery & Mesocolon
Lesser Omentum
Mouth
Reduction of food particle size by grinding or chewing
§ Mixing with saliva
§ Once food is mixed with saliva it is called a bolus
§ Prehension
§ Mastication: mechanical breakdown of food
Accessory structures that aid the mouth include
Salivary glands
Teeth
Tongue
Lips, cheeks, jaws and palates
Salivary Glands
Major glands, well-defined
• Parotid: ventral to the ear in relation to caudal border of mandible
• Mandibular: also called submaxillary; ventral to parotid gland, just caudal to the
mandible (may be deep)
• Sublingual: deep to mucous membrane along the ventral side of the lateral surface of
the tongue near the floor of the mouth
Minor glands, lesser defined
• Labial, buccal, lingual, palatine
• Dog: zygomatic near the eye
Teeth
Ø Mechanically reduce size of food by grinding or chewing
Ø Also used for cutting
Ø Increase surface area of food for chemical and
microbiological degradation
Parts of tooth
Crown: portion above gingiva
covered by enamel Neck: connection between
crown and root
Root: below gingiva; covered in cementum
Pulp cavity: roots and blood supply
Rest of tooth is composed of dentin
Ø teeth with short crown are called Brachydont teeth
Ø teeth with long crown are called Hypsodont teeth
Two sets of teeth
• Deciduous: (milk teeth) erupt first and are replaced
• Permanent: replace deciduous; most accurate way of determining animal’s age
Types of teeth: Incisors, canines, premolars, molars
Incisors (I): front teeth, numbered from the center of mouth (or symphysis) laterally
First pair = I1 or centrals
Next pair = I2 or first intermediates
Next pair = I3 or second intermediates
Last and most lateral pair = I4 or corners
Non-ruminants only have one pair of intermediates
Canines (C): also called eye teeth, bridle teeth, tusks and tushes
• normally not more than 1 pair of canine teeth occur in each jaw at a given time
• may be completely absent in the mare, gelding and ruminant
Premolars (P): cheek teeth
numbered from front to back P1 , P2 , P3 , and P4
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Dental formulas for cattle
Molars (M): caudal to premolars, M1 , M2 , and M3
Deciduous 2(DI 0/4 DC 0/0 DP 3/3)
Permanent 2(I 0/4 C 0/0 P 3/3 M 3/3)
Numerator: upper teeth
Denominator: lower teeth
No upper incisors in cattle
Have a dental pad
Tongue
Ø Muscular organ used to
maneuver food mass
Ø Contains taste receptors;
can discriminate between
good and bad
Ø Cattle use tongue for
prehension
Ø Covered with stratified
squamous epithelial cells
that presents large number
of papillae on dorsal side
Lips, Cheeks, Jaws and Palates
Lips
• sheep, goat and horses are soft and flexible and aid in prehension
• cattle and hogs are stiff and immobile; serve little except to close
mouth
Cheeks
• Aid tongue in positioning food for chewing
Jaws
• opened and closed by powerful muscles that contribute to
grinding movement by protruding the jaw and moving
it side to side
Hard Palate
• forms roof of mouth
Soft Palate
• caudal portion of hard palate; separates mouth from nasopharynx
Pharynx
v common passageway for both food and air
v nasopharynx: pharynx connection with nasal nares and
2 eustachian tubes
v oropharynx: pharynx connection with mouth
v laryngopharynx: pharynx connection with esophagus and larynx
v Deglutition: swallowing
Esophagus
v Muscular tube extending from pharynx to the stomach
v Pierces diaphragm at the hiatus esophagus
v Upper esophageal sphincter
v At stomach, folds within lumen keep it closed
v Function: transport bolus to stomach
v Peristaltic contractions
v Muscle changes from striated to smooth muscle occurs:
caudal 1/3 in the horse esophagus
just in front of diaphragm in pig
Muscle remains striated throughout the entire
length in Dog and ruminant
Non-Ruminant Stomach
Ø located just behind left side of diaphragm
Ø Subdivided into
cardia (entrance)
fundus
body
pylorus (termination)
Ø Cardia and pylorus
have sphincters:
control passage of food
Non-glandular region is
called the esophageal region
Glandular regions include:
cardiac
fundic
pyloric
Glandular surface: surface area is increased
many times by infolding of epithelium into
depressions called gastric pits
Gastric pits contain secretory cells that produce
gastric juice
Parietal cells à HCl
and intrinsic factor
Chief cells à pepsinogen
and gastric lipase
Mucous cells à mucus
Small Intestine
• Peristaltic contractions
•
wave-like mixing that mixes bolus with gastric juice and produce chyme
• Emesis: regurgitation (vomiting); reverse peristalsis or anti-peristalsis
horses cannot regurgitate
• 3 sections
duodenum
jejunum
ileum
Duodenum
first part of small intestine
Ø most absorption and digestion takes place here in animals
not requiring extensive fermentation
Ø lumen contains villi and microvilli (contains brush border)
Jejunum
Ø indistinctly separated from duodenum
Ileum
Ø continuous from jejunum
Ø joins
Cecum (horse)
Colon (dog)
Cecum and colon (ruminant and pig)
S.I. has concentric and longitudinal contractions
Control of movement with S.I. controlled for two reasons:
1. Provide proper mixing
time
•
Provide time for digestion
and absorption
•
Weak peristalsis in comparison to the stomach---chyme remains for 3 to 5 hours
•
Segmentation---local mixing of chyme with intestinal juices---sloshing back & forth
Pancreas
• has both exocrine and endocrine functions
• exocrine produces NaHCO3 and digestive enzymes
• secrete product into duodenum via pancreatic duct
• may also have accessory pancreatic duct
Liver
• produces bile
• receives nutrient blood from hepatic artery
• receives blood from digestive absorption sites
via portal vein
• connected to gallbladder (except horse) for storage of bile
• detoxifies and sorts
Liveràhepatic ductàcystic duct (from gallbladder)
common bile duct à duodenum
Large Intestine
Consists of:
cecum
colon (ascending, transverse, descending)
rectum
anus
v large intestine varies from species to species
v Horse: largest and most complex of domestic animals
v Pig: begins with cecum; ascending colon is a spiral
arrangement of coils, gives a cone-shaped appearance
v Dog: shortest and simplest
short, irregular cecum, short ascending colon
v Ruminants: cecum, colon and rectum
The rectum or final portion of the large intestine
joins the anus.
Anus: junction of the terminal part of the
digestive tract with the skin