Download Biology 233

VT 106
Comparative Anatomy and Physiology
Digestive System
digestive anatomy and physiology depend on diet
herbivores – plant-eaters
carnivores – meat-eaters
omnivores – eat diverse diets
animal must be able to digest (chemically break down) its diet
depends on enzymes available to catalyze reactions
monogastrics – simple stomach
produce most of their own digestive enzymes
ruminants & hindgut fermenters
have fermentation chambers where microbes produce enzymes
to digest plant materials
DIVISIONS OF THE DIGESTIVE SYSTEM
Gastrointestinal (GI) Tract – path that food follows
mouth, pharynx, esophagus, stomach, small intestine,
large intestine, rectum, anus
Accessory Digestive Structures
tongue, teeth, salivary glands
liver, gallbladder, pancreas
FUNCTIONS OF THE DIGESTIVE SYSTEM
1) prehension – grasping food to ingest it
2) mechanical digestion – grinding food into smaller particles and mixing it
mastication – chewing
rumination – regurgitating and rechewing food
GI motility – smooth muscle contractions
churning, segmentation – mix food with digestive secretions
peristalsis – propels food from esophagus to anus
3) chemical digestion (catabolism) – chemically breaking down food into
molecules that can be absorbed (digestive enzymes)
4) absorption – passage of nutrients from GI tract into the blood
5) elimination of wastes
excretion – secretion of cellular wastes into GI tract
defecation – elimination of feces
excreted wastes, undigested or unabsorbed materials
1
ABDOMINAL (PERITONEAL) CAVITY – contains most of GI tract
parietal peritoneum – lines walls
visceral peritoneum – lines organs
peritoneal cavity – space between parietal and visceral peritonea
serous fluid lubricates surfaces
ascites – fluid accumulation in peritoneal cavity
peritonitis – inflammation of peritoneum
GENERAL HISTOLOGY OF THE GI TRACT
4 Main Layers:
1) Mucosa – inner epithelial lining
stratified squamous epithelium – protects from abrasion
mouth, pharynx, esophagus, anus
simple columnar epithelium – secretes and absorbs
stomach, small and large intestines
MALT (mucosa-associated lymphatic tissue) – immune protection
2) Submucosa – connective tissue
contains various digestive glands and many capillaries to absorb nutrients
3) Muscularis
smooth muscle in most regions
skeletal muscle – mouth, pharynx, upper esophagus, anal sphincter
voluntary control of swallowing and defecation
4) Serosa – outer visceral peritoneum
MOUTH (ORAL OR BUCCAL CAVITY)
oral mucosa – stratified squamous epithelium
Functions of Mouth:
Prehension – grasping and manipulating food
lips – skeletal muscle and sensitive tactile receptors in many herbivores
tongue – skeletal muscle anchored to hyoid apparatus
papillae with tactile and taste receptors
Chemical Digestion
salivary glands – secrete saliva
lubricates food and begins chemical digestion
mucus lubricates food and GI tract
digestive enzymes – minor function
amylase – digests starch (omnivores)
lipase – digests fats (nursing animals)
bicarbonate ions – large volumes of basic saliva in ruminants
neutralize acids in the rumen
parotid glands – base of ear
submandibular glands – between mandibles
sublingual glands – base of tongue
2
ANS Regulation of Salivation
parasympathetic n.s. – stimulates salivation
stimulated by taste, feel, smell, sight, or thought of food
sympathetic n.s. – inhibits salivation
Mastication (chewing) – mechanically breaks down food and mixes it with saliva
forms a bolus (ball of food) for swallowing
increases surface area for digestive enzyme function
TEETH
maxillae – upper arcade
mandible – lower arcade
periodontal ligaments – dense fibrous CT that anchors tooth in alveolus
gingiva (gums) – oral mucosa lining alveolar processes
Regions of a Tooth
crown – visible portion above gumline
neck – region where gingiva attaches
root – portion embedded in alveolus
occlusal surface – surface where upper and lower arcade make contact
Layers of a Tooth
pulp – central connective tissue w/blood vessels and nerves
apex – root tip where vessels and nerves enter tooth
dentin – main component of tooth
harder than bone – 70% calcium salts
enamel – outer lining of crown only
hardest substance in body – 95% calcium salts
protects tooth from abrasion and acids
DENTAL FORMULA
4 Types of Teeth:
1) incisors – front teeth
chisel-shaped for cutting food
2) canines
pointed for grasping and tearing food
3) premolars
broader, flatter occlusal surface to crush and grind food
4) molars – back teeth
broad, flat occlusal surface to crush and grind food
deciduous teeth – baby teeth
lost and replaced by adult teeth
permanent teeth – adult teeth
3
basic dental formula
3143
seen in some omnivores (eg. pig)
3143
carnivore teeth are more pointed to grasp and tear prey
herbivore teeth are flatter to grind fibrous plant material
grow continuously to compensate for excessive wear
ruminants
dental pad (thick connective tissue pad) replaces upper incisors
and canines
PHARYNX – conducts food and liquid from mouth to esophagus
bolus of food or liquid is pushed into pharynx by tongue
swallowing reflex – integrated in swallowing center (medulla)
larynx elevated – epiglottis closes glottis
pharyngeal muscles contract – push bolus into esophagus
ESOPHAGUS – muscular tube from pharynx to stomach
lies dorsal to trachea and passes through diaphragm
mucosa – stratified squamous epithelium (abrasion)
peristalsis – coordinated contraction and relaxation of circular and longitudinal
muscularis moves bolus from pharynx to stomach
regulated by swallowing center and local reflexes
lower (gastro)esophageal sphincter – remains closed until bolus arrives
prevents gastroesophageal reflux (acid from stomach damages esophagus)
horses and rabbits – one-way valve; almost impossible to vomit
regurgitation – food comes back up from esophagus or forestomach
MONOGASTRIC STOMACH – simple stomach
roughly J-shaped
greater curvature – large outer curvature
lesser curvature – small inner curvature
rugae – folds in mucosa
mucosa – simple columnar epithelium
gastric glands – deep glands beneath mucosa
3 layers of muscularis
FUNCTIONS OF STOMACH
storage of food – stretches to accommodate food
churning – mechanically breaks down food and mixes with secretions
chyme – soupy mixture of food and secretions
gastric motility – regulated by hormones and ANS
4
secretion of gastric juice
hydrochloric acid (HCl)
denatures proteins – breaks down food tissues
activates digestive enzymes (eg. pepsinogen)
kills bacteria in food
(stimulated by gastrin, parasympathetic n.s., histamine)
digestive enzymes
pepsin (active form or pepsinogen) – digests proteins
mucus – protects mucosa of stomach from HCl
ulcers – erosions of mucosa
prostaglandins – help protect stomach from acid
NSAIDs can cause ulcers
regulatory secretions
intrinsic factor – needed to absorb vit. B12
gastrin – hormone released as stomach fills
stimulates HCl secretion
REGIONS OF STOMACH
1) cardia – attached to esophagus
2) fundus – sac-like region next to cardia
stretches to store large meals; has many gastric glands
3) body – muscular central portion
churns food; has many gastric glands
4) antrum – connects to small intestine
pylorus – sphincter that regulates chyme entering small intestine
antrum secretes gastrin
Regulation of Gastric Activity
1) cephalic phase (head) – taste, smell, sight, thought of food
parasympathetic stimulation of gastric functions
2) gastric phase – food entering stomach
stretch reflexes and gastrin stimulate gastric functions
3) intestinal phase – chyme entering intestine
stretch reflexes and intestinal hormones inhibit gastric functions
SMALL INTESTINE – site of chemical digestion and absorption
FUNCTIONS OF SMALL INTESTINE
motility – regulated by strretch reflexes, hormones, and ANS
segmentation – mixing contractions of circular muscles
peristalsis – slow waves move contents from stomach to large intestine
hypermotility – contents move too rapidly
less absorption causes diarrhea
hypomotility (ileus) – contents move too slowly
bacterial growth – bloat, sepsis (bacteria in blood)
5
chemical digestion
mucus – alkaline to buffer HCl from stomach
digestive enzymes
intestinal (brush border) enzymes – attached to mucosa
pancreatic enzymes – pancreas secretes into duodenum
amylase – digests starch
sucrase, maltase, lactase – digest disaccharides
trypsin, chymotrypsin, peptidases – digest proteins
lipases – digest fats (lipids)
bile – liver secretes into duodenum
emulsifies fats – breaks up lipid droplets so they can be digested
by lipases
regulatory secretions – hormones secreted as chyme enters duodenum
coordinate activities of stomach, intestines, liver, and pancreas
secretin – inhibits gastric function
stimulates liver and pancreatic secretions
cholecystokinin (CCK) – inhibits gastric functions
stimulates gallbladder secretions and intestinal motility
absorption
nutrient molecules pass through mucosa into blood or lymph
villi and microvilli increase surface area for absorption
Substances Absorbed in Small Intestine
into blood capillaries
monosaccharides (carbohydrates)
amino acids (proteins)
electrolytes (ions)
water-soluble vitamins
water
into lymphatic capillaries
micelles – small lipid droplets surrounded by bile salts
dissolve lipids in intestinal lumen and transport them to
mucosa to be absorbed
fatty acids, monoglycerides (lipids)
lipid-soluble vitamins
ANATOMY OF SMALL INTESTINE
duodenum – short, proximal region attached to stomach
bile duct and pancreatic duct empty into intestine here
jejunum – longest, middle region
ileum – short, distal region attached to large intestine
ileocecal valve – opening into colon large intestine
regulated by a smooth muscle sphincter
6
mucosa – simple columnar epithelium w/goblet cells
villi – finger-like extensions of mucosa
increase surface area for absorption
lacteals – large lymphatic capillaries in villi
microvilli – finger-like extensions of epithelial cell membrane
brush border (fuzzy appearance) – increases surface area
Peyer’s patches – MALT
LARGE INTESTINE
FUNCTIONS OF LARGE INTESTINE
motility
haustral churning – haustra (pouches) fill, churn contents, pass contents
to the next haustra
peristalsis – moves contents from ileum to anus
chemical digestion
bacterial flora (resident bacteria) perform minor digestive functions
produce vitamins K and B
absorption
water, vitamins (K and B), bile salts
stores feces
ANATOMY OF THE LARGE INTESTINE
mucosa – simple columnar epithelium
many goblet cells – mucus lubricates feces
cecum – variable-sized blind pouch at proximal end
contains microbes for hindgut fermentation
colon – highly variable in size and appearance
rectum – distal end; expands to store feces
anus – exit for digestive tract (defecation)
internal anal sphincter – involuntary (smooth muscle)
external anal sphincter – voluntary (skeletal muscle)
mucosa – stratified squamous epithelium
REGULATION OF LARGE INTESTINE
gastrocolic reflex – stretch of stomach and duodenum triggers increased
peristalsis
defecation reflex – triggered by stretch receptors in wall of rectum
parasympathetic relaxation of internal anal sphincter
conscious component – feces in anal canal causes urge to defecate
voluntary control of external anal sphincter
contraction of abdominal muscles aids defecation
7
PANCREAS
endocrine function – pancreatic islets regulate blood glucose level
exocrine function – secretes pancreatic juice into small intestine
sodium bicarbonate – neutralizes HCl from stomach
pancreatic enzymes
proenzymes – inactive until activated in intestine
ANATOMY OF PANCREAS
located in mesentery between stomach and duodenum
pancreatic duct – carries pancreatic juice to duodenum
REGULATION OF EXOCRINE PANCREAS
cephalic phase of gastric stimulation
parasympathetic n.s. stimulates secretions
chyme entering duodenum
secretin – stimulates secretions
LIVER
hepatic portal system carries venous blood with absorbed nutrients from GI tract
to liver for processing before it enters the general circulation
liver regulates composition of the blood
FUNCTIONS OF LIVER
carbohydrate metabolism – maintains blood glucose level
glycogenesis – glucose stored as glycogen
glycogenolysis – glycogen broken down to release glucose
gluconeogenesis – amino acids, lactic acid, other nutrients converted to
glucose
synthesizes most plasma proteins
albumin, alpha and beta globulins, fibrinogen, prothrombin
removal or inactivation of many toxins in blood
detoxification – breaks down may drugs and toxins
eg. converts toxic ammonia to urea
excretes or stores other drugs and toxins (eg. antibiotics, pesticides)
eg. excretes bilirubin – from old blood cells
storage
vitamins (A,B12,D,E,K)
minerals (iron, copper)
phagocytosis and antigen presentation
macrophages remove old blood cells, debris, pathogens
synthesis and secretion of bile
8
COMPOSITION OF BILE
bicarbonate ions – neutralize acid from stomach
bile salts – lipids derived from cholesterol
amphipathic – have charged and uncharged regions
emulsification – break large lipid droplets into tiny micelles
lipid droplets coated by a layer of bile salts
aids in lipid digestion and absorption
bile pigments – from bilirubin; give bile its color
Regulation of Bile Secretion
parasympathetic n.s. stimulates bile secretion
chyme in duodenum
CCK – stimulates contraction of gall bladder
ANATOMY OF LIVER AND GALL BLADDER
largest gland of body
hepatic lobes – variable divisions of liver
gallbladder – pear-shaped sac near liver
stores bile produced by liver
(no gallbladder in horses)
common bile duct – empties bile into duodenum
HISTOLOGY OF LIVER
lobules – functional units of liver
central vein – drains into hepatic vein
triads at each corner of lobule
branch of hepatic artery – brings O2 and hormones to lobule
branch of hepatic portal vein – brings GI nutrients to lobule
bile duct – drains bile produced in lobule
hepatic sinusoids – large capillary spaces connecting central vein
to blood vessels of triad
contain macrophages – phagocytosis and present antigens
hepatocytes – liver cells lining sinusoids
adjust content of blood
produce bile
bile canaliculi – ducts which collect bile from hepatocytes
drain into bile ducts
9
RUMINANT DIGESTION – have forestomach that acts as a microbial fermentation
chamber to provide additional nutrition
4 Stomach Compartments:
1) reticulum – 1st chamber
heavy objects swallowed droop into reticulum
hardware disease – sharp metallic objects can penetrate wall of reticulum,
diaphragm, and pericardium of heart
2) rumen – 2nd chamber; fills most of left abdominal cavity
fermentation chamber – normal bacteria and protozoans mixed with
large volumes of basic saliva and ingested plant material
microbe enzymes digest cellulose (plant carbohydrate)
form volatile fatty acids (VFAs)
VFAs are absorbed by rumen and reticulum and converted
to glucose or lipids by ruminant
microbes use ingested proteins and urea from liver to grow and
multiply
(ruminant digests microbes as a protein source)
microbes also synthesize vitamins K and B
rumination – contractions of rumen and reticulum regurgitate fibrous
plant material (cud) to be rechewed
eructation – fermentation produces gases which are expelled (burps)
borborygmi – regular “gut sounds” due to normal motility
bloat – excess gas production, obstruction, reduced motility
3) omasum – 3rd chamber
muscular chamber with many inner folds (leaves)
mechanically breaks down food particles, absorbs water and bicarbonate
4) abomasum – 4th chamber; “true stomach”
similar to monogastric stomach
young ruminants – function as monogastrics
small, nonfunctional rumen and reticulum
nursing stimulates reflex folding of wall of reticulum
reticular groove – funnels milk directly to omasum
reflex stops in weeks to months as young begins grazing
HINDGUT FERMENTERS – large cecum and colon act as fermentation chambers
horse – cecum and large colon (dorsal and ventral colon)
cellulose and urea nourish microbial colonies
VFAs absorbed by cecum and large and small colons
(bicarbonate secreted into colon to neutralize acids)
other hindgut fermenters
rabbit, pig, guinea pig, rat, (ruminants)
10
11