Download The Digestive System

The Digestive System
Overview of the Digestive System
• Organs are divided into
two groups
– Alimentary canal
• Mouth, pharynx, and
esophagus
• Stomach, small intestine,
and large intestine
– Accessory digestive
organs
• Teeth and tongue
• Gallbladder, salivary
glands, liver, and
pancreas
Digestive Processes
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Ingestion – occurs in the mouth
Propulsion – movement of food
– Peristalsis – major means of propulsion
•
Mechanical digestion – prepares food
for chemical digestion
– Chewing, churning of food in stomach,
segmentation
•
Chemical digestion – complex
molecules broken down to chemical
components
– Mouth
– Stomach
– Small intestine
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Absorption – transport of digested
nutrients
Defecation – elimination of indigestible
substances as feces
Digestive Tract Smooth Muscle Movement
• Peristalsis
– Major means of propulsion
– Adjacent segments of the
alimentary canal relax and
contract
• Segmentation
– Rhythmic local contractions of
the intestine
– Mixes food with digestive juices
Figure 22.3a
Histological Organization of Digestive Tract
•
Mucosa lines digestive tract (mucous epithelium)
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Epithelium lines the lumen, moistened by secretions of mucosal glands
Lamina propria – loose areolar tissue
Muscularis mucosa – layer of smooth that creates folds called rugae
Submucosa - layer of dense irregular connective tissue, vascularized (blood and
lymphatic), innervated – nerve plexus, submucosal glands
Muscularis externa - smooth muscle arranged in circular and longitudinal layers
Adventitia or Serosa - serous membrane made of areolar connective tissue with
collagen and elastic fibers, covers most of the muscularis externa
Digestive Tract Membranes
• Peritoneum – a serous membrane
– Parietal peritoneum – lines the body wall
– Visceral peritoneum – surrounds digestive organs
• Peritoneal cavity – a fluid filled space
Digestive Tract Membranes
• Mesentery – a double layer of peritoneum
– Holds organs in place
– Sites of fat storage
– Provides a route for circulatory vessels and nerves
Mesenteries
• Greater omentum – a “fatty apron” of peritoneum
– Attaches the greater curvature of the stomach to the dorsal body
wall
– Covers the transverse colon and a large part of the small intestine
Figure 22.10c
Mesenteries
• Lesser omentum attaches to lesser curvature
of stomach
Figure 22.10b
Oral cavity
• Lined by oral mucosa = stratified
squamous but no serosa
• Roof of cavity = hard and soft
palates
• Uvula guards opening to
pharynx
• Floor of cavity = tongue
– Mechanical processing
– Assistance in chewing and
swallowing
– Sensory analysis by touch,
temperature, and taste receptors
The Oral Cavity
• Its functions include:
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Analysis of material before swallowing
Mechanical processing by the teeth, tongue, and palatal surfaces
Lubrication
Limited digestion
Figure 24.6a, b
Salivary glands (three pairs)
• Parotid, sublingual, and submandibular
• Produce saliva - watery solution includes electrolytes, buffers,
glycoproteins, antibodies, enzymes
• Functions include: lubrication, moistening, and dissolving, Initiation
of digestion of complex carbohydrates
Teeth
• Function in mastication of bolus
• Contain three layers
– Enamel covering crown
– Dentin forms basic structure
– Root - periodontal ligaments hold teeth in alveoli
• 20 primary teeth AKA deciduous teeth
• 32 teeth of secondary dentition
The Pharynx
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Oropharynx and laryngopharynx
Common passageway for food, liquids, and air
Lined with stratified squamous epithelium, no serosa
External muscle layer
– Consists of superior, middle, and inferior pharyngeal constrictors
– Pharyngeal constrictor muscles assist in swallowing
The Esophagus
• Muscular tube - begins as a
continuation of the pharynx
• Carries solids and liquids
from the pharynx to the
stomach
– Passes through esophageal
hiatus in diaphragm
• Joins the stomach inferior to
the diaphragm
– Cardiac sphincter – closes
lumen to prevent stomach acid
from entering esophagus
• The wall of the esophagus
contains mucosal (stratified
squamous), submucosal,
and muscularis layers
The Esophagus
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Epithelium is stratified squamous epithelium
When empty – mucosa and submucosa in longitudinal folds
Mucous glands – primarily compound tubuloalveolar glands
Muscularis externa
• Skeletal muscle first third of length
– Adventitia – most external layer
The Stomach
• Site where food is churned into chyme, Mechanical
breakdown of food
• Breaking of chemical bonds via acids and enzymes,
decretion of pepsin and HCl begins protein digestion
• Bulk storage of undigested food, food remains in
stomach approximately 4 hours
Anatomy Of The Stomach
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Cardia – superior, medial portion
Fundus – portion superior to stomach-esophageal junction
Body – area between the fundus and the curve of the J
Pylorus – antrum and pyloric canal adjacent to the duodenum
Rugae - ridges and folds in relaxed stomach
Microscopic Anatomy of the Stomach
• Muscularis has three layers
– Circular and longitudinal layers and oblique layer
• Epithelium is simple columnar epithelium
• Mucosa dotted with gastric pits holds gastric glands
Microscopic Anatomy of the Stomach
• Gastric glands of fundus and body
– Mucous neck cells - secrete a special mucus
– Parietal (oxyntic) cells - secrete hydrochloric acid and gastric
intrinsic factor
– Chief (zymogenic) cells - secrete pepsinogen
• Glands produce 1500ml juice per day
Small intestine
• Longest portion of the
alimentary canal
• Site of most enzymatic
digestion and absorption
– Secretions and buffers
provided by pancreas, liver,
gall bladder
• Three subdivisions:
– Duodenum
– Jejunum
– Ileum
• Ileocecal sphincter transition between small
and large intestine
The Duodenum
• Receives digestive enzymes and bile
• Main pancreatic duct and common bile duct enter
duodenum
– Sphincters control entry of bile and pancreatic juices
The Small Intestine – Microscopic Anatomy
• Modifications for absorption
– Circular folds (plicae circulares) - transverse ridges of mucosa
and submucosa
– Villi - finger-like projections of the mucosa, covered with simple
columnar epithelium
– Microvilli - further increase surface area for absorption
– Lacteals - terminal lymphatic in villus
Histology of the Intestinal Wall
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Absorptive cells - uptake digested nutrients
Goblet cells - secrete mucus that lubricates chyme
Enteroendocrine cells - secrete hormones
Intestinal crypts - epithelial cells secrete intestinal juice
Large Intestine
• Digested residue contains few nutrients
• Small amount of digestion by bacteria
• Main functions
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absorb water and electrolytes
compact material into feces
Absorb vitamins produced by bacteria
Store fecal matter prior to defecation
Gross Anatomy of Large Intestine
• Four areas of the colon
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Ascending
Transverse
Descending
Sigmoid
• Special features of large
intestine
– Teniae coli - thickening of
longitudinal muscularis
– Haustra - puckering
created by teniae coli
– Cecum - blind pouch,
beginning of large intestine
– Vermiform appendix Contains lymphoid tissue
The Rectum
• Rectum - descends along the
inferior half of the sacrum
• Anal Canal - the last subdivision
of the large intestine
– Lined with stratified squamous
epithelium
– Last portion of the digestive tract
– Terminates at the anal canal
– Internal and external anal
sphincters
– Defecation reflex triggered by
distention of rectal walls
Microscopic Anatomy of Large Intestine
• Villi are absent
• Contains numerous goblet
cells
• Intestinal crypts – simple
tubular glands
• Lined with simple columnar
epithelial tissue
– Epithelium changes at anal
canal, becomes stratified
squamous epithelium
The pancreas
• Pancreatic duct penetrates duodenal wall
• Endocrine functions - insulin and glucagons
• Exocrine functions - majority of pancreatic secretions, pancreatic
juice secreted into small intestine
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Carbohydrases
Lipases
Nucleases
Proteolytic enzymes
The Pancreas
• Exocrine function
– Acinar cells make, store, and secrete pancreatic enzymes
– Enzymes are activated in the duodenum
• Endocrine function
– Produces insulin and glucagon
– Regulates blood sugar
The liver
• Performs metabolic and hematological regulation and produces bile
• Four Lobes: left, right, caudate, quadrate
• Histological organization
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Lobules containing single-cell thick plates of hepatocytes
Lobules empty into bile ducts, bile ducts merge to left/right hepatic ducts
L/R hepatic ducts merge to form common hepatic duct
Common hepatic meets cystic duct to form common bile duct
Liver lobule - basic functional unit
• Hepatocytes form irregular plates arranged in
spoke-like fashion
• Bile canaliculi carry bile to bile ductules
• Bile ductules lead to bile ducts in portal areas
The Gallbladder
• Hollow, pear-shaped organ
• Stores, modifies and concentrates bile
• Bile – bile salts help buffer acids and acts as emulsifier
for lipid digestion
Figure 24.21a, b