BIOL 2304
The Digestive System
Digestive Processes
Ingestion – food enters GI tract at mouth
Propulsion – movement of food through GI tract
Peristalsis – major means of propulsion
Digestion – physical and chemical breakdown of foodstuff
Mechanical digestion – chewing, churning, segmentation
prepares food for chemical digestion
Chemical digestion – complex molecules broken down to
chemical components In mouth, stomach and small
intestine
Absorption – transport of digested nutrients to bloodstream
Defecation – elimination of indigestible substances as feces
Overview of the Digestive System
Digestive organs are divided into two groups:
Alimentary canal
Mouth, pharynx, esophagus, stomach, small
intestine, and large intestine
Accessory digestive organs
Teeth and tongue
Gallbladder, salivary glands, liver, and pancreas
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
Peristalsis
Segmentation
Histological Organization of Digestive Tract
Mucosa - lines digestive tract (mucous epithelium)
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 – outer membranes made of connective tissue, collagen, and elastin
Digestive Tract Membranes
Peritoneum – the serous membrane within the abdominal cavity
Parietal peritoneum – lines the body wall
Visceral peritoneum – covers digestive organs
Peritoneal cavity – the fluid filled space between the parietal and visceral peritonea
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
Lesser omentum attaches to lesser curvature of stomach
Oral cavity
Lined by oral mucosa – stratified squamous, not serosa
Hard and soft palates form roof of oral cavity
Uvula – guards opening to pharynx; assists in some voice production
The Oral Cavity
Floor of cavity – tongue
Mechanical processing
Assistance in chewing and swallowing
Sensory analysis by touch, temperature, and taste receptors
Functions include:
Analysis of material before swallowing
Mechanical processing by the teeth, tongue, and palatal surfaces
Lubrication
Limited digestion
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
(salivary amylase)
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
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 Pharynx
Glottis – the space in between the vocal folds
Epiglottis – a flap of cartilage at the root of the
tongue that is used to cover the opening of the
windpipe during 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
Epithelium is stratified squamous epithelium
When empty, mucosa and submucosa form 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, secretion of pepsin and HCl begins protein digestion
Bulk storage of undigested food, food remains in stomach approximately 4 hours
Anatomy Of The Stomach
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
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
absorb water and electrolytes
compact material into feces
Absorb vitamins produced by bacteria
Store fecal matter prior to defecation
Large Intestine
Gross Anatomy of Large Intestine
Four areas of the colon
Ascending colon
Transverse colon
Descending colon
Sigmoid colon
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 mucus-secreting 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
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
Lobules containing single-cell thick plates of hepatocytes
Lobules empty into bile ducts, bile ducts merge to left/right hepatic ducts
Left and right hepatic ducts merge to form common hepatic duct
Common hepatic meets cystic duct to form common bile duct
The Liver
The Liver
Liver Lobule - basic functional unit
Lobule – hepatocytes arranged in hexagonal plates with vessels permeating 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