BIO2305
Digestive Physiology
The alimentary canal or gastrointestinal (GI) tract digests and absorbs food
- Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine
- Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas
Basic organization of the digestive tract
• Mucosa lines digestive tract (mucous epithelium)
– Moistened by glandular secretions
– Lamina propria and epithelium form mucosa
• Submucosa - layer of dense irregular connective tissue
• Muscularis externa - smooth muscle arranged in circular and longitudinal layers
• Serosa - serous membrane covering most of the muscularis externa
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Digestive System Anatomy
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Digestive System Activities
• The GI tract is a “disassembly” line
• Nutrients become more available to the body in each step
– Ingestion – taking food into the digestive tract
– Mechanical digestion – chewing, churning food, segmentation
– Propulsion – swallowing and peristalsis
– Chemical digestion – catabolic breakdown of food
– Absorption – movement of nutrients from the GI tract to the blood or lymph
– Excretion – elimination of indigestible solid wastes
Basic Processes of the Digestive System
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Motility
Movement of digestive materials
Visceral smooth muscle
Tonic contractions
Sustained
Smooth muscle sphincters and stomach
Phasic contractions
Rhythmic cycles of activity - pacemaker cells
Last a few seconds
Peristalsis moves bolus forward
Segmentation mixes
Regulation of Digestion
2 levels of control
• Intrinsic control regulated by local centers
– Autonomous smooth muscle pacesetter cells
– Intrinsic nerve plexuses and sensory receptors
• Extrinsic control regulated by
– ANS
– GI hormones
Intrinsic Control: Autonomous Smooth Muscle
Pacesetter cells
Slow wave potentials – digestive tracts’s basic electrical rhythm
Wavelike fluctuations in membrane potential
Transmitted throughout smooth muscle via gap junctions
Threshold is reached by the effect of various mechanical, neural and hormonal factors
Intrinsic nerve plexuses
 Digestive tract has its own intramural nervous system – referred to as enteric nervous system.
Initiates reflexes that activate or inhibit digestive glands, mix lumen contents and move them
along
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Motility
• Visceral smooth muscle shows rhythmic cycles of activity - pacemaker cells
• Peristalsis - waves that move a bolus
Motility
• Segmentation – churns and fragments a bolus
Ingestion and Mechanical Digestion
• Food is ingested
• Mechanical digestion begins (chewing)
• Propulsion is initiated by swallowing
• Salivary amylase begins chemical breakdown of starch
• The pharynx and esophagus serve as conduits to pass food from the mouth to the stomach
• Uvula guards opening to pharynx
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Oral Cavity functions include:
Its functions include:
- Analysis of material before swallowing by touch, temperature, and taste receptors
- Mechanical processing by the teeth, tongue, and palatal surfaces
- Lubrication
- Assistance swallowing
- Limited digestion
Digestive Processes in the Mouth
Food is ingested
Mechanical digestion begins (chewing)
Propulsion is initiated by swallowing
Salivary amylase begins chemical breakdown of starch
The pharynx and esophagus serve as conduits to pass food from the mouth to the stomach
Uvula guards opening to pharynx
Salivary Glands (3 Pairs)
Salivary glands (3 pairs) - parotid, sublingual, and submandibular; produce saliva
Stimulated by thought of food or ingested food
Secreted from serous and mucous cells of salivary glands
Produce saliva - watery solution includes electrolytes, buffers, glycoproteins, antibodies, enzymes
Functions include:
lubrication, moistening, dissolving
initiation of digestion of complex carbohydrates (starches)
Strong sympathetic stimulation inhibits salivation and results in dry mouth
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Saliva: Source and Composition
Secreted from serous and mucous cells of salivary glands
A 97-99.5% water, hypo-osmotic, slightly acidic solution containing
Electrolytes – Na+, K+, Cl–, PO42–, HCO3–
Digestive enzyme – salivary amylase
Proteins – mucin, lysozyme, defensins, and IgA
Metabolic wastes – urea and uric acid
Control of Salivation
Intrinsic glands keep the mouth moist, extrinsic salivary glands secrete serous, enzyme-rich saliva in
response to:
- Ingested food which stimulates chemoreceptors and pressoreceptors
- The thought of food
- Strong sympathetic stimulation inhibits salivation and results in dry mouth
Swallowing (Deglutition) Process
• Involves the coordinated activity of the tongue, soft palate, pharynx, esophagus and 22
separate muscle groups
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Three phases
– Buccal phase – tongue pushes bolus against soft palate and forced into the oropharynx,
triggering swallowing reflex – controlled by the medulla and lower pons
– Pharyngeal – esophogeal sphincter relaxes while epiglottis closes
– Esophageal – bolus moves into esophagus propelled by peristalsis and into stomach
The esophagus carries solids and liquids from the pharynx to the stomach, passes through esophageal
hiatus in diaphragm.
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Stomach
5 Functions of the stomach:
- Holds ingested food
- Degrades this food both physically and chemically
- Delivers chyme to the small intestine
- Enzymatically digests proteins with pepsin
- Secretes intrinsic factor required for absorption of vitamin B12
Stomach Lining
The stomach is exposed to the harshest conditions in the digestive tract
To keep from digesting itself, the stomach has a mucosal barrier with:
 A thick coat of bicarbonate-rich mucus on the stomach wall
 Epithelial cells that are joined by tight junctions
 Gastric glands that have cells impermeable to HCl
 Damaged epithelial cells are quickly replaced
Glands of the Stomach
Gastric glands have a variety of secretory cells
 Mucous neck cells – secrete acid mucus
 Parietal cells – secrete HCl and intrinsic factor
 Chief cells – produce pepsinogen , pepsinogen is activated to pepsin by:
1. HCl in the stomach
2. Pepsin itself via a positive feedback mechanism
 Enteroendocrine cells secrete
o gastrin,
o histamine,
o endorphins
o serotonin,
o cholecystokinin (CCK),
o somatostatin
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Regulation of Gastric Secretion
• Neural and hormonal mechanisms regulate the release of gastric juice
• Stimulatory and inhibitory events occur in three phases
– Cephalic (reflex) phase: prior to food entry
– Gastric phase: once food enters the stomach
– Intestinal phase: as partially digested food enters the duodenum
Cephalic (reflex) phase: prior to food entry, prepares stomach to receive ingested material,
ccelerates gastric juices
 Directed by CNS via the vagus nerve
 Stimulated by sight, smell, taste, or thought of food
 Accelerates gastric juices
 Inhibited by loss of appetite, depression
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Excitatory events include:
- Sight or thought of food
- Stimulation of taste or smell receptors
Inhibitory events include:
- Loss of appetite or depression
- Decrease in stimulation of the parasympathetic division
Gastric phase
 begins with the arrival of food in the stomach
• Enhanced secretion of gastric juices due to the arrival of food in the stomach
– Homogenize and acidify chyme
– Production of pepsinogen - digestion of proteins
• Stimulated by
• Stomach distension - activation of stretch receptors
• Chemoreceptors detects - peptides, caffeine, and rising pH
– Neural - plexuses
– Hormonal - secretion of gastrin
• Inhibitory events include:
– A pH lower than 2
– Emotional upset that overrides the parasympathetic division
Intestinal phase:
 as partially digested food enters the duodenum – release of hormones controls (inhibits)
the rate of gastric emptying by releasing
• Intestinal phase – release of hormones controls the rate of gastric emptying
• Excitatory phase – distension of duodenum, presence of partially digested foods
• Releases enterogastrones that inhibit gastric secretion: CCK, GIP, Secretin
• Inhibited by low pH, presence of fatty, acidic, or hypertonic chyme, and/or irritants in
the duodenum
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Release of Gastric Juice
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Regulation of HCl Secretion
• HCl secretion is stimulated by ACh, histamine, and gastrin through second-messenger systems
• Release of hydrochloric acid:
– Is low if only one ligand binds to parietal cells
– Is high if all three ligands bind to parietal cells
• Antihistamines block H2 receptors and decrease HCl release
Response of the stomach to filling
Stomach pressure remains constant until about 1L of food is ingested
Reflex-mediated events include:
Receptive relaxation – as food travels in the esophagus, stomach muscles relax
Adaptive relaxation – the stomach dilates in response to gastric filling
Gastric Contractile Activity
 Peristaltic waves move toward the pylorus at the rate of 3 per minute .
 This basic electrical rhythm BER) is initiated by pacemaker cells (cells of Cajal).
 Most vigorous peristalsis and mixing occurs near the pylorus
 Chyme is either delivered in small amounts to the duodenum or forced backward into the
stomach for further mixing
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Regulation of Gastric Emptying
• Gastric emptying is regulated by:
– The neural enterogastric reflex
– Hormonal (enterogastrone) mechanisms
• These mechanisms inhibit gastric secretion and duodenal filling
• Carbohydrate-rich chyme quickly moves through the duodenum
• Fat-laden chyme is digested more slowly causing food to remain in the stomach longer
Gastrointestinal Hormones
• Gastrin
– Release is stimulated by presence of protein in stomach
– Secretion inhibited by accumulation of acid in stomach
• Acts in several ways to increase secretion of HCl and pepsinogen
• Enhances gastric motility, stimulates ileal motility, relaxes ileocecal sphincter,
induces mass movements in colon
• Helps maintain well-developed, functionally viable digestive tract lining
• Secretin
– Presence of acid in duodenum stimulates release
– Inhibits gastric emptying in order to prevent further acid from entering duodenum until
acid already present is neutralized
– Inhibits gastric secretion to reduce amount of acid being produced
– Stimulates pancreatic duct cells to produce large volume of aqueous NaHCO3 secretion
– Stimulates liver to secrete NaCO3 rich bile which assists in neutralization process
– Along with CCK, is trophic to exocrine pancreas
• CCK
– Inhibits gastric motility and secretion
– Stimulates pancreatic acinar cells to increase secretion of pancreatic enzymes
– Causes contraction of gallbladder
– Along with secretin, is trophic to exocrine pancreas
– Implicated in long-term adaptive changes in proportion of pancreatic enzymes in
response to prolonged diet changes
– Important regulator of food intake
• GIP
– Glucose-dependent insulinotrophic peptide
– Stimulates insulin release by pancreas
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Regulation of Gastric Emptying
Digestion & Absorption in Stomach
Preliminary digestion of proteins via pepsin
Permits digestion of carbohydrates
Very little absorption of nutrients –some drugs, however, are absorbed
Small intestine
• Important digestive and absorptive functions
– Secretions and buffers provided by pancreas, liver, gall bladder
• Three subdivisions:
– Duodenum
– Jejunum
– Ileum
• Ileocecal sphincter - transition between small and large intestine
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Structural modifications of the small intestine wall increase surface area
 Plicae circulares: deep circular folds of the mucosa and submucosa
 Villi: fingerlike extensions of the mucosa
 Microvilli: tiny projections of absorptive mucosal cells’ plasma membranes
The epithelium of the mucosa is made up of:
Absorptive cells and goblet cells
Enteroendocrine cells
Interspersed T cells called intraepithelial lymphocytes (IELs)
Cells of intestinal crypts secrete intestinal juice
Secreted in response to distension or irritation of the mucosa
Slightly alkaline and isotonic with blood plasma
Largely water, enzyme-poor, but contains mucus
Introduction of secretions and buffers provided by:
 pancreas
 liver
 gall bladder
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Functions of Glands of the duodenum:
 moisten chyme,
 help buffer acids,
 maintain digestive material in solution
Hormones
o Secretin - produces alkaline buffers, increase bile by liver and pancreas
o Cholecystokinin (CCK)– increase pancreatic enzymes, stimulates contraction of gall
bladder, reduces hunger sensation
o GIP – stimulates release of insulin, inhibits gastric secretion and motility
Activities of Major Digestive Tract Hormones
The Liver
The largest gland in the body
Performs metabolic and hematological regulation and produces bile
Histological organization
Lobules containing single-cell thick plates of hepatocytes
Lobules unite to form common hepatic duct
Duct meets cystic duct to form common bile duct
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Hexagonal-shaped liver lobules are the structural and functional units of the liver
Composed of hepatocytes
Hepatocytes’ functions include:
– Production of bile
– Processing bloodborne nutrients
– Storage of fat-soluble vitamins
– Detoxification
Secreted bile flows between hepatocytes toward the bile ducts in the portal triads
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Composition of Bile
• A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats,
phospholipids, and electrolytes
• Bile salts are cholesterol derivatives that:
– Emulsify fat
– Facilitate fat and cholesterol absorption
– Help solubilize cholesterol
• Enterohepatic circulation recycles bile salts
• The chief bile pigment is bilirubin, a waste product of heme
The Gallbladder
• Thin-walled, green muscular sac on the ventral surface of the liver
• Stores and concentrates bile by absorbing its water and ions
• Releases bile via the cystic duct, which flows into the bile duct
Regulation of Bile Release
 Acidic, fatty chyme causes the duodenum to release:
o Cholecystokinin (CCK) and secretin into the bloodstream
 Bile salts and secretin transported in blood stimulate the liver to produce bile
 Vagal stimulation causes weak contractions of the gallbladder
 Cholecystokinin causes:
o The gallbladder to contract
o The hepatopancreatic sphincter to relax
 As a result, bile enters the duodenum
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Regulation of Bile Release
The Pancreas
• Pancreatic duct penetrates duodenal wall
• Endocrine functions - insulin and glucagons
• Exocrine functions
– Secretes pancreatic juice secreted into small intestine which breaks down all categories
of foodstuff
– Acini (clusters of secretory cells) contain zymogen granules with digestive enzymes
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Composition of Pancreatic Juice
• Water solution of enzymes and electrolytes (primarily HCO3–)
– Neutralizes acid chyme
– Provides optimal environment for pancreatic enzymes
• Enzymes are released in inactive form and activated in the duodenum
• Examples include
– Trypsinogen is activated to trypsin
– Procarboxypeptidase is activated to carboxypeptidase
• Active enzymes secreted
– Amylase, lipases, and nucleases
– These enzymes require ions or bile for optimal activity
Regulation of Pancreatic Secretion
 Secretin and CCK are released when fatty or acidic chyme enters the duodenum
 CCK and secretin enter the bloodstream
 Upon reaching the pancreas,
o CCK induces the secretion of enzyme-rich pancreatic juice
o Secretin causes secretion of bicarbonate-rich pancreatic juice.
 Vagal stimulation also causes release of pancreatic juice
Regulation of Pancreatic Secretion
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Digestion in the small intestine
• As chyme enters the duodenum:
– Carbohydrates and proteins are only partially digested
– No fat digestion has taken place
• Digestion continues in the small intestine
– Chyme is released slowly into the duodenum
– Because it is hypertonic and has low pH, mixing is required for proper digestion
– Required substances needed are supplied by the liver
– Virtually all nutrient absorption takes place in the small intestine
Motility in the Small Intestine
• The most common motion of the small intestine is segmentation
– It is initiated by intrinsic pacemaker cells (Cajal cells)
– Moves contents steadily toward the ileocecal valve
• After nutrients have been absorbed:
– Peristalsis begins with each wave starting distal to the previous
– Meal remnants, bacteria, mucosal cells, and debris are moved into the large intestine
Control of Motility
• Local enteric neurons of the GI tract coordinate intestinal motility
• Cholinergic neurons cause:
– Contraction and shortening of the circular muscle layer
– Shortening of longitudinal muscle
– Distension of the intestine
• Other impulses relax the circular muscle
• The gastroileal reflex and gastrin:
– Relax the ileocecal sphincter
– Allow chyme to pass into the large intestine
Functions of the Large Intestine
 Reabsorb water and compact material into feces
 Absorb vitamins produced by bacteria
 Store fecal matter prior to defecation
 Other than digestion of enteric bacteria, no further digestion takes place
 Vitamins, water, and electrolytes are reclaimed
 Its major function is propulsion of fecal material toward the anus
 Though essential for comfort, the colon is not essential for life
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Motility of the Large Intestine
Haustral contractions
Slow segmenting movements that move the contents of the colon
Haustra sequentially contract as they are stimulated by distension
Presence of food in the stomach:
Activates the gastrocolic reflex
Initiates peristalsis that forces contents toward the rectum
Rectum
• Last portion of the digestive tract
• Terminates at the anal canal
• Internal and external anal sphincters
• Defecation reflex triggered by distention of rectal walls
Defecation
• Distension of rectal walls caused by feces:
– Stimulates contraction of the rectal walls
– Relaxes the internal anal sphincter
• Voluntary signals stimulate relaxation of the external anal sphincter and defecation occurs
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Regulation of Digestion
• Intrinsic control by local centers
– Autonomous smooth muscle pacesetter cells
– Intrinsic nerve plexuses and sensory receptors
• Extrinsic control
– ANS
– GI hormones
• Mechano- and chemoreceptors respond to:
– Stretch, osmolarity, and pH
– Presence of substrate, and end products of digestion
• They initiate reflexes that:
– Activate or inhibit digestive glands
– Mix lumen contents and move them along
Nervous Control of the GI Tract
• Intrinsic controls
– Nerve plexuses near the GI tract initiate short reflexes
– Short reflexes are mediated by local enteric plexuses (gut brain)
• Extrinsic controls
– Long reflexes arising within or outside the GI tract
– Involve CNS centers and extrinsic autonomic nerves
– Parasympathetic reflexes
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The Enteric Nervous system
Composed of 2 major intrinsic nerve plexuses
Submucosal nerve plexus – regulates glands and smooth muscle in the mucosa
Myenteric nerve plexus – major nerve supply that controls GI tract mobility, initiates
reflexes that activate or inhibit digestive glands, mix lumen contents and move
them along
Segmentation and peristalsis are largely automatic involving local short reflex arcs
Linked to the CNS extrinsic controls via long autonomic reflex arc
Control of Digestion
Neural and hormonal mechanisms coordinate glands
Hormonal mechanisms enhance or inhibit smooth muscle contraction
Local mechanisms coordinate response to changes in pH or chemical stimuli
Digestion and Absorption of Nutrients
Disassembling organic food into smaller fragments
Hydrolyzing carbohydrates, proteins, lipids and nucleic acids for absorption
Chemical Digestion: Carbohydrates
• Begins in the mouth
– Salivary and pancreatic enzymes catabolize into disaccharides and trisaccharides
– Brush border enzymes catabolize into monosaccharides
• Absorption of monosaccharides occurs across the intestinal epithelia
• Absorption: via cotransport with Na+, and facilitated diffusion
– Enter the capillary bed in the villi
– Transported to the liver via the hepatic portal vein
• Enzymes used: salivary amylase, pancreatic amylase, and brush border enzymes
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Chemical Digestion: Proteins
• Low pH destroys tertiary and quaternary structure
• Enzymes used include pepsin, trypsin, chymotrypsin, and elastase
– Liberated amino acids are absorbed
• Absorption: similar to carbohydrates
• Enzymes used: pepsin in the stomach
• Enzymes acting in the small intestine
– Pancreatic enzymes – trypsin, chymotrypsin, and carboxypeptidase
– Brush border enzymes – aminopeptidases, carboxypeptidases, and dipeptidases
Lipid digestion and absorption
 Lipid digestion utilizes lingual and pancreatic lipases
o Bile salts improve chemical digestion by emulsifying lipid drops
o Lipid-bile salt complexes called micelles are formed
o Micelles diffuse into intestinal epithelia which release lipids into the blood as
chylomicrons
Chemical Digestion: Fats
Absorption: Diffusion into intestinal cells where they:
Combine with proteins and extrude chylomicrons
Enter lacteals and are transported to systemic circulation via lymph
Glycerol and short chain fatty acids are:
Absorbed into the capillary blood in villi
Transported via the hepatic portal vein
Enzymes/chemicals used: bile salts and pancreatic lipase
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Fatty Acid Absorption
Fatty acids and monoglycerides enter intestinal cells via diffusion
They are combined with proteins within the cells
Resulting chylomicrons are extruded
They enter lacteals and are transported to the circulation via lymph
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Chemical Digestion: Nucleic Acids
• Absorption: active transport via membrane carriers
• Absorbed in villi and transported to liver via hepatic portal vein
• Enzymes used: pancreatic ribonucleases and deoxyribonuclease in the small intestines
Absorption
• Water - nearly all (95%) that is ingested is reabsorbed
– Net osmosis occurs whenever a concentration gradient is established by active transport
of solutes into the mucosal cells
– Water uptake is coupled with solute uptake, and as water moves into mucosal cells,
substances follow along their concentration gradients
• Vitamins
– Water soluble vitamins are absorbed by diffusion
– Fat soluble vitamins are absorbed as part of micelles
• Vitamin B12 requires intrinsic factor
Electrolyte Absorption
• Most ions are actively absorbed along the length of small intestine
– Na+ is coupled with absorption of glucose and amino acids
– Ionic iron is transported into mucosal cells where it binds to ferritin
• Anions passively follow the electrical potential established by Na+
• K+ diffuses across the intestinal mucosa in response to osmotic gradients
• Ca2+ absorption:
– Is related to blood levels of ionic calcium
– Is regulated by vitamin D and parathyroid hormone (PTH)
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