BIO2305
The Digestive System
The Digestive System
The alimentary canal or gastrointestinal
(GI) tract digests and absorbs food
Alimentary canal:
Oral cavity
Pharynx
Esophagus
Stomach
Small intestine
Large intestine
Rectum
Accessory digestive organs:
Teeth
Tongue
Salivary glands
Liver
Gallbladder
Pancreas
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 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
Motility
Movement of digestive materials
Visceral smooth muscle
Tonic contractions
Sustained, long-lasting
Smooth muscle sphincters and stomach
Phasic contractions
Rhythmic cycles of contraction and relaxation initiated by pacemaker cells (modified
smooth muscles cells)
Last a few seconds
Peristalsis moves bolus forward
Segmentation mixes
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Autonomous Smooth Muscle
GI Pacemaker cells (modified smooth muscles cells) are called Interstitial cells of Cajal
(ICCs)
Slow wave potentials
digestive tract’s basic electrical rhythm
cycles between depolarizations and repolarizations
temporal summation slowly brings the cell membrane to threshold
Action potentials are transmitted throughout smooth muscle via gap junctions
Rate and frequency of action potentials are effected of various mechanical, neural, and hormonal
factors
Motility
Peristalsis – waves that move a bolus forward
Segmentation – to churn and fragment a bolus
No net forward movement
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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Salivary Glands (three types)
Parotid, sublingual, and submandibular glands produce saliva
Stimulated by thought of food or ingested food
Secreted from serous and mucous cells of salivary glands
Watery solution includes electrolytes, buffers, glycoproteins, antibodies, enzymes
Functions include:
Lubrication, moistening, and dissolving
Salivary Amylase – a salivary enzyme that initiates of digestion of complex carbohydrates
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
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 – esophageal sphincter relaxes while epiglottis
closes
Esophageal – bolus moves into esophagus propelled by
peristalsis and into stomach
Functions of the Stomach
Receives and holds ingested food
Digests 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
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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
Glands of the Stomach
Gastric glands have a variety of secretory cells
Mucous neck cells secrete mucus
Parietal cells secrete HCl and intrinsic factor
Chief cells secrete pepsinogen
HCl converts pepsinogen to pepsin in the stomach
Pepsin activates more pepsin via positive feedback
Enteroendocrine cells secrete gastrin, histamine, cholecystokinin (CCK)
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
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Cephalic Phase
Cephalic phase prepares stomach to receive
ingested material
Directed by CNS (parasympathetic nervous system)
via the vagus nerve (CN X)
Stimulated by sight, smell, taste, or thought of food
Accelerates gastric juices
Inhibited by loss of appetite, depression
Gastric Phase
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:
pH lower than 2
Emotional upset that overrides the parasympathetic division
Intestinal phase
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
Regulation of HCl Secretion
Hydrochloric acid (HCl) secretion is stimulated by ACh,
histamine, and gastrin through second-messenger
systems
Release of HCl:
Is low if only one ligand binds to parietal cells
Is high if all three ligands bind to parietal cells
Antihistamines block Histamine H2 receptors and
decrease HCl release
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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
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
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 NaHCO3 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 And Absorption In The Stomach
Preliminary digestion of proteins by pepsin
Permits digestion of carbohydrates
Very little nutrient absorption
Although lipid soluble drugs (e.g.: alcohol, aspirin) can
cross the gastric mucosa and enter the blood stream
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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Small Intestine
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
Small Intestine
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
Small Intestine
Glands of the duodenum
Moisten chyme
Help buffer acids
Maintain digestive material in solution
Hormones
Secretin - produces alkaline buffers, increase bile by liver and pancreas
Cholecystokinin (CCK) – increase pancreatic enzymes, stimulates contraction of gall bladder,
reduces hunger sensation
GIP – stimulates release of insulin, inhibits gastric secretion and motility
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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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The Liver
Hexagonal-shaped liver lobules are the structural
and functional units of the liver
Composed of hepatocytes (liver cells)
Hepatocytes’ functions include:
Production of bile
Processing blood-borne nutrients
Storage of fat-soluble vitamins
Detoxification
Production of plasma proteins
Secreted bile flows between hepatocytes toward
the bile ducts in the portal triads
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 (stercobilin), 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
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Regulation of Bile Release
Acidic, fatty chyme causes the duodenum to release:
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 (CCK):
The gallbladder to contract
The hepatopancreatic sphincter to relax
As a result, bile enters the duodenum
Regulation of Bile Release
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The Pancreas
Pancreatic duct penetrates duodenal wall at duodenal papilla
Endocrine functions:
Modulate Blood Glucose Levels:
Insulin (to ↓BGL) and glucagon (to ↑BGL)
Endocrine hormones enter venous bloodstream
Exocrine functions:
Digestion:
Pancreatic juice secreted into
small intestine to break down all
categories of foodstuff
Acini – clusters of secretory
acinar cells that contain zymogen
granules with digestive enzymes
Exocrine juices enter duodenum through
duodenal papilla, controlled by
hepatopancreatic sphincter
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:
Pancreatic 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:
CCK induces the secretion of enzyme-rich pancreatic juice
Secretin causes secretion of bicarbonate-rich pancreatic juice
Vagal stimulation also causes release of pancreatic juice
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Regulation of Pancreatic Secretion
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
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Functions of the Large Intestine
Reabsorb water and compact material into feces
Absorb vitamins produced by bacteria
Store fecal matter prior to defecation
Functions of the Large Intestine
Other than digestion of enteric bacteria, no further digestion takes place
Vitamins, water, and electrolytes are reclaimed
Major function: propulsion of fecal material toward the anus
Though essential for comfort, the colon is not essential for life
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
The 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
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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
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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Enteric Nervous System
Composed of two 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 motility
Segmentation and peristalsis are largely automatic involving local reflex arcs
Linked to the CNS via long autonomic reflex arc
Control of the Digestive System
Local mechanisms coordinate response to changes in pH, physical, chemical stimuli
Neural and hormonal mechanisms coordinate glands
Hormonal mechanisms enhance or inhibit smooth muscle contraction
Digestion And Absorption Of Nutrients
Disassembling organic food into smaller fragments
Hydrolyzing carbohydrates, proteins, lipids and nucleic acids for absorption
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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
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
Bile salts improve chemical digestion by
emulsifying lipid drops
Lipid-bile salt complexes called micelles
are formed
Micelles diffuse into intestinal epithelia
Small micelles enter bloodstream
and are sent to liver
Large micelles enter lymphatic
system as chylomicrons
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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
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
Chemical Digestion: Nucleic Acids
Enzymatic catabolism:
Pancreatic ribonucleases and deoxyribonuclease in the small intestines
Absorption:
Active transport via carrier proteins
Absorbed in villi and transported to liver via hepatic portal vein
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Absorption of Water
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
Absorption of Vitamins
Vitamins:
Water-soluble vitamins are absorbed by diffusion
Fat-soluble vitamins are absorbed as part of micelles
Vitamin B12 requires intrinsic factor for absorption
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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