Download The Digestive System

The Digestive system
• 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
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
Digestive System Anatomy
Figure 21-2c
Digestive System Anatomy
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
Figure 21-1
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
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
Motility
• Peristalsis - waves that move a bolus
Motility
• Segmentation – a churn and fragment 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
The Oral Cavity
• Mechanical processing by the teeth, tongue, and palatal
surfaces
• Lubrication
• Assistance in swallowing
• Limited digestion
Figure 24.6a, b
Salivary glands (three pairs)
• 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
– Initiation 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 – esophogeal
sphincter relaxes while epiglottis
closes
– Esophageal – bolus moves into
esophagus propelled by
peristalsis and into stomach
Figure 24.11a-h
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
Glands of the Stomach
• Gastric glands have a
variety of secretory cells
– Mucous neck cells – mucus
– Parietal cells – HCl and
intrinsic factor
– Chief cells – pepsinogen
• Pepsinogen is activated to
pepsin by HCl in the stomach
• Pepsin itself via a positive
feedback mechanism
– Enteroendocrine cells –
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
Cephalic Phase
• Cephalic phase prepares stomach to receive ingested
material
• 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
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:
– A 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
Release of Gastric Juice
Figure 23.16
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
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
Gastrointestinal Hormones
• 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
Gastrointestinal Hormones
• 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
Regulation of Gastric Emptying
Figure 23.19
Digestion And Absorption In The Stomach
• Preliminary digestion of proteins - 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
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 – increase pancreatic enzymes,
stimulates contraction of gall bladder, reduces hunger
sensation
– GIP – stimulates release of insulin, inhibits gastric
secretion and motility
Activities of Major Digestive Tract Hormones
Figure 24.22
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
The Liver
• 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
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:
– 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:
– The gallbladder to contract
– The hepatopancreatic sphincter to relax
• As a result, bile enters the duodenum
Regulation of Bile Release
Figure 23.25
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
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:
– 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
Regulation of Pancreatic Secretion
Figure 23.28
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
Functions of the Large Intestine
• 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
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
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
Regulation of digestion
• 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
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 mobility
• Segmentation and peristalsis are largely
automatic involving local reflex arcs
• Linked to the CNS via long autonomic reflex arc
Control of the digestive system
• 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
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 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
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
• 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)