Transcript DIGESTIVE SYSTEM

DIGESTIVE SYSTEM
•
•
•
•
•
•
•
•
•
•
•
•
•
Digestive System
muscular tube
digestive tract
GI tract
gastrointestinal tract
alimentary canal
runs from oral cavity pharynx,
esophagus, stomach, small &
large intestines, rectum to anus
includes accessory organs
teeth
tongue
salivary glands
liver
gall bladder
pancreas
Functions
• provides fuel to keep
cells running
• provides building
blocks for growth &
repair
• removes residue
• protective function for
bacteria
– largest immune
organ
•
•
•
•
•
Functions
four integrated steps
Ingestion
– intake of food
Digestion
– mechanical
• physical manipulations
• teeth tear & crush
• stomach mixes & churns
– chemical breakdown
• physically manipulated materials
broken into smaller fragments by
enzymes & acids
Absorption
– uptake of nutrients
– movement of organic substances,
ions, vitamins & water into blood
Defecation
– excretion of undigested residue
– removal of waste products
Histological Organization
• located in peritoneal
cavity
• lined by serous
membrane
• visceral peritoneum
covers organs
• parietal peritoneum lines
inner body wall surface
• 4 layers
• mucosa
• submucosa
• muscularis externa
• serosa
•
•
•
•
•
Layers of Wall
Mucosa
– innermost epithelial lining
– consists of inner epithelium & thin layer
of smooth muscle-muscularis
mucosae
Epithelium-simple columnar in most of tract
– from oral cavity through esophagus &
lower anal canal-stratified
Submucosa
– loose connective tissue containing
blood & lymph vessels with nerve
plexus
Muscularis externa
– 2 layers of smooth muscle arranged in
inner, circular layer & outer,
longitudinal layer
– important in mechanical processing &
movement of materials along tract.
– lining of muscularis externa is thrown
into foldsincreases surface area
Serosa
– inner layer of loose connective tissue
– adipose tissue & outer epithelial layer
Nervous System Control of
Digestive Functions
• sympathetic
stimulation inhibits
gastrointestinal
secretion, motor
activity & contraction
of gastrointestinal
sphincters & blood
vessels
• parasympathetic
stimuli stimulate
these
Nervous Control of Digestive Functions
• digestive system has its own,
local nervous system
• enteric nervous system
• can function independently of
CNS
• regulates motility, secretion &
blood flow in tract
• has more neurons than spinal
cord
• comprised of two nerve
networks
• submucosal plexus in
submucosa
• myenteric plexus between
two layers of muscularis
externa
•
Motility in Digestive Tract
• Peristalsis
– progressive
contraction of circular
& longitudinal muscles
– propels bolus along
tract
– circular muscles
contract behind bolus
while circular muscle
ahead relaxes
Motility in Digestive Tract
• Segmentation
– contraction &
relaxation of nonadjacent segments of
tract
– moves contents
forwards & backwards
– mixes & churns bolus
– breaks into fragments
– mixes with intestinal
secretions
Ingestion
• eating
• begins food processing
• begins chemical &
mechanical digestion
• oral or buccal cavity
• responsible for analysis of
material prior to swallowing
• mechanical processing via
tongue & teeth
• lubrication-mixing ingested
material with saliva & mucus
• limited digestion of
carbohydrates & lipids
• lined with stratified squamous
epithelium
• roof-hard & soft palate
• floor-tongue
Tongue
• mechanical
processing
• manipulation to
assist chewing
• sensory analysis
– touch, taste,
temperature
• secretion of lingual
lipase
– begins lipid
breakdown
Salivary Glands
• make saliva
• controlled by ANSparasympathetic
nervessalivary reflex
• Lubricating
• Moistening
• Parotid
– serous, watery secretion
containing salivary amylase
(starchmaltose)
• Submandibular
– secretion contains mucus
& amylase
• Sublinguals
– mucus secretion
– buffer & lubricant
Teeth
• aid in mechanical
digestion by mastication
or chewing
• breaks down connective
tissues in plant fibers &
meat
• helps saturate materials
with salivary secretions &
enzymes
• permits easier deglutition
• during mastication,
salivary glands secrete
salivasoften food into a
bolus (semi-solid lump)
Swallowing-Deglutition
• involves over 22 muscles in mouth,
pharynx & esophagus
• controlled by swallowing center in
medulla & pons
• occurs in three phases
• Buccal
• Pharyngeal
• Esophageal
Buccal Phase
• voluntary
• tongue pushes just
formed bolus
toward oropharynx
• bolus stimulates
tactile receptors
• activates next
phase
Pharyngeal Phase
• tactile receptors send
impulses to deglutition
center in medulla
• impulses returning from
center cause soft palate &
uvula to more upward
closing off nasopharynxprevents food from entering
nasal cavity
• epiglottis covers glottisopening to larynx
• bolus driven downward by
constriction of upperthen
middle then lower
pharyngeal constrictors
• as bolus slides into
esophagus
Esophageal Phase
• esophagus
stretchestriggers
peristalsis
pushes bolus
ahead of it
• peristalsis carries
bolus from upper
esophageal
sphincter through
esophagus to lower
esophageal
sphincterstomach
Stomach
• bolus passes
through lower
esophageal
sphincter into
stomach
• expanded section
of digestive tube
between
esophagus & small
intestine
Stomach Functions
• bulk storage
– stores ingested food in upper part
• digestion
– mechanical breakdown of ingested food-lower
parts
– disrupts chemical bonds by acids & enzymes
• produces intrinsic factor
– for vitamin B12 absorption
•
•
•
•
•
Stomach Parts
Cardia
– smallest region
– contains mucous glands
• protects from stomach acids
& enzymes
Fundus
– makes contact with diaphragm
Body
– largest region
– mixing tank
– contains gastric glandsacids &
enzymes
Pyloris
– leads into duodenum of small
intestine
inside empty stomach mucosa &
submucosa are thrown into folds
called rugae
– distended when food is in
stomach
– allow stomach to expand
Stomach Wall
• covered with simple
columnar glandular
epithelium
• gastric mucosa is covered
with numerous small holes
– openings of gastric pits
• two or three tubular glands
open into bottom of each
gastric pit
• gastric glands
– secrete mucus
– secrete acid & enzymes
Gastric Gland Cells
• Mucous Neck
Cells
• Parietal
• Chief
• Enteroendocrine
Parietal Cells
• secrete hydrochloric acid
• assists in break down of
food
• not made in cytoplasm
– too strong
– would dissolve secretory vessel
& destroy cell
• H+ & Cl- are made & then secreted
out of cell & assembled
– maintains pH between 0.8 & 2.0
– kills microorganisms
– breaks down plant cell walls &
connective tissue in meats
– essential for pepsin
• converts pepsinogen to
pepsin
• make intrinsic factor
– needed for absorption of vitamin
B12
Cells of Gastric Pit
• Chief Cells
• secrete pepsinogen
– inactive precursor
of pepsin
– digests proteins
• Enteroendocrine
Cells-G cells
– make gastrin
Absorption in Stomach
• little absorption
– aspirin & ethanol
• absorption does not occur
because
• cells are covered by mucus
blanket & therefore never
contact chyme directly
• cells do not have transport
mechanisms needed to absorb
materials
• gastric lining is impermeable to
water
• digestion has not been
completed
– digested food pieces too big
Digestion in Stomach
• mechanical digestion-churns
bolus & mixes it with digestive
juices
• chemical digestionbreaks
bonds
• food digests in stomach for
several hours
• preliminary digestion of proteins
by pepsin
• not completed
– limited time substances are
in stomach
– pepsin attacks only specific
types of peptide bonds
• digestion of carbohydrates &
lipids by salivary amylase &
lingual lipase
• enzymes continue to digest until
pH falls below 4.5
Chyme Formation
• bolus + secretions
soupy mixture-chyme
• each peristaltic wave
delivers a bit of chyme
to small intestine
through pyloric
sphincter-gastric
emptying
• chemical digestion in
small intestine depends
on activity of pancreas,
liver & gall bladderaccessory digestive
organs
•
•
•
•
•
•
Pancreas
lies posterior to greater curvature of
stomach
exocrine acini cells secrete 2001500 ml of juice/day
secreted into small ducts-unite to
form larger ducts- pancreatic &
accessory
pancreatic duct joins common bile
duct from liver & gall bladder
enters duodenum as
hepatopancreatic ampulla
passage of pancreatic juice & bile
through this into small intestine is
controlled by sphincter of the
hepatopancreatic ampulla or
Sphincter of Oddi.
Pancreatic
Juice
• mixture of water, salt,
enzymes, zymogens &
sodium bicarbonate
• secretions controlled by
hormones of
duodenum
• chymeduodenum
secretin pancreas
watery buffer pH 7.58.8 raises pH of
chyme
• chymeduodenumC
CK pancreas
pancreatic enzymes
Pancreatic Secretions
• pancratic amylase
– starch breakdown
• ribonuclease &
deoxyribonuclease
– nucleic acid breakdown
• pancreatic lipase
– lipid breakdown
• zymogens: trypsinogen &
carboxypeptidase
• a brush border enzymeenterokinase cleaves
trypsinogentrypsin
• trypsin then works on other
inactive precursorsactive
ones
Liver & Gall Baldder
• accessory digestive organs
• liver-inferior to diaphragm
• gall bladder-in a
depression on posterior
surface of liver
• Liver-two principle lobes-a
larger right & a smaller left
lobe
• connected by a mesentery
fold-falciform ligament
• right lobe includes an
inferior quadrate & a
posterior caudate lobe
Liver & Gall Bladder
• hepatocytes-major functioning cells
of liver make 800 -1000 mls of bile
each day
• bile leaves liver via right & left
hepatic ducts which unite as
common hepatic duct
• these join with the cystic duct from
the gall bladder to form the
common bile duct
• bile is stored & modified in the
gall bladder
• enters small intestine via cystic
duct
• does not enter small intestine until
gallbladder contracts
• principal stimulus for releasecholecystokinin CCK
Emulsification
• bile contains water, bile salts,
bile pigments, cholesterol,
lecithin & several ions
• bile salts are important in
digestion of lipids
• lipids are not water soluble
• mechanical processing results
in large drops
• bile salts breaks down large
lipid globules into a suspension
of smaller lipid globules
• process called emulsification
• increases surface area
available for enzymatic attack
• digested lipids are absorbed in
lacteals of the small intestine
•
•
•
•
•
•
•
•
•
Liver Functions
carbohydrate metabolism
– stabilizes blood glucose by
glycogenolysis & gluconeogenesis
lipid metabolism
– removes lipids for storage or breaks
down lipids when needed
amino acid metabolism
– removes excess amino acids
removes waste products
– amino acidsammonia
– neutralizes ammonia by converting it
to urea
– important in drug inactivation
vitamin storage
fat soluble vitamins-A, D, E & K & B12
mineral storage
– stores iron bound to ferritin
phagocytosis & antigen presentation
– Kupffer cells engulf old RBCs,
debris, etc, & stimulates immune
system
synthesis of plasma proteins
Small Intestine
• about 20 feet long
– 90% of nutrient
absorption
• Duodenum
– next to stomach
– mixing bowl
• Jejunum
– bulk of chemical
digestion & absorption
• Ileum
– longest part
– ends at ileocecal valve
• sphincter controlling
release of substances
into large intestine
Small Intestine Lining
• folded into transverse foldsplicae
• permanent
• increase surface area for
absorption
• covered by simple columnar
epithelium
• microvilli project from cells of
epithelium forming brush
border
• increase surface area more
• allow chyme to contact more of
small intestine wall
• increased contact means more
efficient food absorption
Brush Border
Small Intestine Epithelium
• absorptive cells
• digest &
absorb
nutrients in
chyme
• goblet cells
• make mucus
Intestinal Villi
• mucosa is thrown into folds forming
intestinal villi
• Core contains lymph vessel- lacteal
– absorbs products of fat digestion
• at base-entrance to intestinal glandscrypts of Lieberkuhn
– secrete 1-2 liters of intestinal
juice each day
– produce brush border enzymes
• Paneth cells
– secrete lysozyme
• Enteroendocrine cells
– S cells
• make secretin
– CCK cells
• make cholecystokine
– K cells
• make GIP-glucose-dependentinsulinotropic peptide
Duodenum
• contains
duodenal or
Brunners
glandsmucous
• primary function
of duodenum
• receive chyme &
neutralize acids
Ileum
• contains Peyer’s
patches
• aggregates of
lymphoid
nodules
• protection from
bacteria
Movement in Small Intestine
• as chyme enters
duodenumperistaltic
contractions (migrating
motility complexes)
move it toward jejunum
• segmentation-contraction
& relaxation of nonadjacent segments of the
tract mixes & churns the
material breaking it into
fragments & mixing it with
intestinal secretions
Chemical Digestion & Absorption
• begins in mouth
• salivary glandssalivary amylase
• polysaccharides broken into di- &
tri- saccharides
• salivary amylase & pancreatic
amylase continue to break down
in stomach
• a brush border enzyme-α
dextrinase clips off one glucose
at a time making di & tri
saccharides
• monosaccharides are made by
brush border enzymes specific for
specific disacchardies
• maltase splits maltose
• sucrase splits sucrose
• absorbed by facilitated transport
Chemical Digestion & Absorption
• fat digestion begins in mouthlingual lipase
• continues in stomach
• fat enters small intestine as
coarse emulsion created by
lipase digestion & mechanical
mixing
• lipid droplets too big to be
absorbed or further broken
down
• bile salts (phospholipids) coat
emulsion stabilizing fat droplets
• pancreatic lipase digests
triglycerides in emulsion
• triglyceridesmonoglycerides
& free fatty acids
• absorbed by simple diffusion
into lacteals
Chemical Digestion & Absorption
• protein digestion begins in mouth
– mechanical processing
• chemical processing
– takes place in stomach with
HCl
• pepsin continues digestion in
stomach breaking peptide
bonds
• other proteases & peptidases
found on brush border of
intestinal villa cells-trypsin,
elastin, chymotrypsin continue to
breakdown protein bonds
• carboxypeptidases break off
individual amino acids from
ends of peptides
• absorbed via facilitated diffusion
Large Intestine
• begins at end of ileum
• ends at anus
• attached to posterior abdominal wall
by mesocolon
• ileocecal sphincter regulates passage
of chyme from the small to the large
intestine
• relaxes when food leaves stomachgastroileal reflex
• cecum-first part
• hangs inferior to ileocecal valve
• attached-appendix
• functions: reabsorbs water
• compacts intestinal contents into feces
• absorbs vitamins
• stores fecal materials
Large Intestine-Colon
•
•
•
•
•
•
colon-largest part
ascending colon
transverse colon
descending colon
sigmoid colon
muscularis layer has inner
circular layer & longitudinal
muscle layer concentrated into
3 bands called teniae coli
• contraction of these pull wall
into bulging pockets or
haustra
• permits expansion &
elongation of colon
Large Intestine-Rectum & Anus
• Rectum
– short, terminal expandable part
of large intestine
• Anus
– separates large intestine from
outside
• closed by 2 sphincters
• internal smooth muscle sphincter
– not under voluntary control
• external skeletal muscle
sphincter
– under voluntary control
Digestion & Absorption in Large
Intestine
• takes 12 -24 hours to reduce
residue of meal into feces
• no significant digestion of
organic molecules-less than
10%
• reabsorbs electrolytes
• reabsorbs water
• digestive system is one way
body maintains water balance
• receives about 9 liters of water
each day
• 7.5 L reabsorbed by small
intestine
• 1500 ml of material enters
colon each day & only 200 ml
are excreted with feces
Motility in Large Intestine
• movement begins when
• .
substances pass ileocecal
sphincter
• hastral churning
– haustra remain relaxed &
begin to fill upwhen
distension reaches certain
pointwalls contract &
squeeze contents of one
haustra into another
• mass peristalsis
• occur 3-4X/day
• called the gastrocoli reflexes
•
•
•
•
•
•
•
•
Defecation
final digestive process stage
processed feces (undigested waste
products) carried from sigmoid colon
to rectum
distends rectal wallstimulates
stretch receptorssends nerve
impulses to sacral spinal cord
impulses return via parasympathetic
fibers to colon, rectum & anus
causes contraction of longitudinal
muscles
causes rectum to shorten
increases pressure in it & opens
internal anal sphincter
external anal sphincter must relax to
complete the process
feces are expelled
Phases of Gastric Activity
• gastric activity occurs in three
overlapping stages
• Cephalic
• Gastric
• Intestinal
Cephalic Phase
• directed by CNS
• see, smell or anticipate
foodcerebral cortex,
hypothalamus and brain
stemfacial,
glossopharyngeal & vagus
nerves
• facial, glossopharyngeal
nervessalivary
glandssaliva
• vagus nervegastric
glandsgastric juice
production
• lasts only a few minutes
Gastric Phase
• begins with arrival of food in
stomach
• stomach distendsstretch
receptors
• increased pH & undigested
materials in
stomachchemoreceptors
• Stretch &
chemoreceptorssubmucos
al plexus  parasympathetic
& enteric
neureonsincrease
peristaltic waves &
stimulates secretion of
gastric juice
• phase
Intestinal Phase
• begins when chyme enters duodenum
• phase controls rate of gastric emptying
• important for small intestine to slow down gastric
emptying to give time to neutralize acid & to
efficiently absorb incoming nutrients
– duodenum, jejunum & ileum are able to process
only small amounts of material at any one time
• stretching duodenum activates the enterogastric
reflex
• stretch receptorsmedullainhibit
parasympathetic & stimulate sympathetic
fibersinhibits gastric motility
Intestinal
Phase
• controlled by : secretin & cholecystokinin
• chyme containing amino acids & fatty
acidsIntestinal glands (CCK cells)
CCKstsimulates secretion of pancreatic
juice, causes gall bladder to contract,
relaxes Sphincter of Oddi-slows gastric
emptying
• immediate resultsno more chyme
released from stomach
• lowered pH (from chyme) Intestinal
glands (S cells)secretin stimulates
flow of pancreatic juicesbicarbonate
buffers the acidic chyme
• secretininhibits secretion of gastric
juice and enhances effects of CCK
Other Gut Hormones
• tract makes at least 10 other gut hormones
• motilin, substance P, bombesinaffect
motility of intestine
• VIP (vasoactive intestinal
peptide)stimulates secretion of ions &
water;inhibits acid production by stomach
• gastrin releasing peptide  stimulates
gastrin release
• somatostatininhibits gastrin release