Download Digestive Physiology

Digestive Physiology
Overview
• Inside
gastrointestinal
(GI) tract, food is
broken down by
hydrolysis
reactions into
molecular
monomers
• Most digestion of
nutrients and
absorption of
monomers occurs
in small intestine
(90%)
18-3
Functions of the Digestive System
–
–
–
–
Ingestion--taking food into mouth
Mastication--chewing food and mixing it with saliva
Deglutition--swallowing food
Peristalsis--rhythmic wave-like contractions that move
food through GI tract
– Digestion– mechanical and chemical breakdown of
food
– Absorption--Is passage of digested end products into
blood or lymph
– Storage and Elimination--Includes temporary storage
and subsequent elimination of indigestible
components of food
18-6
Functions of Digestive System
• Secretion:
– Includes release of exocrine and endocrine products into GI
tract
– Exocrine secretions include: HCl, H2O, HCO3-, bile, lipase,
pepsin, amylase, trypsin, elastase, and histamine
– Endocrine includes hormones secreted into stomach and
small intestine to help regulate GI system
• e.g. gastrin, secretin, cholescytokinin, gastric inhibitory peptide, and
somatostatin
18-7
Digestive System
• Is composed of GI
tract (alimentary
canal) and
accessory digestive
organs
• Organs include oral
cavity, pharynx,
esophagus, stomach,
small and large intestine
• Accessory organs
include teeth, tongue,
salivary glands, liver,
gallbladder, and
pancreas
Regulation of GI Tract
• Parasympathetic effects, arising from vagus and
spinal nerves, stimulate motility and secretions of GI
tract
• Sympathetic activity reduces peristalsis and
secretory activity
• GI tract contains an intrinsic system that controls its
peristaltic movements--the enteric nervous system
• GI motility is also influenced by paracrine and
hormonal signals
18-18
From Mouth to Stomach continued
• Peristalsis propels
food thru
esophagus and
GI tract
– = wave-like
muscular
contractions
– After food passes
into stomach, the
lower esophageal
sphincter
constricts,
preventing reflux
18-23
Stomach
• Is most distensible part of GI tract
• Empties into the duodenum
• Functions in:
– storage of food;
– initial chemical digestion of proteins
– some mechanical digestion of all nutrients
– kills bacteria with high acidity (HCl)
– moves soupy food mixture (chyme) into intestine
18-25
Stomach
• Gastric
mucosa has
gastric pits in
its folds
• Cells that line
folds deeper in
the mucosa,
are exocrine
gastric glands
18-28
Stomach continued
• Gastric glands
contain cells that
secrete different
products that form
gastric juice
– 1. Goblet cells
secrete mucus
– 2. Parietal cells
secrete HCl and
intrinsic factor
(necessary for B12
absorption in
intestine)
– 3. Chief cells secrete
pepsinogen
(precursor to pepsin)
18-29
Stomach continued
• 4. Enterochromaffinlike cells secrete
histamine and
serotonin
• 5. G cells secrete
gastrin
• 6. D cells secrete
somatostatin
18-30
HCl in Stomach
• Is secreted into stomach lumen by proton
pumps of epithelial parietal cells in response to
the histamine secreted by ECL cells ; and ACh
from vagus (parasympathetic stimulation)
– These are indirect effects since release of histamine
is due to gastrin release from G cells
– Proton pump inhibitors (medicines) are common
and work to reduce stomach acids (treat ulcers)
18-32
HCl in Stomach continued
• Makes gastric
juice very acidic
which denatures
proteins to make
them more
digestible
• Also converts
pepsinogen into
pepsin
– Pepsin is more
active at low pHs
18-33
Digestion and Absorption in Stomach
• Proteins are partially digested by pepsin
• Carbohydrate digestion by salivary amylase
is soon inactivated by acidity
• Water, alcohol and aspirin are the only
commonly ingested substances that are
absorbed here
18-35
Small Intestine (SI) continued
• Surface area
increased by
foldings and
projections
• Large folds are
plicae circulares
• Microscopic
finger-like
projections are villi
• Cell apical hairlike membrane
projections are
microvilli
18-40
Intestinal Enzymes
• Attached to microvilli are brush border enzymes
– Enzyme active sites are exposed to chyme
18-43
Large Intestine or Colon
• Has no digestive function of its own but absorbs
H2O, electrolytes, B and K vitamins, and folic acid
• Internal surface has no villi (relatively smooth)
• Intestinal Flora: Contains large population of
microflora
– 400 different species of commensal bacteria
• Which produce folic acid and vitamin K and ferment
indigestible foods to produce fatty acids
• And reduce ability of pathogenic bacteria to infect colon
• Antibiotics can kill commensals
18-49
Fluid and Electrolyte
Absorption in Colon
• SI absorbs most water but colon absorbs 90%
of water it receives
– Begins with osmotic gradient set up by Na+/K+
pumps
• Water follows by osmosis
• Salt and water reabsorption stimulated by
aldosterone
• Colon can also excrete H2O via active transport
of NaCl into intestinal lumen (an ANP influence)
18-51
Day 28
complete
Accessory Organs:The Liver
Functions of the Liver
• Overall it over 500 functions!
• Lipid metabolism: lipolysis, lipogenesis, synthesis of
cholesterol
• Protein metabolism: synthesizes the plasma proteins
• (albumin, fibrinogen, alpha and beta globulins, and
prothrombin); breaks down proteins and converts the to
carbohydrates or lipid for storage.
• Carbohydrate metabolism: helps to maintain normal
blood glucose levels by;
– breaking down glycogen into glucose and then secreting it into
the blood
– converting serum glucose into glycogen and triglycerides for
storage
Functions of the Liver
• Detoxification: processes drugs and hormones;
detoxifies substances such as alcohol or
excretes drugs such as the antibiotics into bile.
• Synthesis of bile salts: bile salts are used in the
small intestine for the emulsification and
absorption of lipids, cholesterol, phospholipids,
and lipoproteins.
• Storage: stores glycogen, vitamins and minerals.
• Phagocytosis: Kupffer cells phagocytize RBCs,
WBCs, bacteria, and toxins.
Detoxification of Blood
• Liver can remove hormones, drugs, and other
biologically active molecules from blood by:
– Excretion in bile
– Phagocytosis by Kupffer cells
– Chemical alteration of molecules
• e.g. ammonia is produced by deamination of amino acids
in liver
– Liver then converts it to urea which is excreted in urine
18-64
The Gallbladder
• Stores and
concentrates bile
continuously produced
by liver
– When SI is empty,
sphincter
of Oddi in common bile
duct closes and bile is
forced up into
gallbladder
• Expands as it fills with
bile
– When food is in SI,
sphincter of Oddi opens,
gall bladder contracts,
and bile is ejected thru
ducts into duodenum
18-69
The Pancreas
• Has both endocrine
and exocrine
functions
– Endocrine function
performed by Islets
of Langerhans cells
• Secrete insulin and
glucagon
– Exocrine secretions
include bicarbonate
(HCO3-) solution and
digestive enzymes
• These pass in
pancreatic duct to
small intestine
• Exocrine secretory
units are acini
18-70
The Physiology of Digestion
Regulation of Gastric Function
• Extrinsic control of gastric function is
divided into cephalic, gastric, and
intestinal phases
18-78
Cephalic Phase
• Refers to control by brain via vagus nerve
• Stimulated by sight, smell, thought, and taste of
food
• Activation of vagus nerve stimulates:
– Salivary glands to secrete saliva
– Chief cells to secrete pepsinogen
– G cells to secrete gastrin
– ECL cells to secrete histamine
– Parietal cells to secrete HCl
18-79
Gastric Phase
• The presence of
proteins,
polypeptides and
amino acids in the
stomach raises the
pH. This change in
chemical nature,
along with stomach
distension, activates
the gastric
(stomach) phase.
18-81
Gastric Phase
• Proteins, polypeptides
and amino acids present
in the stomach stimulate
G cells to secrete gastrin
and chief cells to secrete
pepsinogen
– Gastrin then stimulates
ECL cells to secrete
histamine which stimulates
parietal cell secretion of
HCl
– This is a positive feedback
mechanism: as more HCl
and pepsinogen are
secreted, more
polypeptides and amino
acids are liberated, and
more digestive processes
are stimulated.
18-81
Gastric Phase
• As polypeptides
leave the stomach
and move into the
duodenum, the pH
begins to drop
again and the
gastric phase
slows.
18-81
Summary of the Interactions Among
Gastric Gland Cells and Secretions
• 1. Presence of proteins in stomach, an increased pH, and
stomach distension (ie: stretch) all stimulate G cells to
secrete gastrin and chief cells to secrete pepsinogen.
• 2. Gastrin stimulates ECL cells to secrete histamine.
• 3. Histamine stimulates parietal cells to secrete HCL.
• 4. HCl denatures proteins and activates pepsin from
pepsinogen.
• 5. Pepsin digests proteins into polypeptides.
• 6. Polypeptides also stimulate G cells to secrete gastrin (a
positive feedback effect).
Intestinal Phase
• Begins when chyme enters the small intestine
• Arrival of chyme in duodenum causes a neural
reflex that inhibits gastric motility and secretion
– Fat in chyme stimulates SI to secrete
enterogasterones--hormones that inhibit gastric
motility and secretion
• Include Somatostatin, Cholecystokinin, Secretin, and
Gastric Inhibitory Peptide
18-83
Intestinal Phase
• Enterogasterones secreted by intestines whem
chyme arrives:
– Somatostatin: inhibits stomach acid secretion.
– Cholecystokinin: stimulates secretion of pancreatic
enzymes; stimulates contraction of gall bladder;
brings about a feeling of fullness after eating; slows
gastric motility and acid secretion.
18-83
Intestinal Phase
• Enterogasterones secreted by intestines whem
chyme arrives:
– Secretin: stimulates secretion of pancreatic HCO3and bile from the liver; inhibits acid production and
gastric motility.
– Gastric Inhibitory Peptide: Inhibits gastric motility
and secretion; stimulates secretion of insulin from
pancreas.
18-83
Digestion and Absorption of
Carbohydrates
• Most carbohydrates
are ingested as
starch-structured of glucose
• Salivary amylase
begins starch
digestion in the
mouth and continues
for awhile in the
stomach (until it is
denatured by the low
pH)
18-90
Digestion and Absorption of
Carbohydrates
• Pancreatic amylase
secreted into
duodenum converts
starch to
oligosaccharides
– Oligosaccharides are
hydrolyzed by small
intestine brush border
enzymes into
monosaccharides
– Monosaccharides are
absorbed directly into
the bloodstream
18-90
Digestion and Absorption of Proteins
• Chemical digestion begins in stomach when pepsin
digests proteins to form polypeptides
• In small intestine, endopeptidases (trypsin,
chymotrypsin, elastase) from pancreas cleave peptide
bonds from interior of polypeptides
• Also in small intestine exopeptidases
(carboxypeptidase, aminopeptidase) cleave peptide
bonds from ends of polypeptides. Carboxypeptidase is
a pancreatic enzyme while aminopeptidase is a brush
border enzyme.
18-91
Digestion and Absorption of Protein
• Protein digestion in
small intestine
results in free amino
acids, dipeptides,
and tripeptides
– Which are absorbed
into small intestine
cells where they are
broken down into
amino acids
– Which are then
secreted directly into
the bloodstream
18-92
Digestion and Absorption of Lipids
• Small amount of lipid chemical digestion begins
in mouth via lingual lipase
• No chemical digestion in stomach (except in
infants who can digest milk fats)
• Arrival of lipids in duodenum causes secretion
of bile from gall bladder
• Fat is emulsified by bile salt micelles
– Form tiny droplets of fat
– Greatly increases surface area for digestion by
pancreatic lipase
18-93
Digestion and Absorption of Lipids
continued
• Pancreatic
lipase then
hydrolyzes
exposed
triglycerides to
free fatty acids
and
monoglycerides
which are then
absorbed into
epithelial cells
18-94
Digestion and Absorption of Lipids
continued
• Products of fat digestion are dissolved in micelles
which move to the brush border
18-95
Digestion and Absorption of Lipids
continued
• Free fatty acids and monoglycerides leave micelles
and are absorbed into epithelial cells
– Inside epithelial cells, they are resynthesized into
triglycerides and phospholipids, and then packaged into
protein transport structures termed chylomicrons.
18-96