digestive physiology -...
TRANSCRIPT
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
– 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
Functions of the Digestive System
18-6
• 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
Functions of Digestive System
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
• 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
Regulation of GI Tract
18-18
• Peristalsis propels food thru esophagus and GI tract– = wave-like
muscular contractions
– After food passes into stomach, the lower esophageal sphincterconstricts, preventing reflux
From Mouth to Stomach continued
18-23
• 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
Stomach
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
• Gastric glands contain cells that secrete different products that form gastric juice– 1. Goblet cells
secrete mucus
– 2. Parietal cellssecrete HCl and intrinsic factor (necessary for B12
absorption in intestine)
– 3. Chief cells secrete pepsinogen(precursor to pepsin)
Stomach continued
18-29
• 4. Enterochromaffin-
like cells secrete
histamine and
serotonin
• 5. G cells secrete
gastrin
• 6. D cells secrete
somatostatin
Stomach continued
18-30
• 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)
HCl in Stomach
18-32
• 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
HCl in Stomach continued
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
• Surface area
increased by
foldings and
projections
• Large folds are
plicae circulares
• Microscopic
finger-like
projections are villi
• Cell apical hair-
like membrane
projections are
microvilli
Small Intestine (SI) continued
18-40
• Attached to microvilli are brush border enzymes
– Enzyme active sites are exposed to chyme
Intestinal Enzymes
18-43
• 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
Large Intestine or Colon
18-49
• 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)
Fluid and Electrolyte
Absorption in Colon
18-51
Accessory Organs:The LiverDay 28
complete
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
• Stores and concentrates bile continuously produced by liver– When SI is empty,
sphincterof 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
The Gallbladder
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
• Extrinsic control of gastric function is
divided into cephalic, gastric, and
intestinal phases
The Physiology of DigestionRegulation of Gastric Function
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
• 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.
Gastric Phase
18-81
• 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.
Gastric Phase
18-81
• As polypeptides
leave the stomach
and move into the
duodenum, the pH
begins to drop
again and the
gastric phase
slows.
Gastric Phase
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).
• 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
Intestinal Phase
18-83
• 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.
Intestinal Phase
18-83
• Enterogasterones secreted by intestines whem chyme arrives:
– Secretin: stimulates secretion of pancreatic HCO3-and bile from the liver; inhibits acid production and gastric motility.
– Gastric Inhibitory Peptide: Inhibits gastric motility and secretion; stimulates secretion of insulin from pancreas.
Intestinal Phase
18-83
Digestion and Absorption of
Carbohydrates• Most carbohydrates
are ingested as starch--structured of glucose
• Salivary amylasebegins 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
• 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.
Digestion and Absorption of Proteins
18-91
• 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
Digestion and Absorption of Protein
18-92
• 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
Digestion and Absorption of Lipids
18-93
• Pancreatic lipase then hydrolyzes exposed triglycerides to free fatty acidsand monoglycerides which are then absorbed into epithelial cells
Digestion and Absorption of Lipids continued
18-94
• Products of fat digestion are dissolved in micelles which move to the brush border
Digestion and Absorption of Lipids continued
18-95
• 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.
Digestion and Absorption of Lipids continued
18-96