Download (17) Digestive system

Digestive system
Dr. Carmen E. Rexach
Physiology
Mt San Antonio College
Functions
• Motility
–
–
–
–
ingestion
mastication
deglutition
peristalsis
• Secretion
– exocrine
– endocrine
• Digestion
• Absorption
Tunics
Innervation
• Autonomic nervous system
• Parasympathetic (vagus nerve)
– motility
– secretion
• Sympathetic
– decreases peristalsis
– secretion
– contraction of sphincters
Esophagus
• Collapsible tube
• first 1/3
– Skeletal muscle
• last 2/3
– Smooth muscle
• gastroesophageal
sphincter
Stomach
• Function
– food storage
– initiates protein
digestion
– kills bacteria
– moves food to SI
– intrinsic factor
– Digestion and
absorption
• proteins
• alcohol/aspirin
Gastric Glands
• Goblet cells
– mucus
• Parietal cells
– HCl
• Chief cells
– pepsinogen
• Argentaffin cells
– Seratonin (correct)
• G cells
– gastrin
pH
• 3 functions of pH in
stomach
– denature ingested
protein
– convert pepsinogen to
pepsin
– destroy bacteria
Protective mechanisms
• Impermeability of gastric mucosa to
CO2 and NH3
• alkaline mucus
• tight junctions
• rapid cell division and replacement
• protective effects of prostaglandins
• In doudenum
– bicarbonate from pancreatic juice
– Brunner’s glands
• Secrete mucinous alkaline solution
Peptic ulcers
– Gastric ulcers
• Weakened defense mechanisms
– Duodenal ulcers
• Increased acid and pepsin production
– Helicobacter pylori
• Major factor in both types
• Present in 100% of gastric ulcer patients and 95% of
duodenal ulcer patients
– Exceptions: Those whose ulcers are the result of overuse of
NSAIDS = inhibits prostaglandin production
• Produces NH3 and urease
– damages epithelium and allows H+ to permeate
• Also linked to increased acid secretion
Small intestines
• Length
• 3 divisions
– duodenum
– jejunum
– ileum
• absorption
– doudenum + jejunum
– ileum
• system of folds
– microvilli, villi, plicae circularis
Villi
• Columnar epithelium
• goblet cells
• crypts of Lieberkuhn
villus
microvilli
capillaries
– Glands that secrete intestinal
juice at base of the villi
• brush border
– surface area
– enzymes
– enterokinase
nerve
arteriole
Lymph
duct
lacteal
vein
Muscularis artery
mucosa
Intestinal motility
• Major types of
contractions
– peristalsis
– segmentation
• Pacesetter potential
– smooth muscle cells
– can lead to a/p
– parasympathetic and
sympathetic
influences
• Relaxation
– NO
Large Intestine
• Structure
• Haustra
• Fluid and electrolyte absorption
– 90% in SI
– passive osmosis
– water secretion
• Defecation
Liver
Liver lobules: functional unit
Functions of liver
•
•
•
•
•
•
•
Exocrine
Endocrine
Clotting functions
Plasma proteins
Organic metabolism
Cholesterol metabolism
Excretory and degradative functions
Exocrine & Endocrine
• Synthesis and
secretion of bile salts
– 250-1500ml/day
• Adds bicarbonate rich
solution to bile
• Secretes IGF-1 =
promotes cell division
• Forms T3 from T4
• Secretes
Angiotensinogen
• Metabolizes hormones
• Secretes immune
cytokines
Clotting and Plasma proteins
• Produces
prothrombin &
fibrinogen
• Produces bile salts
needed for vitamin
K absorption
• Produces plasma
albumin, acute
phase proteins,
binding proteins,
and lipoproteins
Organic metabolism
• Converts plasma glucose to glycogen and
triglycerides
• Converts amino acids to fatty acids
• Produces triglycerides and secretes them
as lipoproteins
• Gluconeogeneisis and glycogenolysis
• Converts fatty acids into ketones
• Produces urea
Cholesterol metabolism/excretory
& degradative functions
• Synthesizes
cholesterol
• Secretes plasma
cholesterol into
bile
• Converts plasma
cholesterol into
bile salts
• Secretes bile
pigments
• Excretes toxins via
bile
• Destroys old
erythrocytes
• …and lots, lots
more!
Gallbladder & pancreas
Hepatic ducts
Common
bile duct
gallbladder
Pancreas
Pancreatic
duct
Duodenal
papillae
Doudenum
Pancreas
• Endocrine = islets of Langerhans
– insulin and glucagon, etc.
• Exocrine = pancreatic acini
– pancreatic juice
– composition
• water
• bicarb
• digestive enzymes
– role of enterokinase
Neural and endocrine regulation
• Long reflex
– preconditioning
• Short reflex
– more local
• GI hormones
• Extrinsic controls
– 3 phases (cephalic, gastric, intestinal)
• some events without neural or
endocrine control
GI hormones
•
•
•
•
Gastrin
Secretin
CCK
glucose-dependent insolinotropic
peptide (GIP)
• Glucagon-like peptide 1 (GLP-1)
• Guanylin
Gastrin
• endocrine cell location
– antrum of stomach
• stimulus for release
– amino acids, peptides in stomach
parasympathetic nervous system
• actions
– stimulates secretion of HCl and
pepsinogen
Secretin
• Endocrine cell location
– small intestine
• stimulus for release
– acid in small intestine
• action
– stimulate pancreatic bicarbonate
secretion
– potentiate CCK-stimulated pancreatic
enzyme secretion
Cholecystokinin (CCK)
• Endocrine cell location
– small intestine
• Stimulus for release
– amino acids, fatty acids in small
intestine
• Action
– contraction and emptying of the gall
bladder
– stimulates release of pancreatic
enzymes
– (in the brain: acts as satiety hormone)
Glucose-dependent
insulinotropic peptide (GIP)
• Endocrine cell location
– small intestine
• Stimulus for release
– glucose, fat in the small intestine
• Action
– inhibits gastric emptying
– potentiates insulin release
Glucagon-like peptide 1
(GLP-1)
• Endocrine cell location
– ileum and colon
• Stimulus for release
– oral ingestion of nutrients
– glucose, fats, amino acids (mixed meals)
• Action
– inhibits gastric motility
– stimulates insulin secretion
Guanylin
• Endocrine gland location
– ileum and colon
• Action
– stimulates intestinal Cl- release
– result: increased NaCl and water in
feces
Cephalic phase
• First 30 minutes
– Smell, sight, and taste of food
stimulates vagus nuclei of brain (long
reflex)
• vagus nerve stimulates:
– parietal and chief cells
– Gastrin secretion by G cells
• Also stimulates parietal and chief cells
Gastric phase
• Begins when food arrives in stomach
• Stimulus
– distension
– chemical composition of chyme
• short polypeptides and amino acids in stomach
– Positive feedback, cells secrete gastrin
– HCl and pepsinogen are released
• Glucose
– No effect
• Lipids
– Inhibition of gastrin secretion
– pH<2.5 negative feedback inhibition of gastrin
Intestinal phase
• Begins when chyme enters small intestines
• neural inhibition of gastric emptying and acid
secretion due to:
– increased osmolality
– stretching
• enterogastrone
– stimulus: fat in chyme
– inhibits gastric acid secretion
• GIP = insulin secretion if glucose is present
• CCK = inhibits gastric emptying when chyme
in duodenum
Intestinal reflexes
• Gastroileal reflex
– increased gastric activity = increased
motility of ileum = increased chyme
through sphincter
• ileogastric reflex
– ileal distension = decreased gastric
motility
• intestino-intestinal reflex
– one segment overdistends = rest relax
Regulation pancreatic juice & bile
• Neural & hormonal
• Pancreatic juice
– secretin
• pH <4.5 in duodenum
• stimulates bicarbonate production by pancreas
• cAMP as a second messenger
– CCK
• fat content in chyme
• production of pancreatic enzymes
• Ca++ as second messenger
• Secretion of bile
– continuous secretion
– gallbladder contraction under influence of CCK and
secretin
Digestion and absorption of
carbohydrates
• Mouth
– salivary amylase
• Stomach
– enzymes denatured
• Small intestines
– pancreatic amylase = maltose, maltriose
– brush border enzymes =
monosaccharides
– secondary active transport with Na+
– Into capillaries in villus
Mouth:
Salivary amylase
Digestion of
Carbohydrates
Stomach:
Enzymes denatured
Duodenum:
Pancreatic amylase
Brush border enzymes
Absorption of carbohydrates
Digestion and absorption of
proteins
• Mouth
– nothing
• Stomach
– Pepsin = short chain polypeptides
• Small Intestines
–
–
–
–
–
trypsin, chymotrypsin, elastase
carboxypeptidase
aminopeptidase = from brushborder
across into blood
Whole proteins
• babies
• botulinum toxin
Free amino acids
Dipeptides
tripeptides
Digestion of proteins
Stomach:
Pepsin and HCl
Small intestines:
Trypsin, chymotrypsin
Carboxypeptidase
Brush border:
aminopeptidases
Protein absorption
aa
Na+
-Amino acids move into enterocytes by countertransport, in exchange for two Na+.
-They are absorbed into a capillary bed on the basal
side of the cell and taken to the liver via the hepatic
portal system.
Digestion and
absorption of lipids
•
•
•
•
Emulsification
pancreatic lipase
Mixed micelles to brushborder
transport
– across into epithelial cells
– can be moved inside cell
– chylomicron into lacteal
Digestion of Fat
Small intestines:
Pancreatic lipase
lacteal
Lipid transport
• From lymphatics to thoracic duct
• Triglycerides removed by lipoprotein lipase
on endothelial membranes
• Free fatty acids and glycerol into tissues
• Leftovers to liver
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–
–
–
Remnant particles contain cholesterol
Combined with apoproteins to produce VLDL’s
Deliver triglycerides to other organs
Later converted to LDL’s when triglyceride has
been removed
Lipoproteins
• What are they?
– Lipid & protein complexes
– Transport cholesterol & triglycerides in blood
– Protein allows hydrophobic lipids to remain in
suspension
• Five classes: Based on density, molecular weight, size,
chemical composition
• Chylomicrons
• VLDL
• IDL
• LDL
– High levels associated with increased risk CVD
• HDL
– Low levels associated with increased risk of CVD
– Best profile = high HDL, low LDL
Treatment for morbid
obesity
Lapband
Roux-en-Y Gastric
Bypass