Post on 22-Dec-2015
transcript
1
Definitions
Digestion:
Breakdown of ingested nutrients into forms which can be absorbed
Absorption:
Transport of small molecules from the GI tract into the blood
Motility:
Patterns of GI contraction and relaxation; tone of sphincters
Secretion:
Control of secretion of digestive enzymes & regulatory hormones
2
Gastrointestinal functions
Digestion: mouth, stomach, small intestine Absorption: small intestine
Secretion: mouth, stomach, small intestine Motility: entire GI tract
fig 15-2
3
GI functions: mouth, esophagus, stomach, pancreas
fig 15-3a
4
GI functions: biliary system, small intestine, large intestine
fig 15-3b
5
Fluid exchanges
fig 15-5
Oral intake: 1200 ml/day
Secretions: 7000 ml/day
Absorption: 8100 ml/day
Feces: 100 ml/day
Significance:
potential for fluid loss if absorption compromised
6
Microanatomy of GI tract
fig 15-6
Mucosa: absorptive epithelium, endocrine/exocrine cells, muscularis mucosa
Submucosa: blood/lymph vessels, submucosal nerve plexus
Muscularis externa: circular muscle, myenteric nerve plexus, longitudinal muscle
Serosa, mesentery
7
Intestinal structure: villi
fig 15-7
Surface area: “tennis court”, villi, microvilli (next)
Life cycle: ~5 day life cycle, generated at base of villi, shed from top
8
The intestinal cell
fig 15-8
Surface enzymes: released into intestine as cells released
Microvilli = “brush border”
9
Carbohydrate digestion & absorption
Source:
~ half of daily caloric intake
~ 70% as starch (potatoes, pasta, rice, bread), ~ 30% as sucrose, lactose
cellulose: glucose links not digestible, forms “bulk” of diet
Starch: 14, 16 links hydrolyzed by salivary & pancreatic amylase maltose, maltotriose
Intestinal disaccharidases:
maltase: hydrolyses maltose, maltotriose glucose
sucrase: hydrolyses sucrose glucose + fructose
lactase: hydrolyses lactose glucose + galactose
Absorption: from small intestine into blood capillaries
Glucose, galactose by Na+ linked cotransport at luminal border
Fructose by facilitated diffusion at luminal border
Glucose, galactose, fructose by facilitated diffusion at basolateral surface
10
Protein digestion & absorption
Source:
~ 20% of daily caloric intake
Digestion:
gastric *pepsin (endopeptidase, acid pH optimum)
pancreatic *trypsin & *chymotrypsin (endopeptidase, alkaline pH optimum)
pancreatic carboxypeptidase (exopeptidase)
intestinal aminopeptidase (exopeptidase)
Absorption:
small intestine, into blood capillaries
aminoacids by Na+ linked cotransport, di- & tri-peptides by H+ linked cotransport at luminal border
peptides hydrolyzed in intestinal cell
aminoacids by facilitated diffusion at basolateral membrane
* secreted as inactive precursor (next slide)
11
Pepsin, trypsin, chymotrypsin activation
fig 15-21 fig 15-26
pepsin
trypsin, chymotrypsin
12
Lipid digestion & absorption
Source:
Triglyceride, ~30% of daily caloric intake
Digestion:
pancreatic lipase (triglyceride 2-monoglyceride + 2 fatty acids)
lipases act at fat water interfaces & depend on fat surface area
Emulsification:
mechanical disruption (chewing, motility of stomach & small intestine
amphipathic emulsifiers (bile salts, bile phospholipid)
13
Anatomy of bile & pancreatic ducts
Bile: produced in liver, stored in gall bladder
Composition: bile salts, phospholipids, cholesterol, bicarbonate, bile pigments (heme metabolism), drug metabolites & trace metals
fig 15-4
14
Bile salt actions
fig 15-9 fig 15-10
Bile: produced in liver, stored in gall bladderEmulsification in duodenum & upper intestine
15
Triglyceride digestion
fig 15-11
Colipase:
from pancreas, binds lipase to emulsion droplet
Micelles:
4-7 nm diameter
store of fatty acids & monoglycerides for transport into epithelial cells
16
Triglyceride absorption
fig 15-12
Resynthesis:
2 FA + MG TG
on smooth endoplasmic reticulum
Chylomicrons:
80-90% triglyceride + phospholipid + cholesterol, coated with amphipathic proteins
released by exocytosis into lacteal
metabolized by fat & liver
17
Vitamins, water absorption
Water:
passively absorbed down osmotic gradient
GI wall permeable, therefore GI contents isosmotic
mostly from small intestine
Vitamins:
lipid soluble (vit A, D, E, K) with lipids (micelles, diffusion, chylomicrons)
water soluble (vit B, vit C) diffusion, mediated transport
vit B12 with intrinsic factor (from stomach parietal cells), endocytosis
18
Regulation of secretion: general
Luminal stimuli:
1. distension of wall (volume stimulates mechanoreceptors)
2. osmolality of chyme (osmoreceptors)
3. acidity of chyme (chemoreceptors)
4. chyme concentration of digestive products: monosaccharides, fatty acids, peptides, aminoacids (chemoreceptors)
19
Neural regulation
fig 15-13
Long reflexes: mostly parasympathetic
Short reflexes: enteric nervous system, myenteric & submucosal plexuses, conduct impulses up & down GI tract
20
Hormonal regulation (simplification of table 15-4)
Gastrin Cholecystokinin Secretin GIP
Structure peptide peptide peptide peptide
Source G cells in stomach
small intestine epithelium
small intestine epithelium
small intestine epithelium
Release stimulus
Parasymp NS, stomach AA’s, peptides ( pH inhibits)
FA’s, AA’s in small intestine; osmolality
pH in small intestine
glucose, AA’s in small intestine
Actions stomach enzymes, H+, motility
secretion of bile, pancreatic enzymes, relaxes sphincter of Oddi
secretion of bile & pancreatic HCO3
-
insulin secretion
GIP = glucose insulinotropic peptide (gastric inhibitory peptide)
Cholecystokinin & secretin potentiate each others’ actions
21
Saliva
Composition:
fluid, mucus, amylase
Functions:
moistening food, preventing tooth decay (lysozyme, IgA), starting starch digestion
Regulation of release:
parasympathetic NS (responding to sight, smell, thought of food)
sympathetic NS (transient); ( saliva flow, blood flow)
acidic fruit juices, mechanical contact
22
Gastric secretion
Structure: fundus, body, antrum; lower esophageal & pyloric sphincters
Goblet cells mucus, Parietal cells HCl & intrinsic factor,
Chief cells pepsinogen, G cells gastrin
fig 15-16
fig 15-17
23
HCl secretion by parietal cells
As HCl is secreted, HCO3- is returned to the GI blood (“alkaline tide”)
Function of HCl: not digestive, kills potential parasites: bacteria etc.
fig 15-18
24
Regulation of HCl secretion
HCl secretion increased by:
parasympathetic NS stimulation (cephalic phase)gastrin (direct & via histamine from mast cells) (gastric phase)
HCl secretion inhibited by somatostatin
fig 15-20
25
Phases of gastric secretion of pepsinogen & HCl
Cephalic phase:
stimulus: sight, smell, thought of food, chewing
mechanism: parasympathetic NS secretion & motility
Gastric phase:
stimulus: aminoacids, peptides in stomach, distension, pH as food enters
mechanism: short/long loop reflexes, gastrin secretion & motility
Intestinal phase:
stimulus: intestinal distension, pH, osmolality, digestive products
mechanism: short/long loop reflexes, secretin, CCK, GIP
26
Pancreatic secretion
Enzymes (a reminder):
trypsin, chymotrypsin, (elastase), carboxypeptidaseamylase, lipase, (phospholipases, nucleases)
Bicarbonate
fig 15-25
27
Regulation of pancreatic secretion
Stimulus: fatty acids, glucose, osmolality, distension in small intestine
Mediator: cholecystokinin
Response: relaxation of sphincter of Oddi, contraction of gall bladdersecretion of pancreatic enzymes, potentiation of secretin actions
Stimulus: pH in small intestine
Mediator: secretin
Response: secretion of bicarbonate in pancreatic juice & bile
Stimulus: glucose in small intestine
Mediator: glucose insulinotropic peptide (GIP)
Response: secretion of insulin, inhibition of gastric secretion
28
Regulation of pancreatic secretion
fig 15-28 fig 15-27
29
Bile
Source:
produced in liver, stored & concentrated in the gall bladder
Composition:
bile salts, phospholipids, cholesterol (emulsify fats)
bicarbonate (neutralizes HCl)
bile pigments (heme metabolism), drug metabolites & trace metals
Regulation of secretion:
intestinal fatty acids cholecystokinin gall bladder contraction
Enterohepatic circulation:
next screen
30
Regulation of bile secretion
fig 15-31
31
Bile enterohepatic circulation
fig 15-30
Bile salts circulate 2-3x per meal
32
Gastrointestinal motility omitted
Omit objectives 16-20
33
Liver functions
1. exocrine function: bile salts, bicarbonate
2. plasma proteins: albumin, binding proteins, angiotensinogen
3. metabolism: gluconeogenesis, urea, ketoacids, lipoproteins
4. cholesterol synthesis, secretion in bile
5. excretion: bile pigments, trace metals, drug metabolites