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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:

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Breakdown of ingested nutrients into forms which can be absorbed


Transport of small molecules from the GI tract into the blood


Patterns of GI contraction and relaxation; tone of sphincters


Control of secretion of digestive enzymes & regulatory hormones

gastrointestinal functions
Gastrointestinal functions

fig 15-2

Digestion: mouth, stomach, small intestine Absorption: small intestine

Secretion: mouth, stomach, small intestine Motility: entire GI tract

fluid exchanges
Fluid exchanges

Oral intake: 1200 ml/day

Secretions: 7000 ml/day

Absorption: 8100 ml/day

Feces: 100 ml/day


potential for  fluid loss if absorption compromised

fig 15-5

microanatomy of gi tract
Microanatomy of GI tract

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

fig 15-6

intestinal structure villi
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

the intestinal cell
The intestinal cell

fig 15-8

Surface enzymes: released into intestine as cells released

Microvilli = “brush border”

carbohydrate digestion absorption
Carbohydrate digestion & absorption


~ 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:  14, 16 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

protein digestion absorption
Protein digestion & absorption


~ 20% of daily caloric intake


gastric *pepsin (endopeptidase, acid pH optimum)

pancreatic *trypsin & *chymotrypsin (endopeptidase, alkaline pH optimum)

pancreatic carboxypeptidase (exopeptidase)

intestinal aminopeptidase (exopeptidase)


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)

pepsin trypsin chymotrypsin activation
Pepsin, trypsin, chymotrypsin activation


trypsin, chymotrypsin

fig 15-21

fig 15-26

lipid digestion absorption
Lipid digestion & absorption


Triglyceride, ~30% of daily caloric intake


pancreatic lipase (triglyceride  2-monoglyceride + 2 fatty acids)

lipases act at fat water interfaces & depend on fat surface area


mechanical disruption (chewing, motility of stomach & small intestine

amphipathic emulsifiers (bile salts, bile phospholipid)

anatomy of bile pancreatic ducts
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

bile salt actions
Bile salt actions

fig 15-10

fig 15-9

Bile: produced in liver, stored in gall bladder

Emulsification in duodenum & upper intestine

triglyceride digestion
Triglyceride digestion


from pancreas, binds lipase to emulsion droplet


4-7 nm diameter

store of fatty acids & monoglycerides for transport into epithelial cells

fig 15-11

triglyceride absorption
Triglyceride absorption


2 FA + MG  TG

on smooth endoplasmic reticulum


80-90% triglyceride + phospholipid + cholesterol, coated with amphipathic proteins

released by exocytosis into lacteal

metabolized by fat & liver

fig 15-12

vitamins water absorption
Vitamins, water absorption


passively absorbed down osmotic gradient

GI wall permeable, therefore GI contents isosmotic

mostly from small intestine


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

regulation of secretion general
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)

neural regulation
Neural regulation

Long reflexes: mostly parasympathetic

Short reflexes:

enteric nervous system, myenteric & submucosal plexuses, conduct impulses up & down GI tract

fig 15-13

hormonal regulation simplification of table 15 4
Hormonal regulation (simplification of table 15-4)

GIP = glucose insulinotropic peptide (gastric inhibitory peptide)

Cholecystokinin & secretin potentiate each others’ actions



fluid, mucus, amylase


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

gastric secretion
Gastric secretion

fig 15-17

fig 15-16

Structure: fundus, body, antrum; lower esophageal & pyloric sphincters

Goblet cells  mucus, Parietal cells  HCl & intrinsic factor,

Chief cells  pepsinogen, G cells  gastrin

hcl secretion by parietal cells
HCl secretion by parietal cells

fig 15-18

As HCl is secreted, HCO3- is returned to the GI blood (“alkaline tide”)

Function of HCl: not digestive, kills potential parasites: bacteria etc.

regulation of hcl secretion
Regulation of HCl secretion

fig 15-20

HCl secretion increased by:

parasympathetic NS stimulation (cephalic phase)

gastrin (direct & via histamine from mast cells) (gastric phase)

HCl secretion inhibited by somatostatin

phases of gastric secretion of pepsinogen hcl
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

pancreatic secretion
Pancreatic secretion

Enzymes (a reminder):

trypsin, chymotrypsin, (elastase), carboxypeptidase

amylase, lipase, (phospholipases, nucleases)


fig 15-25

regulation of pancreatic secretion
Regulation of pancreatic secretion

Stimulus: fatty acids, glucose,  osmolality, distension in small intestine

Mediator: cholecystokinin


relaxation of sphincter of Oddi, contraction of gall bladder

secretion 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



produced in liver, stored & concentrated in the gall bladder


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

bile enterohepatic circulation
Bile enterohepatic circulation

Bile salts circulate

2-3x per meal

fig 15-30

liver functions
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