Chapter 6
DIGESTION & ABSORPTION
§ 1. Introduction
 Concept of digestion & absorption
* Mechanical & chemical digestion
 Characteristics of digestive smooth muscles
* General characteristics
* Electrophysiological characteristics
Resting potential (RP)
Slow wave or basic electrical rhythm (BER)
Action potential (AP)
Electrical activities & contraction of
digestive smooth muscles
 Secretion of digestive glands
* Main functions of digestive fluids
 Gastrointestinal innervation
* Intrinsic (or enteric) nervous system
Submucosal & myenteric plexus
* Extrinsic nervous system
Parasympathetic & sympathetic nerves
 Gastrointestinal hormones
* General functions
* Brain-gut peptides
Intrinsic
(or enteric)
nervous
system
Extrinsic
nervous
system
§ 2. Digestion in the mouth & esophagus
 Saliva & its function
* Nature & Composition of saliva
pH 6.6~7.1, 99% water
Salivary amylase, lysozyme, salt, etc.
* Functions of saliva
Moisten the cavity & solve food; keep the
mouth clean; anti-bacteria; digest starch
* Regulation of salivary secretion
Pure neuroregulation
Conditioned & unconditioned reflex
 Mastication or chewing & its effects
* Definition
* Effects
Break up large food particles; mixes the
food with saliva; aid swallowing
 Deglutition or swallowing
* Definition
* Process
1st, 2nd & 3rd phase
Peristalsis & lower esophageal sphincter
Peristalsis of the esophagus
§ 3. Digestion in the stomach
 Gastric juice & its secretion
* Nature, Compositions & functions
pH 0.9~1.5, colorless, 1.5~2.5 L/day
Hydrochloric acid (HCl)
Secreted by parietal cells
Basic secretion: 0~5 mmol/h
Max secretion: 20~25 mmol/h
Mechanism of HCl secretion
Functions: see next page
Relationship between max volume of HCl
secretion & the number of parietal cells
Functions of HCl:
Turn pepsinogen into pepsin & provide a
medium of low pH favoring pepsin action;
Kill many swallowed virulent organisms;
Promote the secretion of pacreas & bile;
Promote the absorption of calcium & ferrum
Pepsinogen
Secreted by chief cells & activate by HCl
Function: digest proteins into proteoses, peptones,
polypeptides, with little free amino acids
Mucus
Secreted by neck & surface mucous cells,
cardiac & pyloric glands
Function: form mucus-bicarbonate barrier
Intrinsic factor
Secreted by parietal cells
Function: Promote the absorption of Vit B12
* Regulation of gastric secretion
Endogenous releasing substances
Acetylcholine (ACh), gastrin & histamine
Somatostatin (SST) (release-inhibiting)
Mucus-bicarbonate barrier
Action of
acetylcholine,
gastrin &
histamine on
the perietal
cells
Gastric secretion in the digestive phase
Cephalic phase
Mechanisms:
Conditioned & Unconditioned reflex
Vagal efferent & with Gastrin secretion
through gastrin-releasing peptide (GRP)
Experiment: Sham feeding by Pavlov
Characteristics:
Large quantity (30%)
High acidity & digestive power
Experiment of Sham feeding by Pavlov
Gastric phase
Mechanisms:
Distension of gastric fundus & body initiating
vagovagal & local plexus reflexes
Distension of pylorus initiating a release of
gastrin through intrinsic plexus
Chemical stimulation of G cells initiaying a
release of gastrin
Experiment: Pavlov pouch
Characteristics:
Large quantity (60%)
High acidity & digestive power
Heidenhain (R) & Pavlov (L) pouch
Intestinal phase
Mechanisms:
Mainly humoral regulation
Chemical & Mechanical stimulation
initiating releases of Gastrin, Enterooxyntin & Other humoral factors
Experiment: Pavlov pouch
Characteristics:
Small quantity (10%)
Lower acidity & digestive power
Cephalic,
gastric, &
intestinal
phase of
gastric
juice
secretion
Summary of cephalic, gastric, & intestinal
phase of gastric juice secretion
Inhibitory regulation
Hydrochloric acid (HCl)
A typical example of negative feedback
Conditions & Mechanisms:
pH ≤ 1.2~1.5 in the gastric antrum
Inhibition of G cells, Release of SST
pH ≤ 2.5 in the duodenum
Release of secretin, bulbogastrone
Fat: Initiating release of enterogastrone
Hypertonic solution: Entero-gastric reflex
 Gastric motility & emptying
* Main types of gastric motility
Tonic contraction
Maintaining the shape & place of stomach,
as well as the intragastric pressure
Receptive relaxation
Definition & its innervation
Peptide or NO –ergic vagal fibers
Gastric peristalsis
Initiate from middle stomach, 3 times/min
Mix & grind food into chyme & help empty
* Gastric emptying & its control
Definition & some rules
Carbohydrate > proteins > lipids, Mix 4~6 h
Discontinuous emptying
Adapting for digestion & absorption
Control of emptying
Promotion in stomach: vagovagal & plexus
reflex, gastrin release
Inhibition in duodenum: entero-gastric
reflex & enterogastrone
* Gastric motility in the digestive interphase
Migrating motor complex (MMC)
Phase I: resting phase, 90~120 min
Phase II: irregular spike & sporadic
peristalsis, 30~45 min
Phase III: large amount of spikes & regular
strong contraction, 5~10 min
Phase IV: transition period, about 5 min
Significance: act as a street cleaner
Vomiting
Definition: a reflex, benefit & harm
Migrating motor complex (MMC)
§ 4. Digestion in the intestine
 Pancreatic juice & its secretion
* Nature, Compositions & functions
pH 7.8~8.4, colorless & odourless, 1~2 L/day
Bicarbonate (HCO3)
Neutralize HCl & provide a weak basic
medium favoring digestive enzyme action
Pancreatic enzymes: amylase, lipase, colipase,
trypsinogen & chymtrypsinogen, etc.
Turn trypsinogen into trypsin by enterokinase, turn chymtrypsinogen into chymtrypsin by trypsin
Trypsin inhibitor: a polypeptide
* Regulation of pancreatic secretion
Nervous regulation
Vagus nerve: ACh, gastrin
Characteristics: H2O & HCO3, enzymes
Sympathetic nerve: ACh, NA
Characteristics: weak effect
Humoral reulation
Secretin: H2O & HCO3, enzymes
Cholecystokinin (CCK):
Characteristics: H2O & HCO3, enzymes
Feedback: CCK-releasing peptide
 Bile secretion & gallbladder emptying
* Nature, Compositions & functions
Hepatic bile: pH 7.4, golden yellow
Bladder bile: pH 6.8, color become darker
Compositions: H2O, ions, bile acid, bile
pigment, fatty acid, cholesterol, lecithin,
mucoprotein, etc., but no enzyme
Functions of bile (mainly by bile salt):
Fat emulsification; lipid absorption;
Promote the absorption of fat-soluble Vits
* Control of bile secretion & gallbladder emptying
Nervous regulation
Vagus nerve: ACh, gastrin
Hepatic bile secretion (small amounts)
Gallbladder contraction (slightly)
Humoral reulation
Gastrin: direct to hepatic cells & gallbladder;
indirect to stomachHClsecretin 
Secretin: act to bile duct & not to hepatic cells,
so: H2O & HCO3, bile salt ()
Cholecystokinin (CCK): gallbladder
contraction & Oddi’s sphincter dialation
Bile salt: enterohepatic circulation of bile salt
Enterohepatic
circulation of
bile salt
 Secretion of the small intestine
* Intestinal glands
Brunner’s gland (basic mucus)
Lieberkuhn Crypt (main part)
* Nature, Compositions & functions
pH 7.6, iso-osmotic solution, 1~3 L/day
Enterokinase, the only one secreted
Oligopeptidase, oligosaccharidase those
contained in the epithelial cells
* Regulation of secretion of small intestine
Mainly through plexus reflex initiated by
inflation of the intestine
 Movements of the small intestine
* Movement in the digestive interphase
MMC similar to that in the stomach
* Main movement forms in digestive phase
Tonic contraction: the same as in stomach
Segmentation: peculiar to the small intestine
Peristalsis: 0.5~2.0 cm/s
Peristaltic rush: 2~25 cm/s
* Regulation of small intestinal movement
Segmentation of the small intestine
* Regulation of small intestinal movement
Intrinsic plexus: receive mechanical &
chemical stimuli and then cause peristalsis
Extrinsic plexus: parasympathetic nerve
intestinal activities; sympathetic never
intestinal activities
Humoral factors: gastrin, CCK, Enk, 5-HT, etc.
 Functions of the ileocecal sphincter
* Permit some of chyme to squirt into the cecum
each time when a peristalsis wave reaches it
* Prevent reflux of colonic contents into the ileum
§ 5. Digestion in the colon
 Main function of the colon
Absorb water
Absorb Vit B & K synthesized by bacteria
Formation & temporarily storage feces
 Secretion of the colon
pH 8.3~8.4, Mucus & HCO3
Protect mucosa against injuries & lubricate feces
 Movements of the colon
Haustral to and fro movement, Segmentation or
Polyhaustral pushing movement, Peristalsis,
Mass peristalsis
 Feces & Defecation
* Feces: contain inorganic material, undigested
plant fibers, bacteria, water, etc.
* Defecation: a reflex initiated by distension of
the rectum with feces
Reflex process, Primary & higher centers
 Action of colonic bacteria
* Benefit: synthesis Vit. Bco & K
* Harm: produce some poisonous substances
 Action of dietary fibers on the intestinal function
Reduce absorption of water; Enhance colonic
movement; Increase the volume of feces; Low
the caloric ratio of the food
§ 6. Absorption in the small intestine
 Sites of absorption
Oral cavity & Stomach: little
Duodenum & Upper jejunum: most nutrients
Ileum: bile salts & Vit. B12
Colon: water & electrolytes
 Proofs as the main absorptive region
Huge absorptive surface (200 m2)
Plenty of capillaries & lymph capillaries
Large quantity of digestive fluid (6~8 L/day)
Food has almost completely been digested
Sites of absorption
Enlargement
of the surface
area of the
intestine
 Absorption of main nutrients
* Water
8 L/day, passive & iso-osmotic absorbed
Different absorbability in different parts
* Inorganic slats
Sodium: 95%~99%, jejunum>ileum>colon
active transport
Ferrum: 1/10, mainly in duodenum & jejunum,
transferrin dependent, active transport
Calcium: promote by Vit. D, active transport
Anions: mainly Cl  & HCO3, passive transport
* Carbohydrate
Absorptive form: monosarccharide
Mechanism: secondary active transport
* Protein
Absorptive form: amino acid
Mechanism: secondary active transport
* Fats
Absorptive form: glycerol, monoglyceride,
fatty acid, cholesterol
Mechanism: passive diffusion
Pathway: blood & lymph
Digestion & absorption of fats in the intestine