PHYSIOLOGY Chapter 6PHYSIOLOGY Chapter 6

Chunmei Xia, Ph.DDepartment of Physiology and Pathophysiologycmxia@fudan.edu.cnContact number:021-54237612-805

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We require:

carbohydrates (mainly glucose)

proteins (essential amino acids)

fats (but Western diet fats too high)

vitamins

minerals

Nutrition

Carbohydrate 50%

Fat 35%

Protein 15%

Intake (normally 3000-6000kcal per day & depends on

Geography

Occupation

• The GI tract (gastrointestinal tract)

The muscular alimentary canal– Mouth– Pharynx– Esophagus– Stomach– Small intestine– Large intestine– Anus

• The accessory digestive organs

Supply secretions contributing to the breakdown of food– Teeth & tongue– Salivary glands– Gallbladder– Liver– Pancreas

Digestive System Organization

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1.Digestion of food and absorption of nutrients are accomplished in a long tube connected to the external world at both ends

2.Secretion and motility of “the tube” are major themes in understanding the gut.

Main function

The Digestive Process• Ingestion

– Taking in food through the mouth• Propulsion (movement of food) 

– Swallowing– Peristalsis – propulsion by alternate 

contraction &relaxation• Mechanical digestion

– Chewing– Churning in stomach– Mixing by segmentation

• Chemical digestion– By secreted enzymes: see later

• Absorption– Transport of digested end products into 

blood and lymph in wall of canal • Defecation

– Elimination of indigestible substances from body as feces 2014-04-21 5

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Histology/organization of the Gut Wall

From esophagus to anus, GI tract has the same basic arrangement of tissues.

There are 4 layers that can be distinguished

• Mucosa

• Submucosa

• Muscularis

• Serosa

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Layers of Gut Wall

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I REGULATION OF GASTROINTESTINAL TRACT FUNCTIONS

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Cell Hormone Site

G Gastrin (G) autrumn, duodenum

I Cholecystokinin (CCK) duodenum、jejunum

S Secretin duodenum、jejunum

D Somatostatin (SS) Stamoch, duodenum, pancreas, colon

L Enteroglucagon Small intestine, colon

PP Pancreatic polrpeptide (PP)

pancreas

EC1 Substance P (SP) Stamoch, intestine

D1 VIP Stamoch, intestine, pancreas

P bombesin Antrum, duodenum

N neurotensin ileum

B insulin pancreas

A glucagon pancreas

K Gastric inhibitory polypeptide(GIP)

duodenum、jejunum

Endocrine Cell and gut hormoneEndocrine Cell and gut hormone

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Types of secretionTypes of secretion

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Function of GI hormones

1. Regulate the secretion and motility of GI tractGastrin HCl secretion, gastric empty

2. Trophic actionGastrin stomach and duodenum mucosa

3. Regulate the release of other hormonesGIP insulinSS gastrin

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CONTROL OF DIGESTIVE FUNCTIONSBY NERVOUS SYSTEM

1.Autonomic nervous system (ANS) is divided into

- Parasympathetic- Sympathetic

2.Enteric nervous system: ENS

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Innervation of the GI tract

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Intrinsic (or enteric) nervous systemIntrinsic (or enteric) nervous system

exextrinsic trinsic nervous systemnervous system

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1.The autonomic nervous systemSympathetic system:

Noradrenaline

Gut secretions (+)Spinal cord

Pons/medulla

Midbrain

Gut sphincters (-)

Pancreas (+)

ACTION

Rectum (+)defaecation

ACTION

Gut wall (+)

adrenaline

EFFECTSEFFECTS

Salivary glands (+)X IX

VII

Cranial nerves

Parasympathetic system: Acetylcholine (Ach)

β(+) salivary glands

α (+) gut blood β2 (-) vesselsβ1/2 (-) gut wall, α (+) sphincters

(+) secretionAdrenal medulla

Dr. Alzoghaibi 15

CONTROL OF DIGESTIVE FUNCTIONSBY NERVOUS SYSTEM

Parasympathetic Nerves:• Located in brain stem & sacral region

• Projection to the G.I. are preganglionic efferents

• Vagus & pelvic nerves

• Vagus nerves synapse with neurons of ENS in esophagus, stomach, small intestine, colon, gall bladder & pancreas

• Pelvic nerves synapse with ENS in large intestine

• Neurotransmitter is Ach

Dr. Alzoghaibi 16

CONTROL OF DIGESTIVE FUNCTIONSBY NERVOUS SYSTEM

Sympathetic nerves:• Located in thoracic & lumbar regions

• Neurotransmitter is NE

• NE increases sphincter tension

• Inactivate the motility

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Innervation of the GI tract

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Innervation of the GI tract2. Intrinsic (enteric) nerve plexuses

Located

in the submucosa (submucosal or Meissner’s plexus) and between circular and longitudinal muscle layers (myenteric or Auerbach’s plexus)

Control

Motility - Myenteric plexus

Secretion - Submucosal plexus

through release of neurotransmitters

Excitatory - Acetylcholine, Substance P

Inhibitory - VIP, nitric oxide

Excitatory - Acetylcholine

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The enteric nervous system

Deep muscular

plexus

Submucosal

artery

Muscularis mucosa

Submucosalplexus

MUCOSA

Myenteric plexus

Longitudinal muscle

Circular muscle

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Intrinsic (or enteric)Intrinsic (or enteric)nervous systemnervous system

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The enteric nervous system coordinates

digestion,

secretion

motility

to optimize nutrient absorption.

Its activity is modified by information

from the CNS

from local chemical and mechanical sensors.

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Gastrointestinal reflex

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II GASTROINTESTINAL SMOOTH

MUSCLE

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Musculature of the GI tractAll smooth muscle except:

Upper third oesophagus – striated

Middle third of oesophagus – mixed

External anal sphincter – striated

Areas of striated muscle are areas that are under conscious control

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General Functional characteristicsGeneral Functional characteristics1. Lower excitability, slower contraction

and relaxation

2. Higher extensibility

3. Tonic contraction

4. Autorhythmicity5. More sensitive to stretch, chemicals, cold and warm stimulation but not to electric stimulation

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Slow Waves & Action potentials are Forms of Electrical Activity in GI Muscles

1. Resting potential

2. Slow wave or basic electric rhythmThe smooth muscle membrane slowly depolarizes and repolarizes in a cyclic fashion

3. Action potential

4. Relationship to contraction

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Insert fig. 18.16

Cells and Electrical Events in the Muscularis

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Slow waves in GI smooth muscle

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‐ Unknown cause‐ Responsible for triggering AP in G.I.‐ Interstitial cells of Cajal, ICCs (pacemaker)  

Myenteric borderSubmucosa border

‐ Occur at different frequency stomach (3/min) small intestine (duodenum, 12‐18/min) ileum & colon (6‐10/min) 

‐ May or may not accompanied by AP

Electrical activity and muscle contraction

Factors that depolarize the membrane:Stretching of the muscle AchParasympathetic stimulation Hormonal stimulation

Factors that hyperpolarize the membrane:Norepinephrine Sympathetic stimulation 

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GI motilityThere are many types of contractions in different areas of the GI tract.

Some muscles contract and relax in seconds – Phasic Contractions

-Peristalsis and Segmentation

Some maintain contractions over minutes or hours –Tonic Contractions

-Sphincter

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III GASTRIC MOTILITY

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Major Function of Gastric Motility

To serve as a reservoir

To break food into small particles and

mix food with gastric secretions

To empty gastric contents into the duodenum at a controlled rate

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1. Anatomy and innervation of the Stomach

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Stomach AnatomyStomach Anatomy

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The stomach can be divided into three anatomic regions (A)

and two functional regions (B)

Gastric reservoirTonic contractions Gastric reservoirTonic contractions

Gastric pumpPhasic contractionsGastric pumpPhasic contractions

BBFundusFundus

CorpusCorpusAntrumAntrum

PylorusPylorus

AA

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OesophagusLower OesophagealSphincter Fundus

Body

Antrum

DuodenumPylorus

Functional Anatomy of StomachFundus

Body

Antrum

• Storage

• Storage• Mucus• HCl• Pepsinogen• Intrinsic factor

• Mixing/Grinding• Gastrin

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2. Responses to Gastric Filling –Receptive Relaxation

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Receptive relaxationDuring chewing and swallowing food, the stimulation of food to the receptors in mouth, pharynx, and esophagus reflexly causes the smooth muscle of the fundus and body of the stomach to relax,

This process allows the stomach to accommodate a large amounts of food and fluid.

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Inhibitoryvagal fibre(NANC-inhibition)

Nutrients

CCKRelaxation of

gastric reservoir

ACH

Vaguscentre

1. ReceptiverelaxationMechanical

stimuli in the pharynx

3. Feedbackrelaxation

2. Adap tiverelax ation

The relaxation of the gastric reservoir is mainly regulated by reflexes.

receptive, adaptive and feedback-relaxation

NutrientsTensionreceptors

Distension

NO + VIP et al.

Non Adrenergic Non Cholinergic)

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3. Peristalsis of the Gut and Gastric Emptying

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Gastric MotilityPeristaltic waves: Body → Antrum

BodyThin muscle → weak contraction→ No mixing

AntrumThick muscle → powerful contractionA Mixing

B Contraction of pyloric sphincter →

1 Only small quantity of gastric content (chyme) entering duodenum

2 Further mixing as antral contents forced back towards body

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Phase of propulsion Phase of retropulsionPhase of emptying

Bulge

Rapid flow of liquids withsuspended small particlesand delayed flow of large

Emptying of liquids withsmall particles whereaslarge particles are retained

Antrum

The contraction of the gastric pump three phases:A: phase of propulsion, B: phase of emptying,C: phase of retropulsion and grinding

Retropulsion of largeparticles and clearingof the terminal antrum

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Control of gastric motilityVagovagal reflex – fundal relaxation

Myenteric plexus – slow waves –contraction

Parasympathetic and Gastrin – increase contraction force and frequency

Sympathetic – decrease contraction force and frequency

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Gastric emptying1. Def.

The process by which the chyme is expelled from the stomach into the duodenum is called the gastric emptying.

2. Control1) stomach: stimulating factor, neuronal and hormonal

2) duodenum: inhibiting factor

entero-gastric reflex, hormones

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Control of Gastric emptying

Stimulating factors in stomach– Presence of food

– Gastrin

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Control of gastric emptyingControl of gastric emptying

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Inhibitory effects in duodenum and jejunum –through reflexes and hormonesInhibitory reflexes – direct – myenteric plexus

indirect – via extrinsic nerves

Neural reflexes stimulated by:Distension, irritation, acidity, high osmolarity, protein/fat

Fats and acids also stimulate release of humoral factors which reduce gastric emptying

Cholecystokinin (CCK), stimulated by fats

Secretin, stimulated by acids

Control of Gastric emptying

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Enterogastric ReflexRegulates the rate at which chyme leaves the stomach

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non-digestible spheres

“Quality” of food regulates gastric emptying

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4. Vomiting

• Emesis

• Stretching, toxins, alcohol, spicy foods, and drugs may stimulate this.

• Emetic Center of the Medulla

• Diaphragm and abdominal wall contract

• Cardiac sphincter relaxes.

• Soft palate rises

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IV MOTILITY OF THE SMALL INTESTINE

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Function of Intestinal Motility

(1)To mix chyme with digestive secretion

(2)To bring fresh chyme into contact with the absorptive surface of the microvili

(3)To propel chyme toward the colon

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1. Tonic contraction: the base of the other contractions2. Segmentation contractions

(1) def.When a portion of the small intestine becomes distended with chyme,the stretch of the intestinal wall elicits a rhythmical contraction and relaxation of localized circular muscles

spaced at intervals along the intestine, (2) function: mix the chyme with the digestive juice increase its exposure to the mucosal surface

Types of small intestinal movement

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3. Peristalsis: propels the small intestinal contents towards the large intestines peristaltic rush：initiated by the harmful stimulation

4. MMC (migrating motor complex): Occurs during fasting state

Types of small intestinal movement

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Segmentation: mix contents to promote digestion & absorption

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Peristalsis

• Distinctive pattern of smooth muscle contractions that propels foodstuffs distally through the esophagus and intestines

• Mediated by….

– Local, intrinsic nervous system

– Ex: peristalsis is not affect to any significant degree by vagotomy or sympathectomy

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Peristalsis: movement along the tract

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Peristalsis of the small intestine

http://medweb.bham.ac.uk/research/toescu/Teaching/OverviewGITY2.html

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Control of Intestinal Motility – Neuronal

Mixing – segmentationFrequency set by slow waves (12/minute duodunum)

additional control: myenteric plexus

Propulsion – peristalsisLocal reflex – stretch causes relaxation distal and

contraction proximal

Moves bolus through intestines

Intestino-intestinal reflex – extrinsic nerves

Local stretch in one area inhibits contraction in rest of bowel

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Gastrin Secretin

CCK + motility -- Glucagon

5-HT VIP

Motilin GIP

Control of Intestinal Motility – Hormonal

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Ileocecal Valve

• What it is– Opening to large intestines

• Function: • (1) prevent the repulsion • (2) control the emptying• normally closed. • Gastro-ileal reflex: enhances ileal emptying after eating. .

• The hormone gastrin relaxes ileocecal sphincter – Short-range peristalsis in

terminal ileum and distension relaxes IC sphincter

– small amount of chyme is squirted into the cecum.

• Distension of cecum contracts IC sphincter.

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V. GASTROINTESTINAL MOTILITY DURING FASTING STATE

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Gastric motility on fasting“Migrating Motor Complex, MMC”

•Occurs during fasting

•To clear undigested food particles

•Peristaltic contractions sweep down stomach and duodenum – pylorus relaxes

•Pattern of contraction approx. every 90 min

•Slow peristaltic waves sweeping whole of GI tract

•Thought to be controlled by motilin

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MMC(migrating motor complex)

• PhaseⅠ: Almost have no contractions 40-60 min• PhaseⅡ: have contractions, only have few 30-45 min• PhaseⅢ: have continuous contractions 5-10 min

• Originates simultaneously at the stomach and duodenum

• Migrates within 90 to 120 minutes along the small intestine

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Importance of MMC

1.Sweep the contents of the small intestine towards the colon

Housekeeper of the small intestine

2.Inhibit the migration of colonic bacteria into the terminal ileum

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VI MOTILITY OF THE COLON

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Large intestine

• Functions– Absorption of water and electrolytes– Storage of feces– In non-ruminant herbivores, fermentative digestion

and absorption of nutrients• Motility patterns

– mixing (form haustrations)– propulsive (mass movements)

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Segmentation in large intestine

• Haustration: （modified form of segmentation in which intense, local contraction of circular muscle causes large intestine to appear to bulge into sacs

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Mass movement

• Occurs in colon• Period of intense propulsive activity that

moves entire contents of colon distally toward rectum– Contractions progress for long distance such that

long length of colon contracts as a unit– Entry of fecal matter into recturn triggers

defecation reflex

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Mass Movement

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DefecationDefecation Reflex

initiated when rectal walls stretch⇓

parasympathetic reflex⇓

walls of the sigmoid colon and the rectum to contract & relaxation of

the anal sphincter⇓

External sphincter control is voluntary control

⇓If defecation is delayed: the reflex

stops until the next mass movement

PHYSIOLOGY Chapter 6PHYSIOLOGY Chapter 6

Chunmei Xia, Ph.DDepartment of Physiology and Pathophysiologycmxia@fudan.edu.cn

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Gastrointestinal Secretions and absorption

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Exocrine of the GI tractCompositionFunctionA. Digest foodB. Dilute the food into iso-

osmotic fluidC. Provide a favorable pH for

the digestive enzymesD. Provide mucus for

lubrication and protection of all parts of the alimentary tract

Regulation

Saliva 1.5 L/dpH 6.8-7.0Ingest 2

L/d water

Gastric secretion 2 L/d, pH 1.5-3

Bile 0.5 L/d pH 7.8-8.0

Pancreatic juice 1.5 L/d pH 8.0-8.4

Intestinal secretion 1.5 L/d pH 7.8-8.0

Small intestine absorbs 8.5 L/d

Colon absorbs 0.4-1 L/d

0.1 L/d water excreted

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1. Gastric secretion

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OesophagusLower OesophagealSphincter Fundus

Body

Antrum

DuodenumPylorus

Functional Anatomy of StomachFundus

Body

• Storage

• Storage• Mucus• HCl• Pepsinogen

Antrum

• Intrinsic factor

• Mixing/Grinding• Gastrin

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II.1 Gastric gland cells1. Oxyntic gland

Parietal cellChief cellMucous neck cell

2. Pyloric glandMucus cell

3. Cardiac glandMucus cell

4. Endocrine cells (G, D, ECL)

ECL:enterochromaffin-like cell

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Synthesize and secrete the HCl acid responsible for the acidic pH in the gastric lumen.

Synthesize and secrete the protease precursor known as pepsinogen.

Produce alkaline mucus that covers mucosa layer

Exocrine gland cells of gastric pits

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II.2 Composition and function of gastric secretions

1. HClconverts pepsinogen to pepsin for chemical digestion provides optimal pH environment for pepsindestroys some bacteriastimulates the small intestinal mucosa to release secretin and CCKpromotes the absorption of Ca2+ and Fe2+ in small intestine

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Composition and function of gastric secretions

2. Pepsinogen (precursor of pepsin)digestion of proteins

3. Mucusforms a protective barrier: Mucus-bicarbonate

barrier4. Intrinsic factor

combines with vitamin B12 to make it absorbable

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HCl secretion

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HCl secretion

pH<2pH>7.4

• H+ source is carbonic anhydrase H2O + CO2 ---> H2CO3

• H+ is pumped out via H+/K+

ATPase• Lots of mitochondria to generate

the ATP required• HCO3

- passes into the plasma in exchange for Cl-

• Cl- is secreted into the lumen

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Cells

Inactive precursor of pepsin which initiates protein digestion

Is not necessary for complete digestion of dietray protein –

pancretic enzymes are sufficient

Active only when the pH < 3.5

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Physical/chemical barrier to attack by gastric juice

Stimulated by:

• Ach

• Mechanical Stim

• Chemicals (ethanol)

If breached e.g. hypersecretion of acid -ulceration

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Gastric Mucus-Bicarbonate Barrier

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Gastric Mucus-bicarbonate barrier

The insoluble mucus and bicarbonate construct a barrier

prevent hydrogen ions from diffusing to the mucosal layerprotect the stomach mucosa from injury by hydrochloric acid and pepsin,

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Intrinsic Factor•Only gastric secretion that is essential for health

•Secreted from parietal cells in humans, cheif cells in other species

•Forms a complex with vitamin B12 in the gut

•The complex is resistant to digestion and therefore enables absorption of vitamin B12

•Lack of intrinsic factor causes Vit B12 deficiency (pernicious anaemia) – as all the Vit B12 is digested and therefore can not be absorbed

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Control of Gastric Acid SecretionGastric acid secretion is controlled by three

mechanisms:

• Neurocrine (vagus/local reflexes)• Endocrine (gastrin)• Paracrine (histamine)

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Endocrine gland cells of gastric pits

Stimulates acid secretion

Inhibits • acid secretion• gastrin and pepsin release• pancreatic exocrine secretions

Stimulates acid secretion

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Regulation of Gastric SecretionsThe important stimulatory signalsAutonomic nerves

• Release ACh• Stimulates smooth muscle contraction• stimulates Chief , Parietal , ECL and G cells

Gastrin• Stimulates Chief , Parietal , ECL cells

Histamin• Stimulates Parietal cells

Protein products such as peptidesStimulates G-cellsAcids

• Stimulate D cells

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Endogenous substances regulating gastric secretion

协同作用

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Gastric secretion during digesting food

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Gastric secretion in the digestive phaseGastric secretion in the digestive phaseCephalic phaseCephalic phase

Mechanisms:Mechanisms:Conditioned & Unconditioned reflexConditioned & Unconditioned reflexVagal efferent & with Gastrin secretionVagal efferent & with Gastrin secretion

through gastrinthrough gastrin--releasing peptide (GRP)releasing peptide (GRP)Experiment: Sham feeding by PavlovExperiment: Sham feeding by PavlovCharacteristics: Characteristics:

Large quantity (30%)Large quantity (30%)High acidity & digestive powerHigh acidity & digestive power

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Cephalic PhaseUnconditioned and conditioned reflex Only occurs when we want fooddepression dampens this reflexLarge amount of HCL and pepsinogen, high digestive

ability

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Experiment of Sham feeding by Experiment of Sham feeding by PavlovPavlov

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Cephalic Phase

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Mechanisms Stimulating Gastric Acid Secretion in Cephasic Phase

Sight, smell,taste of food

Vagusnerve

ParietalcellsACh

+

G cells Gastrin+

Gastrin/ACh ECLcells

Histamine

+GRP

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Gastric phaseGastric phaseMechanisms: Mechanisms:

•• Distension of gastric fundus & body Distension of gastric fundus & body initiating initiating vagovagal & local plexus reflexesvagovagal & local plexus reflexes

•• Distension of pylorus Distension of pylorus initiating a release of initiating a release of gastringastrin through intrinsic plexusthrough intrinsic plexus

•• Chemical stimulation Chemical stimulation of G cells initiating a of G cells initiating a release of gastrinrelease of gastrin

Characteristics: Characteristics: Large quantity (60%)Large quantity (60%)High acidity & digestive powerHigh acidity & digestive power 26

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Gastric phaseGastric phase

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Gastric Phase

Distensionof stomach

(arrival of food)

Vagal/Entericreflexes

Parietalcells

ACh

Peptidesin lumen

G cells Gastrin

Gastrin/ACh ECLcells

Histamine

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Intestinal phaseIntestinal phase

Mechanisms: Mechanisms: Mainly humoral regulationMainly humoral regulationChemical & Mechanical stimulationChemical & Mechanical stimulation

initiating releases of Gastrin, Enteroinitiating releases of Gastrin, Entero--oxyntin & Other humoral factorsoxyntin & Other humoral factors

Characteristics: Characteristics: Small quantity (10%)Small quantity (10%)Lower acidity & digestive powerLower acidity & digestive power

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Intestinal phaseIntestinal phase

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Regulation of Gastric Secretions occurs via 3 phases

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Hydrochloric acid (HCl)Hydrochloric acid (HCl)A typical example of negative feedbackA typical example of negative feedbackConditions & Mechanisms:Conditions & Mechanisms:

pH pH ≤≤ 1.2~1.5 in the gastric antrum1.2~1.5 in the gastric antrumInhibition of G cells, Release of SSTInhibition of G cells, Release of SST

pH pH ≤≤ 2.5 in the duodenum2.5 in the duodenumRelease of secretin, bulbogastroneRelease of secretin, bulbogastrone

Fat:Fat: Initiating release of enterogastroneInitiating release of enterogastroneHypertonic solutionHypertonic solution: Entero: Entero--gastric reflexgastric reflex

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Inhibitory regulation of gastric secretionInhibitory regulation of gastric secretion

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Enterogastrones

• Hormones released from gland cells in duodenal mucosa - secretin, cholecystokinin (CCK), GIP

• Released in response to acid, hypertonic solutions, fatty acids or monoglycerides in duodenum

• Act collectively to prevent further acid build up in duodenum

• Two strategies:• inhibit gastric acid secretion• reduce gastric emptying (inhibit motility/contract

pyloric sphincter)

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Regulation of gastric secretion

Secretion of Ach or other transmittersby nerve endings

Mechanical stimulationEntero-oxyntin

Fatty acids

Hyperosmotic solution

HCl

Gastric gland

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2. Secretion of the pancreas

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Secretion of the pancreas

Endocrine - insulin & glucagon

Exocrine - enzymes and bicarbonate

essential for digestion

almost under separate hormonal control

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Gall bladder

Sphincter of Oddi

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Cleave peptide bonds

Hydrolyze DNA/RNA

Collagen digestion

Phospholipids to fatty acids

Triglycerides to fatty acids+ glycerol

Starch to maltose + glucose

Categories of Pancreatic Enzymes

Proteases

Nucleases

Elastases

Phospholipases

Lipases

Amylase

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Activation of pancreatic proteases

Trypsinogen TrypsinEnterokinase

TrypsinogenChymotrypsinogen

ProelastaseProcarboxypeptidase

TrypsinChymotrypsin

ElastaseCarboxypeptidase

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** Regulation of pancreatic secretionRegulation of pancreatic secretionNervous regulationNervous regulation

Vagus nerve: ACh, gastrinVagus nerve: ACh, gastrinCharacteristics: HCharacteristics: H22O & HCOO & HCO33

−−↑↑, enzymes, enzymes↑↑↑↑Sympathetic nerve: ACh, NASympathetic nerve: ACh, NA

Characteristics: weak effectCharacteristics: weak effectHumoral reulationHumoral reulation

Secretin: HSecretin: H22O & HCOO & HCO33−−↑↑↑↑, enzymes, enzymes↑↑

Cholecystokinin (CCK): Cholecystokinin (CCK): Characteristics: HCharacteristics: H22O & HCOO & HCO33

−−↑↑, enzymes, enzymes↑↑↑↑Feedback:Feedback: CCKCCK--releasing peptidereleasing peptide

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3. Biliary secretion

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Structure/Function of LiverLiver lobule

Portal triad

Bilecanaliculus

Hepaticartery

Hepaticportal vein

Centralvein Central

vein

Portaltriad

BloodBile

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•• Bile secretion & gallbladder emptyingBile secretion & gallbladder emptying

** Nature, Compositions & functionsNature, Compositions & functions

Hepatic bile: pH 7.4, golden yellow Hepatic bile: pH 7.4, golden yellow

Bladder bile: pH 6.8, color become darkerBladder bile: pH 6.8, color become darker

Compositions: Compositions: HH22O, ions, bile acid, bile O, ions, bile acid, bile

pigment, fatty acid, cholesterol, lecithin, pigment, fatty acid, cholesterol, lecithin,

mucoprotein, etc., but no enzymemucoprotein, etc., but no enzyme

Functions of bile (mainly by bile salt): Functions of bile (mainly by bile salt):

Fat emulsificationFat emulsification; lipid absorption;; lipid absorption;

Promote the absorption of fatPromote the absorption of fat--soluble Vitssoluble Vits43

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** Control of bile secretion & gallbladder emptyingNervous regulation

Vagus nerve: ACh, gastrinHepatic bile secretion↑ (small amounts)Gallbladder contraction↑ (slightly)

Humoral reulationGastrin: 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 44

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Digestion in the intestineDigestion in the intestine•• Pancreatic juice & its secretion Pancreatic juice & its secretion

** Nature, Compositions & functionsNature, Compositions & functionspH 7.8~8.4, colorless & odourless, 1~2 L/daypH 7.8~8.4, colorless & odourless, 1~2 L/dayBicarbonate (HCOBicarbonate (HCO33

−−))Neutralize HCl & provide a weak basic Neutralize HCl & provide a weak basic

medium favoring digestive enzyme actionmedium favoring digestive enzyme actionPancreatic enzymes: Pancreatic enzymes: amylase, lipase, colipase,amylase, lipase, colipase,

trypsinogen & chymtrypsinogen, etc.trypsinogen & chymtrypsinogen, etc.Turn trypsinogen into trypsin by enteroTurn trypsinogen into trypsin by entero--

kinase, turn chymtrypsinogen into chymkinase, turn chymtrypsinogen into chym--trypsin by trypsintrypsin by trypsin

Trypsin inhibitor: Trypsin inhibitor: a polypeptidea polypeptide46

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§§ 66. Absorption in the small intestine• Sites of absorption

Oral cavity & Stomach: littleDuodenum & Upper jejunum: most nutrientsIleum: bile salts & Vit. B12

Colon: water & electrolytes• Proofs as the main absorptive region

Huge absorptive surface (200 m2) Plenty of capillaries & lymph capillariesLarge quantity of digestive fluid (6~8 L/day)Food has almost completely been digested

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Enlargement of Enlargement of the surface the surface area of the area of the intestineintestine

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•• Absorption of main nutrientsAbsorption of main nutrients** WaterWater

8 L/day, passive & iso8 L/day, passive & iso--osmotic absorbedosmotic absorbedDifferent absorbability in different partsDifferent absorbability in different parts

** Inorganic slatsInorganic slatsSodium: 95%~99%, jejunum>ileum>colonSodium: 95%~99%, jejunum>ileum>colonactive transportactive transport

Ferrum: 1/10, mainly in duodenum & jejunum, Ferrum: 1/10, mainly in duodenum & jejunum, transferrin dependent, active transporttransferrin dependent, active transport

Calcium: promote by Vit. D, active transportCalcium: promote by Vit. D, active transportAnions: mainly Cl Anions: mainly Cl −− & HCO& HCO33

−−, passive transport , passive transport

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** CarbohydrateCarbohydrateAbsorptive form: monosarccharideAbsorptive form: monosarccharideMechanism: secondary active transportMechanism: secondary active transport

** ProteinProteinAbsorptive form: amino acidAbsorptive form: amino acidMechanism: secondary active transportMechanism: secondary active transport

** FatsFatsAbsorptive form: glycerol, monoglyceride, Absorptive form: glycerol, monoglyceride, fatty acid, cholesterolfatty acid, cholesterol

Mechanism: passive diffusionMechanism: passive diffusionPathway: blood & lymphPathway: blood & lymph

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Digestion & absorption of fats in the intestineDigestion & absorption of fats in the intestine

Digestive Homeostasis Disorders

• ULCERS – erosion of the surface of the alimentary canal generally associated with some kind of irritant

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•• CONSTIPATIONCONSTIPATION – a condition in which the large intestine is emptied with difficulty. 

• Too much water is reabsorbed

• and the solid waste hardens

Digestive Homeostasis Disorders

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Digestive Homeostasis Disorders

• DIARRHEA – a gastrointestinal disturbance characterized by decreased water absorption and increased peristaltic activity of the large intestine. 

• This results in increased, multiple, watery feces. 

• This condition may result in severe dehydration, especially in infants

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Digestive Homeostasis Disorders

• APPENDICITIS – an inflammation of the appendix due to infection

• Common treatment is removal of the appendix via surgery

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Digestive Homeostasis Disorders

• GALLSTONES – an accumulation of hardened cholesterol and/or calcium deposits in the gallbladder

• Can either be “passed” (OUCH!!) or surgically removed

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Digestive Homeostasis Disorders

• HEART BURN – ACID from the stomach backs up into the esophagus. 

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