Gastrointestinal System

Instructor: Ge Shun

Office: 0850 physiological sciencesEmail: geshun@tom.com

Jining medical collegeDepartment of Physiology

Gastrointestinal System

1. G.I. Physiology: general organization of G.I. tract

2. Mastication and deglutition3. Gastric secretion4. Regulation of gastric secretion5. Pathophysiology of peptic ulcer6. Biliary and pancreatic secretions7. Physiology of colon8. Pathophysiology of diarrhea disease

Gastrointestinal System includes GI tract plus the accessory organs.

Four processes carried out by the GI tract

Digestive secretions from the liver and the pancreas are delivered into the duodenum of the small intestine through

the sphincter of Oddi.

Functions of small intenstine movements

• Mixes the luminal contents with secretions

• Brings the contents nto contact with the epithelial surface

• Slowly advances the luminal material toward the large intenstine

Structure of the GI Tract Wall

Structure of the GI Tract Wall

(I) Physiological properties of gastrointestinal smooth muscle

(1) General properties

- Low excitability - High extensibility - Tonic contraction - Autorhythmicity

- High sensitivity to temperature, stretch & chemical stimulation

(2) Electrophysiological properties

(a) Resting potential: - between -50 and -60 Mv

- Ionic basis -Em (selective membrane permeability to K+)

-Electrogenic Na+-K+ pump

(b) Slow wave (basic electrical rhythm,BER)

- The spontaneous rhythmic, subthreshold depolarizations of the cell membrane (slow wave) of the gastrointestinal tract

- Initiated in the interstitial cells of Cajal (ICC) (pacemaker cell)

Slow wave (basic electrical rhythm)

- Intensity: 5~15 mV

- Frequency: 3~12 cpm - Ionic mechanism

spontaneous rhythmic changes in Na+-K+ pump activity

Frequencies: 3-12 per minute

(c) Spike potentials (action potentials)

• only at the peaks of slow waves

• Threshold: -40 mV

Spike potential (Action potential)

-Duration: 10~20 ms

-Ionic mechanism:

• Depolarization: Ca2+ influx

• Repolarization: K+ efflux

The higher the slow wave potential rises, the greater the frequency of the spike potentials

(II) Neural control of gastrointestinal function

Enteric nervous system

Extrinsic nervous system

Myenteric plexus : control over GI motility Submucous plexus: regulate gastrointestinal blood flow and control GI secretion

(III) Hormonal Regulation --GI hormones

The hormones synthesized by a large number of

endocrine cells within the gastrointestinal tract

Brain-gut peptides: a number of the classical GI hormones are also synthesized in the brai

n

Four main types

• Gastrin

• Cholecystokinin

• Secretin

• Gastric inhibitory polypeptide (GIP)

(IV) Phases of Gastrointenstinal Control

• Cephalic phase

• Gastric phase

• Intenstinal phase

• Three phase of gastric secretion.swf

Summary

• Terms:

- Basic electrical rhythm (BER) or slow waves - Gastrointestinal (GI) hormones

• Three phase of gastric secretion

Mouth, Pharynx, and Esophagus

Secretion of saliva

The Salivary Glands • Parotid gland • Mandibular gland • Sublingual gland

Saliva

1000 ml/d pH 6.0~7.0 Composition of saliva -H2O -Na+, Cl-, K+, HCO3

-

-Mucus (containing mucin) -Amylase, lysozyme, …

Swallowing

Digestion in the Stomach

Chief cells synthesize and secrete the protease precursor known as pepsinogen.

Parietal cells synthesize and secrete the hydrochloric acid responsible for the acidic pH in the gastric lumen.

(I) Gastric juice

(1) Hydrochloric acid

Secreted by the parietal cells

Output

- Basal: 0~5 mmol/h

- Maximal: 20~25 mmol/h

Acid production by the parietal cells in the stomach depends on the generation of carbonic acid; subsequent movement of hydrogen ions into the gastric lumen results from primary active transport.

Four chemical messengers regulate HCl secretion

Role of HCl

- Acid sterilization

- Activation of pepsinogen

- Promotion of secretin secretion

- Assisted effect of iron and calcium absorption

(2) Pepsinogen

(3) Mucus-HCO3- barrier

If the mucus-HCO3- barrier is weak?

(4) Intrinsic factor

A high molecular weight glycoprotein, synthesized and secreted by the parietal cells

•The intrinsic factor binds to Vit B12 and facilitates its absorption

(II) Regulation of gastric secretion

(1) Basic factors that stimulate gastric secretion

- Acetylcholine (+ all secretory cells) - Gastrin (+ parietal cells)

- Histamine (+ parietal cells)

(2) Nervous regulation

- ‘Short’ reflex pathways- ‘Long’ autonomic pathways

(3) Humoral regulation

(4) Phases of gastric secretion

• Cephalic phase

• Gastric phase

• Intestinal phase

Motor Function of the Stomach

Motor Function of the Stomach

• Receptive relaxation - Storage function (1.0~1.5 L) - Vago-vagal reflex • Peristalsis - BER in the stomach

Motor Function of the Stomach

Emptying of the stomach

Emptying rate

- Fluid > viscous

- Small particle > large particle

- Isosmotic > hyper- & hypo-osmotic - Carbohydrates > Protein > Fat - Regular meal 4～ 6 hrs

Summary

Terms:

- Receptive relaxation (Storage function of the stomach)

- Mucus-HCO3- barrier - Intrinsic factor

Gastric secretion (including the composition and physiological function of the gastric juice, the regulation of gastric secretion)

Pancreatic Secretion

(I) Pancreatic juice

• pH 7.8~8.4 • ~1500 ml/day• Isosmotic • Components: - Pancreatic digestive enzymes: secreted by pancreatic

acini - Sodium bicarbonate: secreted by small ductules and

larger ducts

Secretion of bicarbonate ions

Secretion of pancreatic digestive enzymes

(II) Regulation of pancreatic secretion

Basic stimuli that cause pancreatic secretion - Ach - Cholecystokinin: • Secreted by I cells • Stimulates the acinar cells to secrete large amounts o

f enzymes - Secretin: • Released by S cells • Acts primarily on the duct cells to stimulate the secreti

on of a large volume of solution with a high HCO3- concentration

Bile Secretion and Liver Function

Composition of bile

Function of bile

Function of bile

Enterohepatic circulation

Regulation of bile secretion

Small Intestine

Absorption in the gastrointestinal tract

(I) Basic principle of absorption

Absorption in the gastrointestinal tract

Absorption in the gastrointestinal tract

3. Absorption mechanisms

• Active transport

• Diffusion

• Solvent drag

(II) Absorption of major nutrients

2. Carbohydrates

3. Proteins

4. Fats

4. Fats

(1) Absorption through lymph duct to the circulatory blood

(2) Direct absorption into the portal blood

Summary

• Pancreatic secretion (including the composition and physiological function of the pancreatic juice, the regulation of pancreatic secretion)

• Biliary secretion (including the composition andphysiological function of the bile)• Absorption in the small intestine• Describe the digestion of carbohydrates, proteins andfat