Chapter 6 DIGESTION ABSORPTION 1 Introduction Concept of

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 p. H 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 1 st, 2 nd & 3 rd phase Peristalsis & lower esophageal sphincter

Peristalsis of the esophagus

§ 3. Digestion in the stomach Gastric juice & its secretion * Nature, Compositions & functions p. H 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 p. H 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 B 12 * 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: p. H ≤ 1. 2~1. 5 in the gastric antrum Inhibition of G cells, Release of SST p. H ≤ 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 p. H 7. 8~8. 4, colorless & odourless, 1~2 L/day Bicarbonate (HCO 3 ) 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: H 2 O & HCO 3 , enzymes Sympathetic nerve: ACh, NA Characteristics: weak effect Humoral reulation Secretin: H 2 O & HCO 3 , enzymes Cholecystokinin (CCK): Characteristics: H 2 O & HCO 3 , enzymes Feedback: CCK-releasing peptide

Bile secretion & gallbladder emptying * Nature, Compositions & functions Hepatic bile: p. H 7. 4, golden yellow Bladder bile: p. H 6. 8, color become darker Compositions: H 2 O, 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: H 2 O & HCO 3 , 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 p. H 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 p. H 8. 3~8. 4, Mucus & HCO 3 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. B 12 Colon: water & electrolytes Proofs as the main absorptive region Huge absorptive surface (200 m 2) 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 & HCO 3 , 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