Chapt-6 消化与吸收

• 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
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
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
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
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
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
* 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
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
• Gastrointestinal innervation * Intrinsic (or enteric) nervous system Submucosal & myenteric plexus * Extrinsic nervous system Parasympathetic & sympathetic nerves
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
Chapter 6
消化和吸收
DIGESTION & ABSORPTION
§ 1. Introduction
• Concept of digestion & absorption * Mechanical & chemical digestion
• Characteristics of digestive smooth muscles * General characteristics * Electrophysiological characteristics
• 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
Intrinsic factor Secreted by parietal cells Function: Promote the absorption of Vit B12
* Regulation of gastric secretion Endogenous releasing substances Acetylcholine (ACh), gastrin & histamine