Chapter 5 Digestion and Absorption Section 1 Introduction

Chapter 5 Digestion and Absorption Section 1 Introduction Section 2 Ingestion Section 3 Digestion in Mouth Section 4 Digestion in Stomach Section 5 Digestion in Small Intestine Section 6 Section 7 Digestion in Large Intestine Absorption

Section 1 Introduction

GI tract mouth, pharynx(咽), esophagus(食管), stomach, small intestine, large intestine Accessory Glandular Organs salivary glands(唾液腺) liver, Gallbladder(胆囊) pancreas(胰腺)

The conception of digestion and absorption Digestion: Process by which food and large molecules are degraded to produce smaller molecules that can be absorbed across the wall of the gastrointestinal tract. Absorption: Process by which nutrient molecules are absorbed by cell that line the gastrointestinal tract and enter the bloodstream

Patterns of digestion (1)Mechanical Digestion (2)Chemical digestion (3)Microbial digestion

General properties of gastrointestinal smooth muscle 1. Lower excitability, slower contraction and relaxation 2. Autorhythmicity 3. Higher extensibility 4. Tonic contraction(紧张性收缩) 5. More sensitive to stretch, chemicals, cold and warm stimulation but not to electric stimulation

Electrical activity of GI smooth muscle 1. Resting potential 2. Slow wave or basic electric rhythm (BER) Basic electrical rhythm is the spontaneous rhythmic, subthreshold depolarization of the cell membrane in the gastrointestinal tract

Ø Slow waves are changes in resting membrane potential Ø 3 -12 cycles per minute depending on area of GI tract - 3/min in stomach, 12/min small intestine Ø Always present but do not always cause contractions Ø Frequency of contractions dictated by frequency of slow waves

Neural control of gastrointestinal function (1)Extrinsic nervous system Sympathetic nerve ending release NE, inhibits the gastrointestinal movement and secretion of digestive gland 。 parasympathetic nerve ending release Ach, stimulates the gastrointestinal movement and secretion of digestive gland 。

Hormonal regulation The hormonal synthesized by a large number of endocrine cells within the GS tract Physiology functions: (1)Control the digestive function (2)Control the release of other hormones (3)Trophic action(营养作用)

Section 2 Ingestion

Section 3 Digestion in Mouth

Salivary secretion唾液的分泌 Composition of saliva water, ions, mucus(粘 液), enzymes Functions of saliva ①moistening and lubricating the food; ②contains the enzyme amylase , partially digests polysaccharides. ③antibacterial role ④ heat elimination

Section 4 Digestion in Stomach

Gastric Motility Patterns: (1)Receptive relaxation容受性舒张 During 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 Role： accommodate a large amounts of food

Control of gastric motility 1）Neural regulation Vagal nerve(+): increase gastric motility sympathetic nerve (+): inhibit gastric motility 2）Hormonal regulation Gastrin胃泌素: increase gastric motility secretin(促胰液素)、CCK: inhibit gastric motility

Gastric emptying(胃的排空) The process by which the chyme(食糜) is expelled from the stomach into the duodenum(十 二指肠) is called the gastric emptying Factors affect gastric emptying (1)States of animal (2) physicochemical property of food Carbohydrates>protein>fat Isosmotic>hyper- or hypo-osmotic Fluid >viscous(粘性的) 3) stomach: stimulating factor, 4) duodenum: inhibiting factor

Control of Gastric emptying (1)Stimulating factors in stomach Presence of food Gastrin(胃泌素) (2)Inhibitory effects in duodenum • Enterogastric reflexes (肠胃反射) • Fat • CCK

Enterogastric Reflex(肠胃反射) regulates the rate at which chyme leaves the stomach

Gastric secretion cardiac gland贲门腺区: 粘液 fundus gland胃底腺区 chief cell: pepsinogen 主细胞：胃蛋白酶原 Parietal cell: Hcl, Intrinsic factor壁细胞：盐酸、内因子 pyloric gland 幽门腺区 G cell: gastrin胃泌素 粘液细胞：粘液 粘液细胞 壁细胞 G 细胞 主细胞

Gastric juice Properties: • p. H: 0. 9~1. 5 • 1~2. 5 L/day Compositions: • HCl • Pepsinogen • Mucus • Intrinsic factor

(1)HCL • Secreted by parietal cell(壁细胞) • output: Basal: 0~5 mmol/h Max: 20~25 mmol/h

Role of HCl w w w Activated pepsinogen into pepsin for chemical digestion provides optimal p. H environment for pepsin Acid sterilization stimulates the small intestinal mucosa to release secretin and CCK Assisted effect of Ca and Fe absorption in small intestine

Mechanism of HCl secretion 是壁细胞逆浓度差主动分泌的，

(2)胃消化酶 1）Pepsinogen胃蛋白酶原 Secreted by the chief cells as an inactive precursor of pepsin Role： Pepsinogen protein 特点： HCl Pepsin→breakdown 自身激活 ①pepsinogen is inactive ②optimal p. H＝ 2. 0，it is inactive when p. H＞６. 0 ③Is not necessary for complete digestion of dietray protein

(3)Mucus(粘液) Soluble mucus可溶性粘液 Insoluble mucus不溶性粘液 Role ①lubrication of the mucosal surface ②Protection of tissue from mechanical damage by food particles； ③ mucus bicarbonate barrier粘液-碳酸氢盐屏 障

mucus bicarbonate barrier The insoluble mucus and bicarbonate construct a barrier prevent H+ from diffusing to the mucosal layer protect the stomach mucosa from injury by HCl and pepsin,

(4)Intrinsic Factor(内因子) secreted by parietal cells 成分：糖蛋白（有2个亚单位） 亚单位A＋Ｂ 12→复合物: 防Ｂ 12被 水解酶破坏 亚单位B＋结合特异受体: 吸收Ｂ 12 Role： The complex is resistant to digestion and therefore enables absorption of Vit. B 12

Regulation of gastric secretion Basic factors that affecting gastric secretion （1）＋ Ach （2）＋ gastrin （胃泌素） （3）＋ Histamine（组胺） （4）— somatostatin（生长抑素） 副交感神经 壁细胞 Ach 内在神经末梢 G细胞 胃泌素

Gastric secretion during digesting food (1)Cephalic Phase 头期 (2)Gastric Phase 胃期 (3)Intestinal Phase 肠期

Cephalic Phase头期 1） mechanism Unconditioned and 口腔机械、化学刺激 conditioned reflex : release Ach, stimulate gastric secretion 迷走-胃泌素：迷走N的末梢递 质GRP(胃泌素释放肽)引起胃窦部G 细胞分泌胃泌素 2）characters： Large amount of HCl and pepsinogen, high digestive ability

Section 5 Digestion in Small Intestine

Types of small intestinal movement (1) Segmentation分节运动—环形肌节律性收缩与舒张 role： mix the chyme with the digestive juice; increase its exposure to the mucosal surface (2) Peristalsis蠕动: 小肠环形肌推进式的收缩 Role: propels the small intestinal contents towards the large intestines (3) Tonic contraction紧张性收缩：使小肠平滑肌经常处 于紧张性状态 (4) MMC移行运动复合波：消化间期 Role: moves any undigested material still remaining in the small intestine into the large intestine

Control of Intestinal Motility Neuronal regulation： Sympathetic nerve ：inhibit intestinal motility Vagal：increase intestinal motility 壁内神经丛：局部反射性调节胃肠反射 summary：小肠前段的运动会兴奋后段的运动； 小肠后段的运动会抑制前段的运动 Hormonal： (+)：Ach、insulin (-)：NE、resectin(促胰液素 )

Secretion of the pancreas endocrine insulin glucagon pancreas 腺泡�胞： enzymes exocrine 胰液 小�管管壁�胞 : H 2 O、HCO 3

Pancreatic juice Characters： (1)p. H 7. 8～ 8. 4， (2)Isosmotic (3)1~2 L/day Components （1）H 2 O：小导管管壁细胞分泌 （2）HCO 3 -：secreted by small ductules role：①中和胃酸，保护肠粘膜不受胃酸的侵蚀； ②为小肠内多种消化酶活动的提供最适p. H环境。

（3）pancreatic digestive enzymes：secreted by pancreatic acini 1） pancreatic Amylase胰淀粉酶 Starch to maltose + glucose水解淀粉为麦芽糖 和葡萄糖 2）pancreatic lipase胰脂肪酶 Phospholipids to fatty acids 3) Proteases 蛋白水解酶 Cleave peptide bonds

Function Lubricate and protect intestinal surface (Ig A) Dilute digestive products Digest specific food substances (enzymes in enterocytes: peptidase, sucrase, etc ) Control of Small Intestine secretion ： Local stimuli主要受局部因素调节 食物及其消化产物对肠粘膜局部的机械和化学刺激， 通过肠壁内神经丛的局部反射，引起小肠液分泌。 刺激迷走 神经引起十二指肠腺的分泌。人在应激状态下，交感神经活 动增强可抑制小肠腺分泌，从而增加患十二指肠溃疡的机会。

Section 6 Digestion in Large Intestine

Section 7 Microbial Digestion

Microbial digestion in rumen 反刍: 反刍动物把瘤胃中的食团逆呕回口腔再仔细咀嚼混合唾 液回到胃的特殊消化活动, 包括逆呕 再咀嚼 混合唾液 再 吞咽四个阶段 condition for existence of microorganism in rumen 瘤胃内微生物的生存条件 1）temperature： 38 --41 ℃ 2）p. H ： 6 --7之间 3）water content is relative stability 4）anaerobic interspace厌氧环境 5）渗透压: 与血浆相近

Section 7 Absorption

The reasons of small intestine as main absorptive region: ①The large absorptive surface area formed by valvulae connivent, villi and microvilli; ②Chyme stays in small intestine for longer time; ③There abundant capillaries and lymphatic capillaries in intestinal mucosa ④Food has already been digested into small and absorbable molecules.