Ch. 21 The Digestive System - Human Physiology: an Integrated Approach PDF

Preview

Summary

An all-encompassing summary/outline for the chapter dictated by the title of this upload. All work is done independently by the user uploading this document by typing summaries and notes while browsing through the specified chapter. Feel free to use this as you progress through the chapter and add l...

Description

Ch. 21: The Digestive System Anatomy of the Digestive System ● Food enters the GI tract consisting of esophagus, stomach, small intestine, and large intestine ○ Portion of GI tract running from stomach to anus is known as the ‘gut’ ● Digestion: chemical and mechanical breakdown of food ○ Occurs in lumen of gut ■ Secretions are added to ingested food by secretory epithelial cells including the salivary glands, liver, gallbladder, and pancreas to create the soupy mixture known as ‘chyme’ ● The Digestive System is a Tube ○ First stages of digestion occur with chewing and salivary secretion in oral cavity by 3 pairs of salivary glands ■ Sublingual glands → under tongue ■ Submandibular glands → under mandible ■ Parotid glands → near hinge of jaw ○ Swallowed food passes into esophagus (skeletal muscle that transitions into smooth muscle ⅔ way down) ■ Esophagus ends at stomach (holds 2L of food) ● Fundus ● Body ● Antrum ■ Pylorus (gatekeeper between stomach and small intestine) has pyloric valve ● Pyloric Valve is a thickened band of smooth muscle ○ Most digestion and absorption occurs in small intestine ■ Duodenum ● Secretions from pancreas and liver enter here ○ Sphincter of Oddi keeps pancreatic fluid and bile from entering small intestine except during a meal ■ Jejunum ■ Ileum ○ After most food is digested and absorbed in small intestine, chyme passes into large intestine ■ Colon → prox. part of large intestine ● Watery chyme becomes semisolid feces ■ Rectum → terminal section ● The GI Tract Wall has 4 Layers ○ The gut wall is crumpled into folds to increase surface area ■ Rugae in stomach ■ Plicae in small intestine ○ Intestinal mucosa also projects into lumen via Villi ○ Additional surface area is added by tubular invaginations of the surface that extend down into the supporting connective tissue ■ Gastric glands in stomach

■ Crypts in intestine ○ Gut wall has 4 layers: ■ Mucosa → inner lining of GI tract ● Mucosal epithelium ○ On apical surface, cells secrete ions, enzymes, mucus, and paracrine molecules into lumen ○ On serosal (basolateral) surface, substances are absorbed from lumen ● Lamina propria ○ Contains nerve fibers and small blood/lymph vessels ○ Has immune cells (macrophages, lymphocytes, etc.) ● Muscularis mucosae (smooth muscle) ■ Submucosa → middle layer of gut wall ● Connective tissue w/ larger blood/lymph vessels ● Submucosal Plexus (Meissner’s Plexus) → one of two major nerve networks of Enteric Nervous System ■ Muscularis Externa → outer wall of GI tract ● Inner Circular layer ○ Contraction decreases diameter of lumen ● Outer Longitudinal layer ○ Contraction shortens tube ● The stomach has a third Oblique layer between circular and longitudinal layers ● Myenteric Plexus (Auerbach’s Plexus) → other major nerve network of ENS lies between circular and longitudinal layers ○ Controls motor activity of muscularis externa ■ Serosa → outer covering of entire GI tract ● Continuation of the peritoneal membrane lining abdominal cavity (can form sheets of mesentery to hold in place) Digestive Function and Processes ● Primary function is to move nutrients, water, and electrolytes from external environment into body’s internal environment via digestion, absorption, secretion, and motility ● Digestive system faces 3 significant challenges: ○ Avoiding autodigestion ■ Enzymes secreted to digest food must not digest the cells of the GI tract itself (autodigestion) ● If protective mechanisms against autodigestion fail, raw patches known as peptic ulcers develop on GI tract walls ○ Mass balance ■ Maintaining mass balance by matching fluid input with output ● Vomiting and diarrhea can be dangerous ○ Defense ■ Protecting the body from foreign invaders ● 80% of body’s lymphocytes are found in small intestine

● We Secrete More Fluid than We Ingest ○ Digestive Enzymes ■ Secreted by exocrine glands (salivary glands, pancreas) or by epithelial cells in stomach and small intestine ■ Many remain bound to apical membrane until needed ○ Mucus ■ Viscous secretion of mucins to form protective coating over GI mucosa and to lubricate ● Made in mucus cells of stomach and salivary glands ● Made in goblet cells of intestine ● Motility: GI Smooth Muscle Contracts Spontaneously ○ Motility in GI tract serves two purposes: moving food from mouth to anus, and mechanically mixing food to break it into particles ■ Tonic Contractions: sustained for minutes-hours ● Occur in smooth muscle sphincters and anterior portion of stomach ■ Phasic Contractions: last only a few seconds ● Occur in small intestine and posterior region of stomach ○ Slow Wave Potentials: cycles of smooth muscle contraction and relaxation (depolarization and repolarization) ■ Occur in Interstitial Cells of Cajal (ICCs) ● Modified smooth muscle cells to act as an intermediary ● Serve as pacemakers for slow wave activity in GI tract ● GI Smooth Muscle Exhibits Different Patterns of Contraction ○ Migrating Motor Complex: ‘housekeeping’ function that sweeps food remnants and bacteria out of upper GI and into large intestine ○ Peristalsis: progressive waves of contraction ■ Circular muscle just behind a bolus contracts to push it forward to a receiving segment where it is more relaxed than usual (then continues on) ○ Segmental Contractions: short segments of intestine alternately contract and relax ■ Circular muscles contract, longitudinal muscles relax ■ Alternating contracts churn intestinal contents by mixing them Regulation of GI Function ● The ENS Can Act Independently ○ Intrinsic neurons lie in wall of gut (similar to CNS neurons) ■ Extrinsic neurons send autonomic signals from CNS to digestive system ○ Neurotransmitters like serotonin, vasoactive intestinal peptide, and nitric oxide ○ Glial support cells ○ Diffusion barrier ○ Integrating center ● Short Reflexes Integrate in the ENS

○ Enteric nerve plexuses in gut act like a “little brain” to allow local reflexes begin, integrate, and end completely in GI tract ■ Known as Short Reflexes ● Long Reflexes Integrate in the CNS ○ Although ENS can work in isolation, it also sends sensory info to CNS and receives autonomic neuron input from CNS ■ Known as Long Reflexes ● When long reflexes originate in the brain instead of the ENS, known as Cephalic Reflexes (i.e. smell dinner cooking and mouth waters then stomach growls) ● Parasympathetic division is excitatory and enhances GI functions → rest and digest ○ Parasympathetic neurons to GI tract are found in vagus nerve ○ Sympathetic neurons usually inhibit GI function ● GI Peptides Include Hormones, Neuropeptides, and Cytokines ○ Peptides secreted by digestive tract act as hormones or paracrine signals ■ Excite or inhibit motility and secretion ■ Can also act outside GI tract ● i.e. Cholecystokinin (CCK) increases satiety, Ghrelin acts on brain to increase food intake ○ GI Hormones ■ Secreted into blood and transported throughout body ● Act on GI tract, accessory organs, and distant targets like the brain ■ Hormone Families ● Gastrin ○ Gastrin ○ CCK ● Secretin ○ Secretin ○ Vasoactive intestinal peptide (VIP) ○ GIP ○ GLP-1 ● Motilin

Integrated Function: The Cephalic Phase ● Anticipatory stimuli and stimulus of food in oral cavity activates neurons in medulla oblongata which sends efferent motor signal to salivary glands via autonomic neurons and to ENS via vagus nerve ○ In response, stomach, intestine, and accessory organs begin secretion and increase motility in anticipation → Cephalic Phase of Digestion ● Chemical & Mechanical Digestion Begins in the Mouth ○ When food enters mouth, it is met by saliva ■ Saliva softens/moistens food, digests starch (salivary amylase), allows for taste, defense ○ Mechanical digestion begins w/ chewing which is known as Mastication ● Saliva Is an Exocrine Secretion ○ Saliva is complex hyposmotic fluid secreted under autonomic control ● Swallowing Moves Food From Mouth to Stomach ○ Swallowing is known as Deglutition ■ Reflex action pushing bolus into esophagus ● Soft palate elevates to close off nasopharynx, larynx closes off trachea, and upper esophageal sphincter opens ● As bolus moves down toward esophagus, epiglottis folds down to cover upper airways Integrated Function: The Gastric Phase ● Stomach has 3 general functions: ○ Storage ■ Stomach stores and regulates passage of food ○ Digestion ■ Stomach chemically and mechanically digests food into soupy mixture called chyme ○ Defense ■ Stomach protects body by destroying bacteria and other pathogens ■ Also, protects itself from being damaged by its own secretions ● Once food enters stomach, stimuli in gastric lumen initiate series of short reflexes that make up the Gastric Phase of Digestion

● The Stomach Stores Food ○ Stomach relaxes and expands to hold increasing volume when food arrives from esophagus → Receptive Relaxation ○ Upper stomach holds food while lower stomach digests food ● Gastric Secretions Protect and Digest ○ Lumen of stomach is lined w/ openings of gastric pits which lead to gastric glands deep within mucosal layer ■ Produce HCl, enzymes, hormones, and paracrine molecules ○ Gastrin Secretions ■ G cells (in gastric glands) secrete Gastrin ● Gastrin is stimulated by presence of amino acids and peptides in stomach and by stomach distension ● Coffee also stimulates Gastrin release ● Neural reflexes (Gastrin-Releasing Peptide, GRP) trigger Gastrin release ■ Gastrin promotes acid release by acting on parietal cells and by indirectly stimulating histamine release ○ Acid Secretion ■ P cells (in gastric glands) secrete HCl into lumen of stomach ● Can create pH as low as 1 ■ HCl causes release/activation of pepsin to digest protein, triggers somatostatin release from D cells, denatures proteins for digestion, kills bacteria, inactivates salivary amylase ○ Enzyme Secretion ■ Pepsin and Gastric Lipase are secreted by stomach ● Pepsin carries out initial digestion of proteins ○ Secreted by chief cells as Pepsinogen which is activated to pepsin by H+ ● Gastric Lipase breaks down triglycerides ○ Paracrine Secretion ■ Paracrine secretions from mucosa include histamine, somatostatin, and intrinsic factor ● Histamine is secreted by ECL cells in response to gastrin or ACh stimulation ○ Diffuses to P cells (target) and stimulates acid secretion by combining with H2 receptors on P cells ● Intrinsic Factor is secreted by P cells to combine with vitamin B12 ● Somatostatin (SS) is secreted by D cells as a negativefeedback signal for gastric phase secretion

○ Shuts down acid secretion by decreasing gastrin and histamine secretion ○ Inhibits pepsinogen secretion ● The Stomach Balances Digestion and Defense ○ Mucosa protects itself from autodigestion by acid and enzymes via mucus-bicarbonate barrier ■ Mucus forms physical barrier, bicarbonate creates chemical buffer barrier Integrated Function: The Intestinal Phase ● Intestinal Phase of Digestion begins when chyme passes into small intestine ● Surface of intestinal epithelium is known as the ‘brush border’ from bristle-like appearance of microvilli ● Intestinal Secretions Promote Digestion ○ Digestive Enzymes ■ Produced by intestinal epithelium and exocrine pancreas ○ Bile ■ Made in liver and released from gallbladder ■ Facilitates fat digestion ○ Bicarbonate ■ Neutralizes highly acidic chyme that enters from stomach ■ Comes from pancreas and is released in response to neural stimuli and secretin ○ Mucus ■ From intestinal goblet cells ■ Lubricates gut contents ○ Isotonic NaCL solution ■ Mixes with mucus to help lubricate gut contents ■ Secreted by Crypt cells in small intestine and colon ● Pancreas Secretes Enzymes and Bicarbonate ○ Endocrine secretions of pancreas include insulin and glucagon ○ Exocrine secretions include digestive enzymes and watery soln of sodium bicarbonate ○ Enzyme Secretion ■ Brush border enteropeptidase converts trypsinogen to active trypsin ■ Trypsin converts pancreatic zymogens (initial pancreatic secretions) into their active forms ○ Bicarbonate Secretion ● The Liver Secretes Bile ○ Bile is secreted from Hepatocytes (liver cells) ■ Made up of bile salts, bile pigments, and cholesterol ○ Bile secreted travels to gallbladder in which it is then stored

○ Gallbladder sends bile to duodenum of small intestine via Common Bile Duct ● Most Digestion Occurs in the Small Intestine ○ When chyme enters small intestine, protein digestion stops when pepsin is inactivated at the more alkaline pH ■ Pancreatic and brush border enzymes finish digestion of peptides, carbs, and fats into smaller molecules ● Bile Salts Facilitate Fat Digestion ○ Bile salts help emulsify fat into smaller particles ● Carbohydrates are Absorbed as Monosaccharides ○ Starch digestion begins in mouth via salivary amylase ■ Salivary amylase is destroyed in acidic stomach ○ Pancreatic amylase resumes starch digestion and then they are broken down further by intestinal brush border enzymes into glucose, fructose, and galactose ○ Carbohydrate Absorption ● Proteins are Digested into Small Peptides and Amino Acids ○ Plant proteins are least digestible ○ Endopeptidases and Exopeptidases ■ Endopeptidases (Proteases): secreted as zymogens then activated once they reach GI tract lumen ● Attack peptide bonds to break down proteins ● Examples of proteases include pepsin (stomach), trypsin/chymotrypsin (pancreas) ■ Exopeptidases: release single amino acids from peptides by chopping them off the ends one at a time ● Secreted by pancreas ○ Protein Absorption ○ Free amino acids, dipeptides, and tripeptides remain after digestion to be absorbed ○ Amino acids are carried in by Na+ dependent cotransport ○ Dipeptides/Tripeptides are carried into enterocytes on oligopeptide transporter PepT1 that uses H+ dependent cotransport ● Some Larger Peptides Can Be Absorbed Intact ○ Larger peptides are absorbed via transcytosis ● Nucleic Acids are Digested into Bases and Monosaccharides ○ DNA and RNA and digested into their component nucleotides and then into nitrogenous bases and monosaccharides ■ Bases absorbed via active transport ■ Monosaccharides absorbed via facilitated diffusion and secondary active transport Intestine Absorbs Vitamins and Minerals ● Fat-soluble vitamins (ADEK) are absorbed along w/ fats in small intestine

● Water-soluble vitamins absorbed via mediated transport ○ Vitamin B12 is only absorbed in ileum when it is combined w/ Intrinsic Factor ● The Intestine Absorbs Ions and Water ○ Most water absorption occurs in small intestine by following solutes ■ Na+ is absorbed via enterocytes (small intestine) and colonocytes (large intestine) by apical Na+ channels, Na+ symporter, and NaH exchanger Regulation of Intestinal Phase ● Regulation comes from signals that control motility and secretion ○ Sensors trigger neural and endocrine reflexes ■ Chyme entering intestine activates ENS to decrease gastric motility/secretion and slows gastric emptying ● Also, secretin, CCK, and GIP decrease motility ■ Secretin is released when acidic chyme present in duodenum to inhibit acid production and decrease gastric motility ■ CCK secreted into bloodstream to slow gastric motility and acid secretion ■ GIP and GLP-1 are released when carbs present to promote insulin release for glucose absorption ● GIP and GLP-1 also slow entry of food into intestine by decreasing gastric motility/secretion

The Large Intestine Concentrates Waste ● Chyme enters large intestine via Ileocecal Valve which leads to cecum ○ Tonically contracted region of muscularis ● Material moves from cecum upward via ascending colon, horizontally across transverse colon, down descending colon and sigmoid colon and into rectum ○ Rectum separated from external environment by anus ■ Closed by two sphincters: inner smooth muscle sphincter and external skeletal muscle sphincter ● Wall of colon differs from that of small intestine ○ Muscularis of large intestine has an inner circular layer but discontinuous longitudinal muscle layer concentrated into 3 bands called Taenia Coli ■ Contractions of Taeniae Coli pull wall into pockets called Haustra ● Luminal surface of colon lacks villi and appears smooth ● Motility in Large Intestine ○ Chyme that enters colon continues to be mixed via segmental contractions but movement forward depends on Mass Movement ○ Defecation Reflex removes undigested feces from body

■ Smooth muscle of internal sphincter relaxes while external sphincter skeletal muscle is also consciously relaxed if it is time to excrete ● Digestion and Absorption in the Large Intestine ○ Colonic bacteria can produce significant amounts of absorbable vitamins like vitamin K

Immune Functions of the GI Tract ● Immune cells of GALT must prevent pathogens from preventing body ● M Cells Sample Gut Contents ○ Immune system of intestinal mucosa consist of immune cells scattered across the mucosa, clusters within Peyer’s Patches, and M Cells ■ M cells provide info about contents of lumen to immune cells of GALT ● Vomiting (Emesis) is a Protective Reflex ○ Forceful expulsion of gastric and duodenal contents from mouth ○ Removes toxic materials from GI tract before they can be absorbed...