Chapter 18 Lecture Outline

Chapter 18 Lecture Outline
See separate PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

I. Introduction to the Digestive System

Introduction From food, humans must get basic organic molecules to make ATP, build tissues, and serve as cofactors and coenzymes. Digestion breaks polymers (carbohydrates, fats, and proteins) into monomer building blocks via hydrolysis reactions Absorption takes these monomers into the bloodstream to be used by the cells

Digestion of food through hydrolysis reactions
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Carbohydrate O O O O H H H H H H H H Carbohydrate- digesting enzymes + H2O + HO O OH HO OH HO OH Maltose + Water Glucose + Glucose Disaccharide + Water Monosaccharides Protein H R O H O H R O H R O H N C C N C C OH + H2O Protein- digesting enzyme H N C C OH + H N C C OH H H R H H Peptide (portion of protein molecule) + Water Amino acid + Amino acid Lipid O H C17H35C H OH HO C H C17H35COO C H Triglyceride- digesting enzyme O C17H35COO C H + 3H2O C17H35C + OH HO C H C17H35COO C H H O C17H35C OH HO C H H Fat + Water Fatty acids + Glycerol

Introduction, cont The digestive tract is open at both ends and is continuous with the environment Considered “outside” the body Materials that cannot be digested (cellulose) never actually “enter” the body. One-way transport allows for specialization of function along the tract

Digestive Tract Functions
Motility – movement of food through the tract Ingestion: taking food into the mouth ________: chewing and mixing food with saliva Deglutination: swallowing Peristalsis: wave-like, one-way movement through tract ____________: churning and mixing while moving forward

Digestive Tract Functions, cont
Secretion ________: digestive enzymes, hydrochloric acid, mucus, water, and bicarbonate Endocrine: hormones to regulate digestion Digestion - breaking food down into smaller units, both physically and chemically __________ - passing broken-down food into blood or lymph

Digestive Tract Functions, cont
Storage and elimination - temporary storage and subsequent elimination of undigested food molecules Immune barrier Simple columnar epithelium with tight junctions prevents swallowed pathogens from entering body. Immune cells in connective tissue of the tract promote immune responses

Digestive System Divisions
Gastrointestinal tract (GI tract, alimentary canal): ____ feet long, from mouth to anus Oral cavity  Pharynx  Esophagus  Stomach  Small intestines  Large intestines  Anus Accessory organs: teeth, tongue, salivary glands, liver, gallbladder, pancreas

Organs of the Digestive System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parotid gland Oral cavity Teeth Pharynx Tongue Submandibular gland Sublingual gland Esophagus Diaphragm Stomach Liver Common bile duct Gallbladder Pancreas Duodenum Transverse colon Ascending colon Descending colon Small intestine Sigmoid colon Cecum Rectum Appendix Anal canal Anus

GI Tract Layers Also called tunics There are four tunics:
Mucosa: inner secretory and absorptive layer; may be folded to increase surface area __________: very vascular, to pick up nutrients; also has some glands and nerve plexuses Muscularis: smooth muscle; responsible for peristalsis and segmentation; myenteric plexus for control by the ANS ________: outer binding and protective layer

Layers of the Digestive Tract
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Artery Vein Mesentery Serosa Plica circularis Mucosa Submucosa Longitudinal muscle (a) Circular muscle Serosa Outer longitudinal Myenteric plexus Muscularis externa Inner circular Submucosal plexus Gland in submucosa Muscularis mucosae Lymph nodule Intestinal crypt (of Lieberkühn) Lamina propria Lacteal (b) Villi Epithelium

Regulation of the GI Tract
Parasympathetic division (extrinsic regulation) Stimulates esophagus, stomach, small intestine, pancreas, gallbladder, and first part of large intestine via vagus nerve Spinal nerves in sacral region stimulate lower large intestine. Preganglionic neurons synapse on __________ and _________ plexuses.

Regulation of the GI Tract, cont
Sympathetic division (extrinsic regulation) Inhibits peristalsis and secretion Stimulates contraction of sphincters Hormones - from brain or other digestive organs

Regulation of the GI Tract, cont
Intrinsic regulation Intrinsic sensory neurons in gut wall help in intrinsic regulation via separate enteric nervous system Paracrine signals

II. From Mouth to Stomach

Mouth Mastication: Chewing breaks food down into smaller pieces for deglutition and mixes it with saliva. Saliva: contains mucus, an antimicrobial agent, and salivary amylase to start digestion of starch.

Deglutition Involves coordinated contraction of 25 pairs of muscles
Three parts: Oral: voluntary; muscles of mouth and tongue mix food with saliva to form a bolus. Pharyngeal: involuntary; initiated by receptors in the posterior oral cavity and oropharynx Uvula (soft palate) lifts to cover nasopharynx, and the epiglottis covers vocal cords. Upper esophageal sphincter relaxes.

Deglutition, cont __________: automatic; controlled by the swallowing center of brain stem; bolus is moved down esophagus to stomach via peristalsis

Peristalsis in the esophagus
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Peristaltic wave Esophagus Bolus Stomach

Esophagus About 10 inches long
Passes through the diaphragm via the esophageal hiatus Lined with nonkeratinized stratified squamous epithelium Mouth, pharynx, and upper esophagus lined with skeletal muscles innervated by somatic motor neurons Lower esophagus lined with smooth muscle controlled by autonomic nervous system Lower esophageal sphincter opens to allow food to pass into stomach. It stays closed to prevent regurgitation.

Stomach Functions Stores food
Churns food to mix with gastric secretions Begins protein digestion Kills bacteria in the food (acid) Moves food into small intestine in the form of chyme

Stomach Structure Regions
Food is delivered from the esophagus to the cardiac region. Upper region = fundus Lower region = body Distal region = pyloric region; ends at pyloric sphincter Lining has folds called _______.

Regions and Structure of the Stomach
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cardia Esophagus Fundus Adventitia Longitudinal muscle Body Circular muscle Pyloric sphincter Pyloric antrum Oblique muscle Duodenum Submucosa Pylorus Mucosa Gastric rugae

Stomach Structure, cont
Gastric pits at base of folds lead to gastric glands that contain several types of secretory cells: Mucus neck cells secrete mucus to help protect stomach lining from acid. ________ cells secrete HCl acid and intrinsic factor (helps small intestine absorb vitamin B12). _____ (zygomatic) cells secrete pepsinogen, the inactive form of a protein digesting enzyme

Stomach Structure, cont
Enterochromaffin-like (ECL) cells secrete histamine and serotonin (paracrine signals). G cells secrete _______ (hormone). D cells secrete somatostatin (hormone). Also secretes the hormone ______ that signals the brain to regulate hunger Serotonin activates system to increase intestinal motility (increased in diarrhea, decreased in constipation) SSRIs then cause diarrhea, nausea, nervousness due to improper levels of plasma 5-HT receptors Gastrin tells chief cells and parietal cells to release corresponding hormones Somatostatin suppresses release of GI hormones gastrin, CCK, secretin, motilin, etc. Ghrelin plays a role in meal initiation; induces appetite and feeding behaviors; causes increase in food intake in humans and rats injected with ghrelin

Gastric Pits & Gastric Glands
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gastric pits (a) Mucous cell Mucosa Gastric gland Parietal cell Submucosa Chief cell (b)

Pepsin and HCl Secretion
Formation of HCl Primary active transport of H+ via H+/K+ATPase pumps Facilitated diffusion of Cl− coupled to downhill movement of bicarbonate Parietal cells secrete Cl- as well as H+ into gastric juice while secreting bicarbonate into the blood

Secretion of gastric acid by parietal cells
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H+ Lumen of stomach Cl– Facilitative diffusion K+ Primary active transport (ATPase carrier) Parietal cell Cl– H+ K+ CO2 + H2O CA H+ H2CO3 HCO3– Cl– Secondary active transport Cl– HCO3– Blood capillaries

Stimulation of HCl Secretion
Gastrin: made in G cells; carried to parietal cells in blood; Also stimulates ECL cells to make histamine Histamine: also stimulates parietal cells via H2 histamine receptors Examples: Tagamet and Zantac block H2 receptors. Parasympathetic neurons and ACh: stimulate parietal and ECL cells

Functions of HCl Drops pH to 2
Ingested proteins are denatured (allows enzymes access). Pepsinogen is converted to active pepsin (digests proteins). Serves as the optimal pH for pepsin activity Function of ______ – catalyzes the hydrolysis of peptide bonds in the ingested proteins

Activation of pepsin Lumen of stomach Ingested protein Short peptides
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lumen of stomach Ingested protein Short peptides Pepsin Pepsinogen Pepsin HCI Gastric mucosa

Stomach Defenses Acid and pepsin could damage the stomach lining.
Defenses that help prevent this: Adherent layer of mucus with alkaline bicarbonate Tight junctions between epithelial cells Rapid epithelial mitosis that replaces epithelium every three days

Digestion and Absorption in the Stomach
Proteins begin digestion in the stomach. Starches begin digestion in the _____, but salivary amylase is not active at pH 2, so this activity stops in the stomach. Alcohol and NSAIDs (aspirin) are the only common substances absorbed in the stomach (due to high lipid solubility).

Peptic Ulcers Peptic ulcers: erosions of the mucosa of the stomach or duodenum produce by HCl Helicobacter pylori: bacterium that reduces mucosal barriers to acid Treatment for ulcers combines K+/H+ pump inhibitors (Prilosec) and antibiotics.

Acute Gastritis Inflammation of the submucosa caused by acid
Histamine released as part of the inflammatory response can stimulate more acid release. Prostaglandins are needed to stimulate protective alkaline mucus production. NSAIDs inhibit prostaglandin activity and can lead to gastritis. Tagamet and Zantac inhibit H2receptors.

Duodenal ulcers Protected by adherent layer of mucus with bicarbonate
Bicarbonate secreted by Brunner’s glands Acidic chyme neutralized by bicarbonate from the pancreas Zollinger-Ellison syndrome – ulcers due to high amounts of gastrin produced by a tumor

III. Small Intestine

Small Intestine Structure
Starts at the pyloric sphincter and ends at the ileocecal valve Three sections: ________ – first 10 inches ________ – middle 2/5 ______ – last 3/5 Mucosa and submucosa folded into plicae circulares; mucosa further folded into villi; and epithelial plasma membranes folded into microvilli; these greatly increase the surface area for absorption of nutrients

Small Intestine Structure
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Plica circulares Villi Duodenum Stomach Ascending colon Jejunum Mucosa Submucosa Mesentery Ileum Muscularis externa Cecum Appendix Plica circulares (a) Circular muscle Longitudinal muscle (b) Serosa

Small Intestine Functions
Complete digestion of carbohydrates, proteins, and fats Absorption of nutrients Sugars, lipids, amino acids, calcium, and iron absorbed in duodenum and jejunum Bile salts, vitamin B12, water, and electrolytes in ileum Very rapid due to villi and microvilli

Villi and Microvilli Villi
Columnar epithelium with goblet cells (mucus) Capillaries absorb monosaccharides and amino acids, and lacteals absorb fats. Intestinal crypts (Crypts of Lieberkuhn) with Paneth cells (secrete antibacterial molecules of lysozyme and defensin) and mitotic stem cells (divide by mitosis to replenish intestinal cells every 4 to 5 days) __________ (brush border) – folding of the apical surface of each epithelial cell

Structure of an intestinal villus
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Simple columnar epithelium Lacteal Villus Capillary network Goblet cells Intestinal crypt Lymph vessel Arteriole Venule

Intestinal Enzymes Called brush border enzymes
Not released into lumen, but stay attached to plasma membrane with active site exposed to chyme Hydrolyze disaccharides, polypeptides and other substrates to simple nutrient molecules

Location of Brush Border Enzymes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lumen of small intestine Enz Enz Enz Enz Enz Microvilli Epithelial cell of duodenum

Brush Border Enzymes

Intestinal Contractions and Motility
Peristalsis is weak. Movement of food is much slower due to pressure at pyloric end. Segmentation is stronger and serves to mix the chyme. Smooth muscle contractions occur automatically due to endogenous pacemaker activity Graded depolarizations called slow waves produced by pacemaker cells called interstitial cells of Cajal produce action potentials in muscle cells.

Segmentation of the small intestine
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Intestinal Contractions/Motility, cont
Depolarization from the interstitial cells of Cajal opens voltage-gated Ca2+ channels in muscle cells  action potential and contraction. Stimulation travels short distances through gap junctions but must be regenerated in neighboring pacemaker regions. Produces contractions needed for segmentation

Slow waves in the intestines
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slow waves –28 mV –62 5 sec

Regulation of Contraction
Autonomic nerves influence enteric nervous system to stimulate or inhibit cells of Cajal. Acetylcholine from parasympathetic system interacts with muscarinic ACh receptors to increase amplitude and duration of slow waves.

Regulation of Contraction
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Structures Functions Production of slow waves Interstitial cells of Cajal Conduction of slow waves to smooth muscle Depolarization and opening of Ca2+ channels, production of action potentials Smooth muscle cells Neural input to ICC and smooth muscle Autonomic axon

IV. Large Intestine

Large Intestine Structure
Regions Through the ileocecal valve into the Cecum Ascending colon  Transverse colon  Descending colon  Sigmoid colon  Rectum  Anal canal  Anus

The large intestine Transverse colon Ascending colon Descending colon
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Transverse colon Ascending colon Descending colon Ileocecal valve Ileum Cecum Haustrum Appendix Sigmoid colon Rectum Anal canal

Large intestine structure, cont
Mucosa – columnar epithelial cells with goblet cells, crypts, lymphatic nodules, but no villi Outer surface forms pouches called haustra

Large Intestine Function
Absorption of water, electrolytes, vitamin K, and some B vitamins Production of vitamin _______ vitamins via microbial organisms Storage of feces

Intestinal Microbiota
Several hundred different species of bacteria live in the large intestine; also called microflora Some are commensal. The bacteria benefit, and we aren’t harmed or benefitted. Others are mutualistic. Both bacteria and us benefit. Bacteria are mostly anaerobic. An infant receives its initial colonization of bacteria from its mother during birth.

Benefits from Microbes
Microbes make vitamin K and some B vitamins. They also make fatty acids from cellulose. Some of these are used for energy by large intestine epithelial cells. We can’t absorb the fatty acids, but they help absorb electrolytes such as sodium, calcium, bicarbonate, magnesium, and iron. They outcompete harmful species of bacteria. Disruption of normal microflora can lead to inflammatory bowel disease.

Protection from intestinal bacteria
Mucus from goblet cells prevents contact with epithelial cells. Secretion of _______ by Paneth cells in the crypts. Secretion of IgA antibodies by plasma cells Defensin active against bacteria, fungi

Intestinal Microbiota, cont
Normal intestinal microbiota helps maintain a healthy epithelial barrier, limits inflammation, and promotes repair of damaged epithelium. Commensal bacteria promote production of regulatory T lymphocytes rather than killer T cells Intestinal dendritic cells are antigen-presenting cells to activate T cells.

Fluid and electrolyte absorption
Most absorption occurs in small intestine, but some is left for large intestine. Not all water is absorbed; about 200 ml is left per day to be excreted with feces. Water is absorbed passively following an osmotic gradient set up by active Na+/K+pumps. ________ stimulates greater salt and water absorption here. Some minor secretion of water occurs by osmosis.

Defecation As material passes to the rectum, pressure there increases, the internal anal sphincter relaxes, and the need to defecate rises. The external anal sphincter controls defecation voluntarily. During defecation, longitudinal rectal muscles contract to increase pressure as the anal sphincters relax. Aided by contraction of abdominal and pelvic skeletal muscles – Valsalva’s maneuver

V. Liver, Gallbladder, and Pancreas

Liver Structure Largest abdominal organ; immediately beneath the diaphragm, mostly on the right side Has amazing regenerative abilities due to mitosis of hepatocytes Composed of hepatocytes that form hepatic plates separated by capillaries called sinusoids Capillaries have fenestrae with no diaphragm or basement membrane Very permeable, allowing passage of blood proteins, fat, and cholesterol

Structure, cont Kupffer cells are also in the sinusoids.
Hepatic damage due to alcohol or viral hepatitis causes liver fibrosis that can lead to cirrhosis of the liver.

Microscopic structure of the liver
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Hepatic sinusoids Central vein Hepatic plate Bile ductule Branch of hepatic portal vein Branch of hepatic artery Portal triad Bile canaliculi Bile ductule

Hepatic Portal System Products of digestion absorbed in intestines are delivered to the liver via the hepatic portal vein. Veins from the pancreas, gallbladder, stomach, omentum, and spleen also join with the hepatic portal vein. After circulating through liver capillaries, the blood leaves via the hepatic vein to join regular venous circulation

Hepatic Portal System, cont
_____________– a unique pattern of capillaries  veins  capillaries  veins Total hepatic blood flow is about 25% of cardiac output; needed to maintain hepatic clearance.

Liver Lobules Hepatic plates are arranged as liver lobules with hepatic arteries, hepatic portal veins, and a central vein. Hepatic artery and hepatic portal vein open into the sinusoids between the plates Bile secreted by the hepatocytes is released into bile canaliculi, which drain into bile ducts, then to hepatic ducts away from the liver.

Flow of blood and bile in a liver lobule
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Branch of portal vein Bile ductule Bile canaliculus Sinusoids Branch of hepatic artery Hepatic plate Central vein

Enterohepatic circulation
Aside from bile, the liver secretes other substances into the bile ducts to clear them from the blood. Some of the molecules released into the bile are absorbed again in the small intestine and returned to the liver. These molecules are part of enterohepatic circulation; analogous to renal clearance. Eventually molecules are excreted in feces.

Enterohepatic Circulation
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Liver Hepatic portal vein Gallbladder Common bile duct Small intestine

Compounds excreted by the liver into bile

Liver Functions

Bile production and secretion
The liver makes __________ ml of bile per day. Bile is composed of: Bile pigment (_______) Bile salts Phospholipids (lecithin) ____________ Inorganic ions

Bilirubin Produced in ______, liver, and bone marrow
Derived from _____ (minus iron) from hemoglobin Not water-soluble Carried attached to _______ in the blood Not directly filtered by kidneys or secreted into bile Some free bilirubin is conjugated with glucuronic acid to make it water- soluble

Bilirubin, cont Conjugated bilirubin is secreted into the bile, where it is taken to the small intestine. ________ there turn it into urobilinogen, which makes feces brown. 30−50% is absorbed by the intestines and taken back to the liver. Some is used to make bile, and some remains in blood to be filtered by the kidneys where it gives an amber color to the urine.

Pathway for the metabolism of heme & bilirubin
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Red Blood Cells Hemolysis in spleen, liver, bone marrow Hemoglobin Heme Fe2+ Recycled to bone marrow Carbon monoxide (CO) Biliverdin NADPH Hemoglobin NADP Bilirubin Carboxyhemoglobin O2 Oxyhemoglobin Liver Carbon monoxide eliminated in exhaled breath Conjugated bilirubin (bilirubin glucuronide) Bilirubin eliminated in bile excreted in feces

Enterohepatic circulation of urobilinogen
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. General circulation Kidney Vena cava Liver Hepatic portal vein Common bile duct Urobilinogen in urine Gallbladder Small intestine Bacteria Bilirubin Urobilinogen Urobilinogen in feces

Bile Salts Made from bile acids conjugated with glycine or taurine
Bile acids: derived from __________ Four polar groups on each molecule Cholic acid and chenodeoxycholic acid Most is recycled in enterohepatic circulation. ½ gram of cholesterol is broken down and lost in the feces through this pathway.

Bile Salts, cont Form ________ with polar groups toward water and nonpolar groups inward toward the fat Fats enter the micelle and are emulsified. Provides a greater surface area for fat digestion by lipase

Bile acids form micelles
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. OH CH3 CH CH2 CH2 COOH HO OH (a) Cholic acid (a bile acid) Polar Nonpolar (b) Simplified representation of bile acid (c) Micelle of bile acids

Detoxification of Blood
The liver can remove hormones, drugs, and other substances in three ways: Excreted into _____ Phagocytized by ________ lining sinusoids Chemically altered by hepatocytes ________ is converted into urea, porphyrins into bilirubin, and purines into uric acid Urea is returned to the blood to be filtered by the kidneys. Steroids and xenobiotics are altered and then secreted into bile.

Secretion of Glucose, triglycerides, and ketone bodies
The liver helps balance blood glucose levels by removing glucose and storing it as ________ (glycogenesis) and triglycerides (________) or by breaking down glycogen (glycogenolysis) and releasing it into the blood. The liver can also make glucose from amino acids (_____________) and convert fatty acids into ketones (ketogenesis).

Production of plasma proteins
Plasma albumin and most plasma globulins, clotting factors and angiotensinogen are produced by the liver. Albumin provides colloid osmotic pressure and transport Globulins provide transport and blood clotting Clotting factors I, II, III, V, VII, IX, XI

Gallbladder Sac-like organ attached to the inferior surface of the liver Stores and concentrates bile from the liver: Liver  Bile ducts  Hepatic duct  Cystic duct  Gallbladder  Common bile duct  Sphincter of Ampulla or Oddi into the duodenum

Pancreatic juice and bile secretion into the duodenum
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Hepatic ducts Pancreatic acinus Cystic duct Common bile duct Pancreatic juice Pancreatic islet (of Langerhans) Gallbladder Pancreas Pancreatic duct Duodenal papilla Duodenum

a: © Dr. Sheril Burton; b: © Martin/Custom Medical Stock Photo
Gallstones Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (a) (b) a: © Dr. Sheril Burton; b: © Martin/Custom Medical Stock Photo

Pancreas Has endocrine and exocrine functions
Endocrine: Islets of Langerhans cells (Pancreatic Islets) make insulin and glucagon. Exocrine: Acinar cells make pancreatic juice, which is delivered to the duodenum via the pancreatic duct.

Pancreas Structure Zymogen granules Acinar cells Endocrine portion
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Zymogen granules Acinar cells Endocrine portion Exocrine portion Pancreatic islet (of Langerhans) Pancreatic acini Inactive enzymes Bicarbonate To pancreatic duct and duodenum (b) Duct Acinus (a) a: © Ed Reschke

Pancreatic Juice Made up of bicarbonate + 20 digestive enzymes
Enzymes for all three classes of macromolecules

Bicarbonate formation
Made by cells lining the ductules Made from CO2 from the blood First, carbonic acid is made. This dissociates to form H+ and bicarbonate. The bicarbonate is secreted into pancreatic juice, and H+ goes back into the blood. Bicarbonate is countertransported with Cl−. People with cystic fibrosis have trouble secreting bicarbonate because of defective chloride carriers, which can lead to destruction of the pancreas.

Formation and Secretion of Bicarbonate
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lumen Blood K+ K+ K+ HCO3– ATP Na+ Na+ HCO3– CI– Na+ HCO3– CI– CI– H2CO3 CO2 Na+ H+ H+

Pancreatic Enzymes Most are inactive (zymogens) until they reach the small intestine. Enterokinase activates trypsinogen  trypsin (to digest protein). Trypsin activates other enzymes.

Activation of Pancreatic Enzymes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Other inactive enzymes (zymogens) Active enzymes Pancreatic juice Trypsin En En En En En Microvilli Epithelial cell of duodenum

VI. Regulation of the Digestive System

Introduction Neural and endocrine control mechanisms
Modifies GI tract functioning Sight, smell, taste, or thought of food can stimulate salivation and gastric secretions to “prime” the digestive tract for food. Stimulation goes from brain to organ via vagus nerve; a conditioned reflex Short reflexes – do not involve the brain GI tract produces some hormones and is the target for action

Effects of GI hormones

Regulation of gastric function
Motility and secretion are somewhat automatic. Contractions are stimulated spontaneously by pacesetter cells in greater curvature of stomach (intrinsic regulation). Secretion of HCl and pepsinogen occurs when proteins enter the stomach. Initiated and regulated by G cells (gastrin), D cells (somatostatin), and ECL cells (histamine)

Three phases of extrinsic gastric regulation
Cephalic phase: control by brain via vagus nerves Stimulates ECL (major response), chief cells, and parietal cells Lasts for the first 30 minutes of a meal

Three phases of extrinsic gastric regulation, cont
Gastric phase: triggered by arrival of food into stomach Gastric secretion is stimulated by stomach distension (amount of food that enters) and the chemical nature of the chyme. Positive feedback occurs; as more proteins are broken down, more secretions are released to break them down (glucose has no effect and fat inhibits gastric secretion).

Gastric phase, cont There is also a negative-feedback system. As pH drops (due to more HCl), somatostatin is released. This inhibits gastrin secretion. Large amounts of proteins and polypeptides buffer pH, so secretion matches protein concentration.

Regulation of gastric secretion
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gastric epithelium Vagus nerve Amino acids + + Gastrin – Stomach lumen G cell Systemic circulation + ECL cell + Histamine + HCl Parietal cell

Three phases of extrinsic gastric regulation, cont
________________: inhibition of gastric activity when chyme enters the small intestine Slows the movement of chyme into the duodenum to allow enough time for digestion and absorption Stretch when food enters the duodenum stimulates a neural reflex that inhibits gastric stimulation via the vagus nerve. The presence of fats stimulates the duodenum to make enterogastrone which inhibits gastric function.

Enterogastric hormones
GIP – glucose-dependent insulinotropic peptide – stimulates insulin secretion and inhibits gastric function Somatostatin CCK – cholecystokinin – secreted by the duodenum in response to chyme GLP-1 – glucagon-like peptide-1 – secreted by the ileum – insulin stimulator

Three Phases of Gastric Secretion

Regulation of Intestinal Function
Enteric nervous system (ENS): neurons and glial cells that innervate the intestines Includes myenteric plexus and submucosal plexus Acts independently from CNS but with some feedback to CNS via vagus nerve (extrinsic afferents) Involved in regulation by the ANS Innervates interstitial cells of Cajal and smooth muscle to intrinsically regulate peristalsis

Enteric Nervous System Coordinates Peristalsis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2. Motor neurons stimulate contraction behind bolus Excitation Smooth muscle + ACh Interneurons activate motor neurons Contraction Substance P ACh 1. Bolus of food stimulates sensory neurons Distention Interneurons activate motor neurons Mucosal stimulus Inhibition 2. Motor neurons produce relaxation in front of bolus Bolus ACh Relaxation – NO Motor neurons VIP Interneurons Sensory Lumen Mucosa and submucosa Circular muscle Longitudinal muscle

Paracrine Regulation Enterochromaffin-like cells in intestinal mucosa secrete serotonin and motilin in response to pressure (filling) and chemicals in the food. This stimulates muscle contractions. ________: made in ileum and colon; stimulates the secretion of water and Cl− and inhibits absorption of Na+. More water and salt are lost in feces. Acts by activating guanylate cyclase to produce second messenger cGMP

Intestinal reflexes Gastroileal reflex: increased gastric activity, increased ileum activity and movement of food through ileocecal valve ___________ reflex: distension of ileum causes a decrease in gastric motility Intestino-intestinal reflex: Over-distension of one portion of the intestine causes relaxation of other portions.

Regulation of pancreatic juice and bile secretion
When chyme enters the duodenum, two hormones are produced: Secretin is produced in response to a drop in pH. (Production stops with a rise in pH.) Cholecystokinin (CCK) is produced in response to the presence of partially digested proteins and fats in chyme. (Production stops when food leaves small intestine.)

Secretion of Pancreatic Juice
Pancreatic enzyme production of trypsin, lipase, and amylase is stimulated by ACh from vagus nerve, CCK, and secretin. ACh and CCK use Ca2+ as a second messenger. Secretin uses cAMP as a second messenger. Bicarbonate production is stimulated by secretin.

Secretion of Bile The liver produces bile continuously, but the arrival of food into the duodenum stimulates increased production of bile. Happens when: Bile acids are returned to the liver after intestinal absorption via enterohepatic circulation. Secretin and CCK stimulate increased bicarbonate secretion into bile. CCK (in response to the presence of fat in chyme) stimulates gallbladder contraction.

Trophic effects of GI hormones
GI hormones have a trophic effect for maintenance of their target organs Structure of the gastric mucosa is dependent on gastrin Structure of the acinar cells of the pancreas depends on CCK

VII. Digestion and Absorption of Food

Digestion & Absorption of Carbohydrates
Most carbohydrates are ingested as starch or sugars such as sucrose and lactose Starch digestion begins in mouth with salivary amylase; polysaccharides  shorter chains No digestion in the stomach – too acidic Continues in intestines with pancreatic amylase; short chains  disaccharides & maltriose Brush border enzymes finish breaking down disaccharides (maltose, sucrose, lactose) to simple sugars (mainly glucose)

Action of pancreatic amylase
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Short oligosaccharide Glucose Amylase Maltriose Starch Maltose

Absorption of Carbohydrates
Monosaccharides are absorbed across the epithelium via: Secondary active transport with sodium (Cl- follows) Facilitated diffusion through GLUT carriers into interstitial fluid and then to capillary blood of the villus Water follows the NaCl through a paracellular route and is absorbed with glucose and NaCl

Digestion & Absorption of Proteins
Digestion of proteins Begins in ______ with pepsin and hydrochloric acid to produce short-chain polypeptides Finishes in duodenum and jejunum with pancreatic trypsin, chymotrypsin, elastase, and carboxypeptidase, and the brush border enzyme aminopeptidase. Final products are amino acids, some dipeptides and tripeptides

Absorption of Proteins
Free amino acids cotransported with Na+ Dipeptides and tripeptides cross via secondary active transport using a H+ gradient. Hydrolyzed into free amino acids within the cytoplasm of the epithelial cells Free amino acids move by facilitated diffusion into interstitial fluid, then to the blood capillaries of the villi

Digestions & Absorption of Proteins
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lumen Polypeptide chain Exopeptidase Endopeptidase Amino acids Brush border enzymes (microvilli) Tripeptidase Dipeptidase Epithelial cell Capillary blood

Digestion & Absorption of Fats
Digestion of fats Fat digestion begins in ________ when bile emulsifies the fat and the pancreatic enzyme lipase, aided by colipase, breaks it down into fatty acids and glycerol Phospholipase A (from pancreas) digests phospholipids into fatty acids and lysolecithin.

Digestion of Fats H O H C O C O H C O C Triglyceride O H C O C H
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H O H C O C O H C O C Triglyceride O H C O C H Glycerol Fatty acids Lipase + 2 HOH H O H C OH C O HO C O C + O H C OH C HO H Monoglyceride Free fatty acids

Fat Emulsification and Digestion
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Step 1: Emulsification of fat droplets by bile salts From liver and gallbladder Step 2: Hydrolysis of triglycerides in emulsified fat droplets into fatty acid and monoglycerides Bile duct Micelles of bile salts, cholesterol, and lecithin Step 3: Dissolving of fatty acids and monoglycerides into micelles to produce “mixed micelles” 1 2 3 Free fatty acids From stomach Into micelles + Lipase Monoglycerides Fat droplets (triglycerides) Emulsified fat droplets (triglycerides)

Absorption of Fats Fatty acids, monoglycerides, and lysolecithin move into bile ________ and are transported to brush border. The fat molecules then leave the micelles and diffuse into the epithelial cells of the villi. Inside the epithelial cells, they are regenerated into triglycerides, cholesterol, and phospholipids and combined with proteins to form ____________. Chylomicrons are secreted by exocytosis into the central lacteal of the villus

Fat Absorption Mixed Micelles with Lumen of small intestine
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mixed Micelles with Lumen of small intestine Triglycerides Bile salts, lipase Fatty acids Monoglycerides Fatty acids Monoglycerides Triglycerides + Protein Chylomicrons Lacteal To thoracic duct

Transport of Lipids in Blood
The lymphatic system drops chylomicrons into the bloodstream at the thoracic duct. They pick up an apolipoprotein (ApoE), which allows them to bind to receptors on the capillary endothelium within muscles and adipose tissue. Here they are digested by lipoprotein lipase, which releases free fatty acids for use by muscle cells or for storage by fat cells. The remaining remnant particle containing cholesterol is released and travels in the blood to the liver.

Transport of Lipids in Blood, cont
Cholesterol and triglycerides made in the liver are combined with other apolipoproteins to form very-low-density lipoproteins (VLDLs) to deliver triglycerides to organs. Once triglycerides are removed, they are low- density lipoproteins (LDLs), which transport cholesterol to organs. Excess cholesterol is returned to the liver on high-density lipids (HDL).

Transport of Lipids in Blood
HDL particles bind to receptors in the blood vessel walls and capture phospholipids and free cholesterol, reducing cholesterol amounts. Once the HDL is full, it travels to the liver and unloads the cholesterol.

Lipid Carrier Proteins

Characteristics of Major Digestive Enzymes

Chapter 18 Lecture Outline

Similar presentations

Presentation on theme: "Chapter 18 Lecture Outline"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Chapter 18 Lecture Outline

Similar presentations

Presentation on theme: "Chapter 18 Lecture Outline"— Presentation transcript:

Similar presentations

About project

Feedback