Chapter 23B Digestive System Slides by Barbara Heard and W. Rose.

Chapter 23B Digestive System Slides by Barbara Heard and W. Rose.
HESC310 4/16/2017 Chapter 23B Digestive System Slides by Barbara Heard and W. Rose. figures from Marieb & Hoehn 9th ed. Portions copyright Pearson Education Axial Skeleton

Digestive System Introduction/Overview Functional Anatomy
HESC310 4/16/2017 Digestive System Introduction/Overview Functional Anatomy Mouth, Pharynx, Esophagus Digestive Processes in the Mouth Stomach Small Intestine & Related Organs Large Intestine Physiology of Digestion & Absorption Axial Skeleton

© 2013 Pearson Education, Inc.
Mouth Oral (buccal) cavity Bounded by lips, cheeks, palate, and tongue Lined with stratified squamous epithelium Functions Ingestion Mechanical digestion Chemical digestion Propulsion © 2013 Pearson Education, Inc.

Sagittal section of the oral cavity and pharynx
Figure 23.7a Anatomy of the oral cavity (mouth). Soft palate Palatoglossal arch Uvula Hard palate Oral cavity Palatine tonsil Tongue Oropharynx Lingual tonsil Epiglottis Hyoid bone Laryngopharynx Esophagus Trachea Sagittal section of the oral cavity and pharynx © 2013 Pearson Education, Inc.

Upper lip Gingivae (gums) Superior labial frenulum Palatine raphe
Figure 23.7b Anatomy of the oral cavity (mouth). Upper lip Gingivae (gums) Superior labial frenulum Palatine raphe Palatoglossal arch Hard palate Palatopharyngeal arch Soft palate Uvula Palatine tonsil Posterior wall of oropharynx Tongue Sublingual fold with openings of sublingual ducts Lingual frenulum Opening of Submandibular duct Gingivae (gums) Oral vestibule Inferior labial frenulum Lower lip Anterior view © 2013 Pearson Education, Inc.

Tongue Skeletal muscle Functions include Repositioning and mixing food during chewing Formation of bolus Initiation of swallowing, speech, and taste Intrinsic muscles change shape of tongue Extrinsic muscles alter tongue's position Lingual frenulum: attachment to floor of mouth © 2013 Pearson Education, Inc.

Epiglottis Palatopharyngeal arch Palatine tonsil Lingual tonsil
Figure Dorsal surface of the tongue, and the tonsils. Epiglottis Palatopharyngeal arch Palatine tonsil Lingual tonsil Palatoglossal arch Terminal sulcus Foliate papillae Vallate papilla Medial sulcus of the tongue Dorsum of tongue Fungiform papilla Filiform papilla © 2013 Pearson Education, Inc.

Salivary Glands Lie outside oral cavity Parotid Submandibular Sublingual Functions of saliva Cleanses mouth Dissolves food chemicals for taste Moistens food; compacts into bolus Begins breakdown of starch with enzymes © 2013 Pearson Education, Inc.

Figure 23.9 The salivary glands.
Tongue Teeth Parotid gland Ducts of sublingual gland Parotid duct Masseter muscle Frenulum of tongue Body of mandible (cut) Sublingual gland Posterior belly of digastric muscle Mylohyoid muscle (cut) Submandibular duct Anterior belly of digastric muscle Submandibular gland Mucous cells Serous cells forming demilunes © 2013 Pearson Education, Inc.

Two types of secretory cells
Salivary Glands Two types of secretory cells Serous cells Watery, enzymes, ions, bit of mucin Mucous cells Mucus Parotid, submandibular glands mostly serous; sublingual mostly mucous © 2013 Pearson Education, Inc.

97–99.5% water, slightly acidic
Composition of Saliva 97–99.5% water, slightly acidic Electrolytes—Na+, K+, Cl–, PO4 2–, HCO3– Salivary amylase and lingual lipase Mucin Metabolic wastes—urea and uric acid Lysozyme, IgA, defensins, and a cyanide compound protect against microorganisms PLAY Animation: Rotating head © 2013 Pearson Education, Inc.

Control of Salivation 1500 ml/day
Intrinsic glands continuously keep mouth moist Major salivary glands activated by parasympathetic nervous system when Ingested food stimulates chemoreceptors and mechanoreceptors in mouth  Salivatory nuclei in brain stem send impulses along parasympathetic fibers in cranial nerves VII and IX Strong sympathetic stimulation inhibits salivation and results in dry mouth (xerostomia) © 2013 Pearson Education, Inc.

Figure 23.10 Human dentition.
Incisors Central (6–8 mo) Lateral (8–10 mo) Canine (eyetooth) (16–20 mo) Molars First molar (10–15 mo) Deciduous (milk) teeth Second molar (about 2 yr) Incisors Central (7 yr) Lateral (8 yr) Canine (eyetooth) (11 yr) Premolars (bicuspids) First premolar (11 yr) Second premolar (12–13 yr) Molars First molar (6–7 yr) Second molar (12–13 yr) Third molar (wisdom tooth) (17–25 yr) Permanent teeth © 2013 Pearson Education, Inc.

Number and types of Teeth
Primary: Permanent: © 2013 Pearson Education, Inc.

Enamel Dentin Cement Root canal Bone
Figure Longitudinal section of a canine tooth within its bony socket (alveolus). Enamel Dentin Crown Dentinal tubules Pulp cavity (contains blood vessels and nerves) Neck Gingival sulcus Gingiva (gum) Cement Root canal Root Periodontal ligament Apical foramen Bone © 2013 Pearson Education, Inc.

College or Department name here
Tooth & gum disease Cavities (dental caries) due to destruction of enamel & dentin by lactic acid made by bacteria digesting sugar in a film on tooth surface. Periodontal (gum) disease: affects up to 95% over age 35, accounts for 80-90% of adult tooth loss. Due to bacterial infection btn tooth & gum, preventable by flossing & brushing. College or Department name here

Pharynx = throat, passes air and food via sequential contraction of muscles College or Department name here

Esophagus Carries food from throat to stomach, collapsed when empty Diaphragm and esophageal sphincter (and gravity) keep food in stomach Heartburn: often due to "gastroesophageal (GE) reflux" (stomach contents spurting up into esophagus), which can be caused by hiatal hernia (top of stomach protrudes above diaphragm) College or Department name here

Involves tongue, soft palate, pharynx, esophagus
Deglutition Involves tongue, soft palate, pharynx, esophagus Requires coordination of 22 muscle groups Buccal phase Voluntary contraction of tongue Pharyngeal-esophageal phase Involuntary – primarily vagus nerve Control center in the medulla and lower pons © 2013 Pearson Education, Inc.

Figure 23.13 Deglutition (swallowing)
Bolus of food Tongue Uvula Pharynx Bolus Epiglottis Epiglottis Glottis Trachea Upper esophageal sphincter Bolus Esophagus During the buccal phase, the upper esophageal sphincter is contracted. The tongue presses against the hard palate, forcing the food bolus into the oropharynx. The constrictor muscles of the pharynx contract, forcing food into the esophagus inferiorly. The upper esophageal sphincter contracts (closes) after food enters. 1 The pharyngeal-esophageal phase begins as the uvula and larynx rise to prevent food from entering respiratory passageways. The tongue blocks off the mouth. The upper esophageal sphincter relaxes, allowing food to enter the esophagus. 2 3 Relaxed muscles The gastroesophageal sphincter surrounding the cardial oriface opens, and food enters the stomach. 5 Relaxed muscles Circular muscles contract Peristalsis moves food through the esophagus to the stomach. 4 Bolus of food Longitudinal muscles contract Circular muscles contract Gastroesophageal sphincter closed Gastroesophageal sphincter opens Stomach

Stomach: Gross Anatomy
Cardia Fundus Body Pyloric region Pyloric sphincter (valve) © 2013 Pearson Education, Inc.

Cardia Fundus Esophagus Muscularis externa Serosa Longitudinal layer
Figure 23.14a Anatomy of the stomach. Cardia Fundus Esophagus Muscularis externa Serosa Longitudinal layer Circular layer Oblique layer Body Lumen Lesser curvature Rugae of mucosa Greater curvature Pyloric sphincter (valve) at pylorus Pyloric canal Pyloric antrum Duodenum © 2013 Pearson Education, Inc.

Liver Gallbladder Lesser omentum Stomach Duodenum Transverse colon
Figure 23.30b Mesenteries of the abdominal digestive organs. Liver Gallbladder Lesser omentum Stomach Duodenum Transverse colon Small intestine Cecum Urinary bladder © 2013 Pearson Education, Inc.

Greater omentum Transverse colon Transverse mesocolon Descending colon
Figure 23.30c Mesenteries of the abdominal digestive organs. Greater omentum Transverse colon Transverse mesocolon Descending colon Jejunum Mesentery Sigmoid mesocolon Sigmoid colon Ileum © 2013 Pearson Education, Inc.

Layers of the stomach wall
Figure 23.15a Microscopic anatomy of the stomach. Surface epithelium Mucosa Lamina propria Muscularis mucosae Submucosa (contains submucosal plexus) Oblique layer Muscularis externa (contains myenteric plexus) Circular layer Longitudinal layer Serosa Stomach wall Layers of the stomach wall © 2013 Pearson Education, Inc.

Enlarged view of gastric pits and gastric glands
Figure 23.15b Microscopic anatomy of the stomach. Gastric pits Surface epithelium (mucous cells) Gastric pit Mucous neck cells Parietal cell Gastric gland Chief cell Enteroendocrine cell Enlarged view of gastric pits and gastric glands © 2013 Pearson Education, Inc.

Location of the HCl-producing parietal cells
Figure 23.15c Microscopic anatomy of the stomach. Pepsinogen Pepsin HCI Mitochondria Parietal cell Chief cell Enteroendocrine cell Location of the HCl-producing parietal cells and pepsin-secreting chief cells in a gastric gland © 2013 Pearson Education, Inc.

Gastric Gland Secretions
Parietal cells Hydrochloric acid (HCl): pH 1.5–3.5 denatures protein, activates pepsin, breaks down plant cell walls, kills bacteria Intrinsic factor Glycoprotein required for vitamin B12 absorption Chief cells Pepsinogen: Inactive protease; activated to pepsin by HCl and by pepsin itself (positive feedback) Lipases Enteroendocrine cells Serotonin1, histamine1, somatostatin1,2, gastrin2 1. Paracrine (acts locally); 2. Hormone © 2013 Pearson Education, Inc.

Harsh digestive conditions in stomach Has mucosal barrier to protect
Thick layer of bicarbonate-rich mucus Tight junctions between epithelial cells Prevent juice seeping underneath tissue Damaged epithelial cells quickly replaced by division of stem cells Surface cells replaced every 3–6 days © 2013 Pearson Education, Inc.

Homeostatic Imbalance
Gastritis Inflammation caused by anything that breaches mucosal barrier Peptic or gastric ulcers Erosions of stomach wall Can perforate  peritonitis; hemorrhage Most caused by Helicobacter pylori bacteria Some by NSAIDs © 2013 Pearson Education, Inc.

Bacteria Mucosa layer of stomach A gastric ulcer lesion
Figure Photographs of a gastric ulcer and the H. pylori bacteria that most commonly cause it. Bacteria Mucosa layer of stomach A gastric ulcer lesion H. pylori bacteria © 2013 Pearson Education, Inc.

Digestive Processes in the Stomach
Mechanical breakdown Chemical digestion Denaturation of proteins by HCl Enzymatic digestion of proteins by pepsin Propulsion: delivers chyme to small intestine Absorption: alcohol, aspirin, secretion of intrinsic factor for B12 absorption in S.I. © 2013 Pearson Education, Inc.

Neural & Hormonal Regulation of Gastric Secretion
Vagus nerve activity  secretion  Sympathetic activity  secretion  Hormonal Gastrin Gastrin  ⇒ enzyme and HCl secretion  Most small intestine secretions are gastrin antagonists © 2013 Pearson Education, Inc.

Phases of Gastric Secretion
Cephalic (reflex) phase: conditioned reflex triggered by aroma, taste, sight, thought Gastric phase: 3–4 hours Stimulated by gastrin (from G cells), distension, peptides, low acidity Intestinal phase Chyme entering S.I. is initially and briefly stimulatory to stomach Later, chyme in SI inhibits stomach secretion: enterogastric reflex Too much chyme entering SI ⇒ dumping syndrome (nausea, vomiting), common after gastric reduction surgery © 2013 Pearson Education, Inc.

Figure 23.17 Neural and hormonal regulation of gastric secretion
Stimulatory events Inhibitory events Cephalic phase Sight and thought of food 1 Cerebral cortex Lack of stimulatory impulses to parasympathetic center Cerebral cortex Loss of appetite, depression 1 Conditioned reflex Stimulation of taste and smell receptors 2 Hypothalamus and medulla oblongata Vagus nerve Stomach distension activates stretch receptors 1 Vagovagal reflexes Medulla Vagus nerve Gastrin secretion declines G cells Excessive acidity (pH < 2) in stomach 1 Gastric phase Local reflexes Overrides parasympathetic controls Sympathetic nervous system activation Emotional stress 2 Food chemicals (especially peptides and caffeine) and rising pH activate chemoreceptors 2 G cells Gastrin release to blood Stomach secretory activity Entero- gastric reflex Local reflexes Distension of duodenum; presence of fatty, acidic, or hypertonic chyme; and/or irritants in the duodenum 1 Presence of partially digested foods in duodenum or distension of the duodenum when stomach begins to empty 1 Intestinal (enteric) gastrin release to blood Vagal nuclei in medulla Brief effect Intestinal phase Pyloric sphincter Release of enterogastrones (secretin, cholecystokinin, vasoactive intestinal peptide) Distension; presence of fatty, acidic, partially digested food in the duodenum 2 Stimulate Inhibit

Response of the Stomach to Filling
Stretches to accommodate incoming food Pressure constant until 1.5 L food ingested Reflex-mediated receptive relaxation Coordinated by swallowing center of brain stem Gastric accommodation Plasticity (stress-relaxation response) of smooth muscle (see Chapter 9) © 2013 Pearson Education, Inc.

Gastric Contractile Activity
Peristaltic waves move toward pylorus at rate of 3 per minute Distension and gastrin increase force of contraction Most vigorous near pylorus © 2013 Pearson Education, Inc.

Pyloric Pyloric Pyloric valve valve valve slightly closed closed
Figure Deglutition (swallowing). Slide 1 Pyloric valve slightly opened Pyloric valve closed Pyloric valve closed Propulsion: Peristaltic waves move from the fundus toward the pylorus. 1 Grinding: The most vigorous peristalsis and mixing action occur close to the pylorus. 2 Retropulsion: The pyloric end of the stomach acts as a pump that delivers small amounts of chyme into the duodenum, simultaneously forcing most of its contained material backward into the stomach. 3 © 2013 Pearson Education, Inc.

Small Intestine: Gross Anatomy
Major organ of digestion and absorption 2-4 m long; from pyloric sphincter to ileocecal valve Subdivisions Duodenum (retroperitoneal) Jejunum (attached posteriorly by mesentery) Ileum (attached posteriorly by mesentery) © 2013 Pearson Education, Inc.

Mouth (oral cavity) Parotid gland Sublingual gland Salivary Tongue*
Figure Alimentary canal and related accessory digestive organs. Mouth (oral cavity) Parotid gland Sublingual gland Salivary glands* Tongue* Submandibular gland Pharynx Esophagus Stomach Pancreas* (Spleen) Liver* Gallbladder* Transverse colon Duodenum Descending colon Small intestine Jejunum Ascending colon Ileum Cecum Large intestine Sigmoid colon Rectum Appendix Anus Anal canal © 2013 Pearson Education, Inc.

Bile duct and sphincter Accessory pancreatic duct
Figure The duodenum of the small intestine, and related organs. Right and left hepatic ducts of liver Common hepatic duct Bile duct and sphincter Accessory pancreatic duct Tail of pancreas Pancreas Jejunum Main pancreatic duct and sphincter Head of pancreas Hepatopancreatic ampulla and sphincter Duodenum Mucosa with folds Gallbladder Major duodenal papilla Cystic duct © 2013 Pearson Education, Inc.

Small Intestine Duodenum Jejunum Ileum
0.25 m long; receives bile & pancreatic enzymes; bile duct, main pancreatic duct join at hepatopancreatic ampulla, enter duodenum at major duodenal papilla, controlled by hepatopancreatic sphincter Jejunum 2.5 m long Ileum 3.6 m long; joins large intestine at ileocecal valve © 2013 Pearson Education, Inc.

Small Intestine Innervated by vagus nerve (parasympathetic) and sympathetics from thoracic splanchnic nerves Blood supply: Superior mesenteric artery Portal circulation Capillaries to portal vein to more capillaries: Nutrient-rich blood from small intestine caps goes by hepatic portal vein to liver & liver capillaries © 2013 Pearson Education, Inc.

Structural Features to
Increase Surface Area Circular folds Permanent folds (~1 cm deep) force chyme to slowly spiral through lumen, increases nutrient absorption Villi Extensions (~1 mm high) of mucosa with capillary bed and lacteal for absorption Microvilli (brush border) Contain enzymes for carbohydrate and protein digestion © 2013 Pearson Education, Inc.

Vein carrying blood to hepatic portal vessel Muscle layers Lumen
Figure 23.22a Structural modifications of the small intestine that increase its surface area for digestion and absorption. Vein carrying blood to hepatic portal vessel Muscle layers Lumen Circular folds Villi © 2013 Pearson Education, Inc.

Villus Venule Lymphatic vessel Submucosa
Figure 23.22b Structural modifications of the small intestine that increase its surface area for digestion and absorption. Microvilli (brush border) Absorptive cells Lacteal Villus Goblet cell Blood capillaries Mucosa- associated lymphoid tissue Intestinal crypt Enteroendocrine cells Venule Muscularis mucosae Lymphatic vessel Duodenal gland Submucosa © 2013 Pearson Education, Inc.

Absorptive cells Villi Intestinal crypt
Figure 23.22c Structural modifications of the small intestine that increase its surface area for digestion and absorption. Absorptive cells Goblet cells Villi Intestinal crypt © 2013 Pearson Education, Inc.

Mucus granules Microvilli forming the brush border Absorptive cell
Figure Microvilli of Small Intestine Mucus granules Microvilli forming the brush border Absorptive cell

Peyer's patches protect especially distal part against bacteria
Mucosa Peyer's patches protect especially distal part against bacteria May protrude into submucosa B lymphocytes leave intestine, enter blood, protect intestinal lamina propria with their IgA Duodenal (Brunner's) glands of the duodenum secrete alkaline mucus to neutralize acidic chyme © 2013 Pearson Education, Inc.

1-2 L secreted daily in response to distension or irritation of mucosa
Intestinal Juice 1-2 L secreted daily in response to distension or irritation of mucosa Slightly alkaline; isotonic with blood plasma Largely water; enzyme-poor (enzymes of small intestine only in brush border); contains mucus Facilitates transport and absorption of nutrients © 2013 Pearson Education, Inc.

Accessory Organs: Liver and Gallbladder Liver Gallbladder
Many functions Digestive function: produce bile (emulsify fats) Largest gland Gallbladder Chief function: store bile © 2013 Pearson Education, Inc.

Sternum Bare area Liver Right lobe of liver Gallbladder
Figure 23.24a Liver Sternum Bare area Liver Falciform ligament Left lobe of liver Right lobe of liver Round ligament (ligamentum teres) Gallbladder

Bile duct and sphincter Accessory pancreatic duct
Figure Liver, Pancreas, Duodenum Right and left hepatic ducts of liver Cystic duct Common hepatic duct Bile duct and sphincter Accessory pancreatic duct Mucosa with folds Tail of pancreas Pancreas Gallbladder Jejunum Major duodenal papilla Main pancreatic duct and sphincter Hepatopancreatic ampulla and sphincter Duodenum Head of pancreas

Lobule Central vein Connective tissue septum
Figure 23.25a–b Microscopic anatomy of the liver. Lobule Central vein Connective tissue septum © 2013 Pearson Education, Inc.

Liver: Microscopic Anatomy
Portal triad at each corner of lobule Branch of hepatic artery supplies oxygen Branch of hepatic portal vein brings nutrient-rich blood Bile duct receives bile from bile canaliculi Liver sinusoids - leaky capillaries between hepatic plates Stellate macrophages (hepatic macrophages or Kupffer cells) in liver sinusoids remove debris & old RBCs © 2013 Pearson Education, Inc.

Interlobular veins (to hepatic vein) Central vein Sinusoids
Figure 23.25c Microscopic anatomy of the liver. Interlobular veins (to hepatic vein) Central vein Sinusoids Plates of hepatocytes Portal vein Stellate macrophages in sinusoid walls Bile canaliculi Bile duct (receives bile from bile canaliculi) Fenestrated lining (endothelial cells) of sinusoids Bile duct Portal venule Portal arteriole Portal triad © 2013 Pearson Education, Inc.

Hepatocytes Abundant rough & smooth ER, Golgi, mitochondria
Process bloodborne nutrients Store fat-soluble vitamins Detoxification Produce ~900 ml bile per day Regenerative capacity Jean Louis Cesar Lair, 1819

Hepatitis Usually viral infection, drug toxicity, wild mushroom poisoning Cirrhosis Progressive, chronic inflammation from chronic hepatitis or alcoholism Liver  fatty, fibrous  portal hypertension Liver transplants successful, but livers scarce © 2013 Pearson Education, Inc.

Yellow-green, alkaline solution containing
Bile Yellow-green, alkaline solution containing Bile salts - cholesterol derivatives that function in fat emulsification and absorption Bilirubin - pigment formed from heme Bacteria break down in intestine to stercobilin  brown color of feces Cholesterol, triglycerides, phospholipids, and electrolytes © 2013 Pearson Education, Inc.

Thin-walled muscular sac on ventral surface of liver
The Gallbladder Thin-walled muscular sac on ventral surface of liver Stores and concentrates bile by absorbing water and ions Muscular contractions release bile via cystic duct, which flows into bile duct © 2013 Pearson Education, Inc.

High cholesterol; too few bile salts  gallstones (biliary calculi)
The Gallbladder High cholesterol; too few bile salts  gallstones (biliary calculi) Obstruct flow of bile from gallbladder May cause obstructive jaundice Gallbladder contracts against sharp crystals  pain Treated with drugs, ultrasound vibrations (lithotripsy), laser vaporization, surgery © 2013 Pearson Education, Inc.

Pancreas Mostly retroperitoneal, deep to greater curvature of stomach; head by duodenum, tail by spleen Endocrine: pancreatic islets secrete insulin and glucagon Exocrine: secretes pancreatic juice To duodenum via main pancreatic duct Zymogen granules contain proenzymes Secreted in inactive form, activated in gut © 2013 Pearson Education, Inc.

Small duct Acinar cell Basement membrane Zymogen granules Rough
Figure 23.26a Enzyme-producing tissue of pancreas Small duct Acinar cell Basement membrane Zymogen granules Rough endoplasmic reticulum Duct cell One acinus

Pancreatic Juice ml/day, alkaline (pH 8, HCO3-) neutralizes chyme Enzymes Amylase, lipases, nucleases secreted in active form but require ions or bile for optimal activity Proteases secreted in inactive form, activated in duodenum Trypsinogen activated to trypsin by brush border enzyme enteropeptidase Procarboxypeptidase, chymotrypsinogen activated by trypsin © 2013 Pearson Education, Inc.

Figure 23.27 Activation of Pancreatic Proteases in Small Intestine
Stomach Pancreas Epithelial cells Membrane-bound enteropeptidase Trypsinogen (inactive) Chymotrypsinogen Procarboxypeptidase Trypsin Chymotrypsin Carboxypeptidase © 2013 Pearson Education, Inc.

Regulation of Bile Secretion
Bile secretion stimulated by secretin from intestinal cells Gallbladder contraction stimulated by Cholecystokinin (CCK) from intestinal cells exposed to acidic, fatty chyme Vagal activity CCK also causes Secretion of pancreatic juice Hepatopancreatic sphincter to relax © 2013 Pearson Education, Inc.

Regulation of Pancreatic Secretion
CCK induces secretion of enzyme-rich pancreatic juice by acini Secretin causes secretion of bicarbonate-rich pancreatic juice by duct cells Vagal stimulation also causes release of pancreatic juice (minor stimulus) © 2013 Pearson Education, Inc.

Promotion of Bile and Pancreatic Juice
Secretion and Release Figure 23.28 1. Chyme entering duodenum causes duodenal enteroendocrine cells to release cholecystokinin (CCK) and secretin. 4. Bile salts & secretin stimulate liver to produce more bile. 5. CCK causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum. 2. CCK (red dots) and secretin (yellow dots) enter blood. 6. During cephalic and gastric phases, vagus nerve activity causes weak gallbladder contractions. 3. CCK, secretin induce pancreatic juice secretion. CCK secretion Secretin secretion

Digestion in the Small Intestine
Chyme from stomach contains Partially digested carbohydrates and proteins Undigested fats 3–6 hours in small intestine Most water absorbed ~ All nutrients absorbed Small intestine, like stomach, no role in ingestion or defecation © 2013 Pearson Education, Inc.

Requirements for Digestion and Absorption in the Small Intestine
Slow delivery of acidic, hypertonic chyme Delivery of bile, enzymes, and bicarbonate ions from liver and pancreas Mixing © 2013 Pearson Education, Inc.

Motility of the Small Intestine
Segmentation Most common motion of small intestine Initiated by intrinsic pacemaker cells Mixes/moves contents toward ileocecal valve Intensity altered by long & short reflexes; hormones Parasympathetic  ; sympathetic  Wanes in late intestinal (fasting) phase © 2013 Pearson Education, Inc.

Peristalsis Initiated by rise in hormone motilin in late intestinal phase; every 90–120 minutes Each wave starts distal to previous Migrating motor complex Meal remnants, bacteria, and debris moved to large intestine From duodenum  ileum ~ 2 hours © 2013 Pearson Education, Inc.

Figure 23.3a Peristalsis and segmentation.
From mouth Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, moving food along the tract distally. © 2013 Pearson Education, Inc.

Local enteric neurons coordinate intestinal motility Cholinergic sensory neurons may activate myenteric plexus Causes contraction of circular muscle proximally and of longitudinal muscle distally Forces chyme along tract © 2013 Pearson Education, Inc.

Ileocecal sphincter relaxes, admits chyme into large intestine when Gastroileal reflex enhances force of segmentation in ileum Gastrin increases motility of ileum Ileocecal valve flaps close when chyme exerts backward pressure Prevents regurgitation into ileum © 2013 Pearson Education, Inc.

Cecum: connects to ileum at ileocecal valve
Large Intestine Cecum: connects to ileum at ileocecal valve Appendix: part of MALT of immune system Colon: more below Rectum Anal canal Opens to body exterior at anus Internal anal sphincter—smooth muscle External anal sphincter—skeletal muscle © 2013 Pearson Education, Inc.

Unique features of Large Intestine
Teniae coli Three bands of longitudinal smooth muscle in muscularis Haustra Pocketlike sacs created by tone of teniae coli Epiploic appendages Fat-filled pouches of visceral peritoneum © 2013 Pearson Education, Inc.

External anal sphincter
Figure 23.29a Large Intestine Left colic (splenic) flexure Transverse mesocolon Right colic (hepatic) flexure Epiploic appendages Transverse colon Superior mesenteric artery Descending colon Haustrum Ascending colon IIeum Cut edge of mesentery IIeocecal valve Tenia coli Sigmoid colon Cecum Appendix Rectum Anal canal External anal sphincter © 2013 Pearson Education, Inc.

Colon Retroperitoneal except for transverse and sigmoid regions
Ascending colon (right side – to level of right kidney) Transverse colon Descending colon Sigmoid colon in pelvis  rectum © 2013 Pearson Education, Inc.

Greater omentum Transverse colon Descending colon Jejunum Mesentery
Figure 23.30c Mesenteries of the Abdomen Greater omentum Transverse colon Descending colon Jejunum Mesentery Sigmoid colon Ileum

Liver Lesser omentum Pancreas Stomach Duodenum Transverse mesocolon
Figure 23.30d Mesenteries of the Abdomen Liver Lesser omentum Pancreas Stomach Duodenum Transverse mesocolon Transverse colon Mesentery Greater omentum Jejunum Ileum Visceral peritoneum Parietal peritoneum Urinary bladder Rectum

Rectum Hemorrhoidal veins Anal canal External anal sphincter
Figure 23.29b Rectum and Anus Rectum Hemorrhoidal veins Anal canal External anal sphincter Internal anal sphincter Anus

Enter from small intestine or anus
Bacterial Flora Enter from small intestine or anus Colonize colon Synthesize B complex vitamins and vitamin K Metabolize some host-derived molecules (mucin, heparin, hyaluronic acid) Ferment indigestible carbohydrates Release irritating acids and gases (~500 ml/day) © 2013 Pearson Education, Inc.

Digestive Processes in the Large Intestine
Residue remains in large intestine 12–24 hours No food breakdown except by enteric bacteria Vitamins (made by bacterial flora), water, and electrolytes (especially Na+ and Cl–) reclaimed Major functions - propulsion of feces to anus; defecation Colon not essential for life © 2013 Pearson Education, Inc.

Large Intestinal Motility
Most colonic activity is haustral contractions Slow segmenting movements Gastrocolic reflex Initiated by presence of food in stomach Activates three to four slow powerful peristaltic waves per day in colon (mass movements) © 2013 Pearson Education, Inc.

Irritable bowel syndrome Functional GI disorder Recurring abdominal pain, stool changes, bloating, flatulence, nausea, depression Stress common precipitating factor Stress management important in treatment © 2013 Pearson Education, Inc.

stimulating stretch receptors there. The receptors transmit
Figure Defecation Reflex Impulses from cerebral cortex (conscious control) Sensory nerve fibers Feces move into and distend the rectum, stimulating stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons. 1 Voluntary motor nerve to external anal sphincter Sigmoid colon Stretch receptors in wall A spinal reflex is initiated in which parasympathetic motor (efferent) fibers stimulate contraction of the rectum and sigmoid colon, and relaxation of the internal anal sphincter. 2 Rectum External anal sphincter (skeletal muscle) Involuntary motor nerve (parasympathetic division) Internal anal sphincter (smooth muscle) If it is convenient to defecate, voluntary motor neurons are inhibited, allowing the external anal sphincter to relax so feces may pass. 3

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