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Figure 23.1 Alimentary canal and related accessory digestive organs. Mouth (oral cavity) Tongue Esophagus Liver Gallbladder Duodenum Jejunum Small intestine Ileum Anus Copyright © 2010 Pearson Education, Inc. Parotid gland Sublingual gland Salivary Submandibular glands gland Pharynx Stomach Pancreas (Spleen) Transverse colon Descending colon Ascending colon Large Cecum intestine Sigmoid colon Rectum Vermiform appendix Anal canal Figure 23.2 Gastrointestinal tract activities. Ingestion Mechanical digestion • Chewing (mouth) • Churning (stomach) • Segmentation (small intestine) Chemical digestion Food Pharynx Esophagus Propulsion • Swallowing (oropharynx) • Peristalsis Stomach (esophagus, stomach, small intestine, large intestine) Absorption Lymph vessel Small intestine Large intestine Defecation Copyright © 2010 Pearson Education, Inc. Blood vessel Mainly H2O Feces Anus Figure 23.3 Peristalsis and segmentation. From mouth (a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally. Copyright © 2010 Pearson Education, Inc. (b) Segmentation: Nonadjacent segments of alimentary tract organs alternately contract and relax, moving the food forward then backward. Food mixing and slow food propulsion occurs. Figure 23.3a Peristalsis and segmentation. From mouth (a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally. Copyright © 2010 Pearson Education, Inc. Figure 23.3b Peristalsis and segmentation. (b) Segmentation: Nonadjacent segments of alimentary tract organs alternately contract and relax, moving the food forward then backward. Food mixing and slow food propulsion occurs. Copyright © 2010 Pearson Education, Inc. Figure 23.4 Neural reflex pathways initiated by stimuli inside or outside the gastrointestinal tract. External stimuli (sight, smell, taste, thought of food) Central nervous system and extrinsic autonomic nerves Long reflexes Afferent impulses Internal (GI tract) stimuli Efferent impulses Chemoreceptors, osmoreceptors, or mechanoreceptors Local (intrinsic) nerve plexus (“gut brain”) Effectors: Smooth muscle or glands Short reflexes Gastrointestinal wall (site of short reflexes) Lumen of the alimentary canal Copyright © 2010 Pearson Education, Inc. Response: Change in contractile or secretory activity Figure 23.5 The peritoneum and the peritoneal cavity. Abdominopelvic cavity Vertebra Dorsal mesentery Parietal peritoneum Ventral mesentery Visceral peritoneum Peritoneal cavity Alimentary canal organ Mesentery resorbed and lost Liver (a) Schematic cross sections of abdominal cavity illustrate the peritoneums and mesenteries. Alimentary Alimentary canal organ in canal organ a retroperitoneal position (b) Some organs lose their mesentery and become retroperitoneal during development. Copyright © 2010 Pearson Education, Inc. Figure 23.5a The peritoneum and the peritoneal cavity. Abdominopelvic cavity Vertebra Dorsal mesentery Parietal peritoneum Ventral mesentery Visceral peritoneum Peritoneal cavity Alimentary canal organ Liver (a) Schematic cross sections of abdominal cavity illustrate the peritoneums and mesenteries. Copyright © 2010 Pearson Education, Inc. Figure 23.5b The peritoneum and the peritoneal cavity. Abdominopelvic cavity Mesentery resorbed and lost Alimentary Alimentary canal organ in canal organ a retroperitoneal position (b) Some organs lose their mesentery and become retroperitoneal during development. Copyright © 2010 Pearson Education, Inc. Figure 23.6 Basic structure of the alimentary canal. Intrinsic nerve plexuses • Myenteric nerve plexus • Submucosal nerve plexus Glands in submucosa Mucosa • Epithelium • Lamina propria • Muscularis mucosae Submucosa Muscularis externa Nerve Artery Vein Mesentery Gland in mucosa Lymphatic Duct of gland outside vessel alimentary canal Copyright © 2010 Pearson Education, Inc. • Longitudinal muscle • Circular muscle Serosa • Epithelium • Connective tissue Lumen Mucosa-associated lymphoid tissue Figure 23.7 Anatomy of the oral cavity (mouth). Soft palate Palatoglossal arch Hard palate Uvula Upper lip Oral cavity Palatine tonsil Tongue Oropharynx Lingual tonsil Epiglottis Hyoid bone Laryngopharynx Gingivae (gums) Palatine raphe Hard palate Soft palate Uvula Palatine tonsil Trachea (a) Sagittal section of the oral cavity and pharynx Copyright © 2010 Pearson Education, Inc. Palatoglossal arch Palatopharyngeal arch Posterior wall of oropharynx Tongue Sublingual fold with openings of sublingual ducts Esophagus Superior labial frenulum Lingual frenulum Opening of submandibular duct Gingivae (gums) Vestibule Lower lip (b) Anterior view Inferior labial frenulum Figure 23.7a Anatomy of the oral cavity (mouth). Soft palate Palatoglossal arch Hard palate Uvula Oral cavity Palatine tonsil Tongue Oropharynx Lingual tonsil Epiglottis Hyoid bone Laryngopharynx Esophagus Trachea (a) Sagittal section of the oral cavity and pharynx Copyright © 2010 Pearson Education, Inc. Figure 23.7b Anatomy of the oral cavity (mouth). Gingivae (gums) Palatine raphe Hard palate Soft palate Uvula Palatine tonsil Sublingual fold with openings of sublingual ducts Vestibule Lower lip (b) Anterior view Copyright © 2010 Pearson Education, Inc. Upper lip Superior labial frenulum Palatoglossal arch Palatopharyngeal arch Posterior wall of oropharynx Tongue Lingual frenulum Opening of submandibular duct Gingivae (gums) Inferior labial frenulum Figure 23.8 Dorsal surface of the tongue, and the tonsils. Epiglottis Palatopharyngeal arch Palatine tonsil Lingual tonsil Palatoglossal arch Terminal sulcus Foliate papillae Circumvallate papilla Midline groove of tongue Dorsum of tongue Fungiform papilla Filiform papilla Copyright © 2010 Pearson Education, Inc. Figure 23.9 The salivary glands. Ducts of sublingual gland Tongue Teeth Lingual frenulum Sublingual gland Mylohyoid muscle (cut) Anterior belly of digastric muscle (a) Copyright © 2010 Pearson Education, Inc. Parotid gland Parotid duct Masseter muscle Body of mandible (cut) Posterior belly of digastric muscle Submandibular duct Submandibular gland Mucous cells (b) Serous cells forming demilunes Figure 23.9a The salivary glands. Ducts of sublingual gland Tongue Teeth Lingual frenulum Sublingual gland Mylohyoid muscle (cut) Anterior belly of digastric muscle (a) Copyright © 2010 Pearson Education, Inc. Parotid gland Parotid duct Masseter muscle Body of mandible (cut) Posterior belly of digastric muscle Submandibular duct Submandibular gland Figure 23.9b The salivary glands. (b) Copyright © 2010 Pearson Education, Inc. Mucous cells Serous cells forming demilunes Figure 23.10a Human dentition. Incisors Central (6–8 mo) Lateral (8–10 mo) Canine (eyetooth) (16–20 mo) Molars First molar (10–15 mo) Second molar (about 2 yr) (a) Copyright © 2010 Pearson Education, Inc. Deciduous (milk) teeth 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 Figure 23.10b Human dentition. (b) Deciduous teeth Copyright © 2010 Pearson Education, Inc. Permanent teeth Figure 23.11 Longitudinal section of a canine tooth within its bony alveolus. Crown Neck Enamel Dentin Dentinal tubules Pulp cavity (contains blood vessels and nerves) Gingiva (gum) Cementum Root Root canal Periodontal ligament Apical foramen Bone Copyright © 2010 Pearson Education, Inc. Figure 23.12 Microscopic structure of the esophagus. Mucosa (contains a stratified squamous epithelium) Submucosa (areolar connective tissue) Lumen Muscularis externa • Circular layer • Longitudinal layer Adventitia (fibrous connective tissue) (a) Copyright © 2010 Pearson Education, Inc. (b) Figure 23.12a Microscopic structure of the esophagus. (a) Copyright © 2010 Pearson Education, Inc. Mucosa (contains a stratified squamous epithelium) Submucosa (areolar connective tissue) Lumen Muscularis externa • Longitudinal layer • Circular layer Adventitia (fibrous connective tissue) Figure 23.12b Microscopic structure of the esophagus. Mucosa (contains a stratified squamous epithelium) (b) Copyright © 2010 Pearson Education, Inc. Figure 23.13 Deglutition (swallowing) (1 of 5). Bolus of food Tongue Pharynx Epiglottis Glottis Trachea 1 Upper esophageal sphincter is contracted. During the buccal phase, the tongue presses against the hard palate, forcing the food bolus into the oropharynx where the involuntary phase begins. Copyright © 2010 Pearson Education, Inc. Figure 23.13 Deglutition (swallowing) (2 of 5). Uvula Bolus Epiglottis Esophagus 2 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. Copyright © 2010 Pearson Education, Inc. Figure 23.13 Deglutition (swallowing) (3 of 5). Bolus 3 The constrictor muscles of the pharynx contract, forcing food into the esophagus inferiorly. The upper esophageal sphincter contracts (closes) after entry. Copyright © 2010 Pearson Education, Inc. Figure 23.13 Deglutition (swallowing) (4 of 5). Relaxed muscles Circular muscles contract 4 Food is moved through the esophagus to the stomach by peristalsis. Bolus of food Longitudinal muscles contract Gastroesophageal sphincter closed Stomach Copyright © 2010 Pearson Education, Inc. Figure 23.13 Deglutition (swallowing) (5 of 5). Relaxed muscles 5 The gastroesophageal sphincter opens, and food enters the stomach. Gastroesophageal sphincter opens Copyright © 2010 Pearson Education, Inc. Figure 23.14a Anatomy of the stomach. Cardia Esophagus Muscularis externa • Longitudinal layer • Circular layer • Oblique layer Lesser curvature Fundus Serosa Body Lumen Rugae of mucosa Greater curvature Duodenum (a) Copyright © 2010 Pearson Education, Inc. Pyloric Pyloric canal antrum Pyloric sphincter (valve) at pylorus Figure 23.14b Anatomy of the stomach. Fundus Liver (cut) Body Spleen Lesser curvature Greater curvature (b) Copyright © 2010 Pearson Education, Inc. Figure 23.15a Microscopic anatomy of the stomach. Surface epithelium Mucosa Lamina propria Submucosa (contains submucosal plexus) Muscularis externa (contains myenteric plexus) Serosa Muscularis mucosae Oblique layer Circular layer Longitudinal layer (a) Layers of the stomach wall (l.s.) Copyright © 2010 Pearson Education, Inc. Stomach wall Figure 23.15b Microscopic anatomy of the stomach. Gastric pits Surface epithelium (mucous cells) Gastric pit Mucous neck cells Parietal cell Chief cell Gastric gland Enteroendocrine cell (b) Enlarged view of gastric pits and gastric glands Copyright © 2010 Pearson Education, Inc. Figure 23.15c Microscopic anatomy of the stomach. Pepsinogen HCl Pepsin Mitochondria Parietal cell Chief cell Enteroendocrine cell (c) Location of the HCl-producing parietal cells and pepsin-secreting chief cells in a gastric gland Copyright © 2010 Pearson Education, Inc. Figure 23.16 Photographs of a gastric ulcer lesion and of the bacteria that most commonly cause it. Bacteria Mucosa layer of stomach (a) A gastric ulcer lesion Copyright © 2010 Pearson Education, Inc. (b) H. pylori bacteria Figure 23.16a Photographs of a gastric ulcer lesion and of the bacteria that most commonly cause it. Copyright © 2010 Pearson Education, Inc. Figure 23.16b Photographs of a gastric ulcer lesion and of the bacteria that most commonly cause it. Bacteria Mucosa layer of stomach (b) H. pylori bacteria Copyright © 2010 Pearson Education, Inc. Figure 23.17 Neural and hormonal mechanisms that regulate release of gastric juice. Stimulatory events Cephalic phase Gastric phase 1 Sight and thought of food Cerebral cortex Conditioned reflex 2 Stimulation of taste and smell receptors Hypothalamus and medulla oblongata 1 Stomach distension activates stretch receptors Vagovagal reflexes 1 Presence of low pH, partially digested foods, fats, or hypertonic solution in duodenum when stomach begins to empty Stimulate Inhibit Copyright © 2010 Pearson Education, Inc. Medulla Vagus nerve Vagus nerve Local reflexes 2 Food chemicals G cells (especially peptides and caffeine) and rising pH activate chemoreceptors Intestinal phase Inhibitory events Gastrin release to blood Intestinal (enteric) gastrin release to blood Lack of stimulatory impulses to parasympathetic center Cerebral cortex Gastrin secretion declines G cells Overrides parasympathetic controls Sympathetic nervous system activation 1 Excessive acidity (pH <2) in stomach 2 Emotional upset Stomach secretory activity Enterogastric reflex Brief effect 1 Loss of appetite, depression Local reflexes Vagal nuclei in medulla Pyloric sphincter 1 Distension of duodenum; presence of fatty, acidic, hypertonic chyme, and/or irritants in the duodenum 2 Distension; Release of intestinal presence of hormones (secretin, cholecystokinin, vasoactive fatty, acidic, partially intestinal peptide) digested food in the duodenum Figure 23.18 Mechanism of HCl secretion by parietal cells. Blood capillary Chief cell CO2 CO2 + H2O Carbonic H2CO3 anhydrase H+ K+ Stomach lumen H+-K+ ATPase H+ K+ HCO3– Alkaline tide HCI Parietal cell HCO3– Cl– Interstitial fluid Copyright © 2010 Pearson Education, Inc. Cl– HCO3–- Cl– antiporter Cll– Figure 23.19 Peristaltic waves in the stomach. Pyloric valve closed 1 Propulsion: Peristaltic waves move from the fundus toward the pylorus. Copyright © 2010 Pearson Education, Inc. Pyloric valve closed 2 Grinding: The most vigorous peristalsis and mixing action occur close to the pylorus. Pyloric valve slightly opened 3 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. Figure 23.20 Neural and hormonal factors inhibiting gastric emptying. Presence of fatty, hypertonic, acidic chyme in duodenum Duodenal enteroendocrine cells Chemoreceptors and stretch receptors Secrete Enterogastrones (secretin, cholecystokinin, vasoactive intestinal peptide) Duodenal stimuli decline Initial stimulus Physiological response Result Copyright © 2010 Pearson Education, Inc. Target Via short reflexes Enteric neurons Contractile force and rate of stomach emptying decline Via long reflexes CNS centers sympathetic activity; parasympathetic activity Stimulate Inhibit Figure 23.21 The duodenum of the small intestine, and related organs. Right and left hepatic ducts of liver Cystic duct Common hepatic duct Bile duct and sphincter Accessory pancreatic duct Mucosa with folds Gallbladder Major duodenal papilla Hepatopancreatic ampulla and sphincter Copyright © 2010 Pearson Education, Inc. Tail of pancreas Pancreas Jejunum Duodenum Main pancreatic duct and sphincter Head of pancreas 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 Circular folds Villi (a) Copyright © 2010 Pearson Education, Inc. Lumen Figure 23.22b Structural modifications of the small intestine that increase its surface area for digestion and absorption. Microvilli (brush border) Absorptive cells Lacteal Goblet cell Blood capillaries Mucosa associated lymphoid tissue Intestinal crypt Muscularis mucosae Duodenal gland (b) Copyright © 2010 Pearson Education, Inc. Vilus Enteroendocrine cells Venule Lymphatic vessel Submucosa Figure 23.22c Structural modifications of the small intestine that increase its surface area for digestion and absorption. Absorptive cells Goblet cells Villi (c) Intestinal crypt Copyright © 2010 Pearson Education, Inc. Figure 23.23 Villi and microvilli of the small intestine. Villi Microvilli Desquamating cells (a) Copyright © 2010 Pearson Education, Inc. (b) Absorptive cell Figure 23.23a Villi and microvilli of the small intestine. Villi Desquamating cells (a) Copyright © 2010 Pearson Education, Inc. Figure 23.23b Villi and microvilli of the small intestine. Microvilli (b) Copyright © 2010 Pearson Education, Inc. Absorptive cell Figure 23.24a Gross anatomy of the human liver. Sternum Nipple Liver Bare area Falciform ligament Left lobe of liver Right lobe of liver Gallbladder (a) Copyright © 2010 Pearson Education, Inc. Round ligament (ligamentum teres) Figure 23.24b Gross anatomy of the human liver. Sternum Nipple Liver Lesser omentum (in fissure) Left lobe of liver Porta hepatis containing hepatic artery (left) and hepatic portal vein (right) Quadrate lobe of liver Ligamentum teres (b) Copyright © 2010 Pearson Education, Inc. Bare area Caudate lobe of liver Sulcus for inferior vena cava Hepatic vein (cut) Bile duct (cut) Right lobe of liver Gallbladder Figure 23.25a-b Microscopic anatomy of the liver. (a) Lobule Copyright © 2010 Pearson Education, Inc. (b) Central vein Connective tissue septum Figure 23.25c Microscopic anatomy of the liver. Interlobular veins (to hepatic vein) Central vein Sinusoids Bile canaliculi Plates of hepatocytes Bile duct (receives bile from bile canaliculi) Fenestrated lining (endothelial cells) of sinusoids Hepatic macrophages in sinusoid walls Portal vein (c) Copyright © 2010 Pearson Education, Inc. Bile duct Portal venule Portal arteriole Portal triad Figure 23.26 Structure of the enzyme-producing tissue of the pancreas. Small duct Acinar cells Basement membrane Zymogen granules (a) Rough endoplasmic reticulum Acinar cells (b) Copyright © 2010 Pearson Education, Inc. Figure 23.26a Structure of the enzyme-producing tissue of the pancreas. Small duct Acinar cells Basement membrane Zymogen granules Rough endoplasmic reticulum (a) Copyright © 2010 Pearson Education, Inc. Figure 23.26b Structure of the enzyme-producing tissue of the pancreas. Acinar cells (b) Copyright © 2010 Pearson Education, Inc. Figure 23.27 Activation of pancreatic proteases in the small intestine. Stomach Pancreas Epithelial cells Membrane-bound enteropeptidase Trypsinogen Trypsin (inactive) Chymotrypsinogen Chymotrypsin (inactive) Procarboxypeptidase Carboxypeptidase (inactive) Copyright © 2010 Pearson Education, Inc. Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice. 1 Chyme entering duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. 2 CCK (red dots) and secretin (yellow dots) enter the bloodstream. 3 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3–-rich pancreatic juice. Copyright © 2010 Pearson Education, Inc. 4 Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly. 5 CCK (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum. 6 During cephalic and gastric phases, vagal nerve stimulation causes weak contractions of gallbladder. Figure 23.29a Gross anatomy of the large intestine. Left colic (splenic) flexure Transverse mesocolon Epiploic appendages Right colic (hepatic) flexure Transverse colon Superior mesenteric artery Haustrum Descending colon Ascending colon IIeum Cut edge of mesentery Teniae coli IIeocecal valve Cecum Vermiform appendix Sigmoid colon Rectum Anal canal (a) Copyright © 2010 Pearson Education, Inc. External anal sphincter Figure 23.29b Gross anatomy of the large intestine. Rectal valve Rectum Hemorrhoidal veins Levator ani muscle Anal canal External anal sphincter Internal anal sphincter Anal columns Pectinate line Anal sinuses Anus (b) Copyright © 2010 Pearson Education, Inc. Figure 23.30a Mesenteries of the abdominal digestive organs. Falciform ligament Liver Gallbladder Spleen Stomach Ligamentum teres Greater omentum Small intestine Cecum (a) Copyright © 2010 Pearson Education, Inc. Figure 23.30b Mesenteries of the abdominal digestive organs. Liver Gallbladder Lesser omentum Stomach Duodenum Transverse colon Small intestine Cecum Urinary bladder Copyright © 2010 Pearson Education, Inc. (b) Figure 23.30c Mesenteries of the abdominal digestive organs. Greater omentum Transverse colon Transverse mesocolon Descending colon Jejunum Mesentery Sigmoid mesocolon Sigmoid colon Ileum (c) Copyright © 2010 Pearson Education, Inc. Figure 23.30d Mesenteries of the abdominal digestive organs. Liver Lesser omentum Pancreas Stomach Transverse mesocolon Duodenum Transverse colon Mesentery Greater omentum Jejunum Ileum Visceral peritoneum Parietal peritoneum (d) Copyright © 2010 Pearson Education, Inc. Urinary bladder Rectum Figure 23.31 Defecation reflex. Impulses from cerebral cortex (conscious control) 1 Sensory nerve fibers Distension, or stretch, of the rectal walls due to movement of feces into the rectum stimulates stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons. 2 Voluntary motor nerve to external anal sphincter Sigmoid colon A spinal reflex is initiated in which parasympathetic motor (efferent) fibers stimulate contraction of the rectal walls and relaxation of the internal anal sphincter. Stretch receptors in wall Rectum External anal sphincter (skeletal muscle) 3 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 that feces may pass. Copyright © 2010 Pearson Education, Inc. Figure 23.32 Flowchart of chemical digestion and absorption of foodstuffs (1 of 4). Carbohydrate digestion Foodstuff Enzyme(s) and source Site of action Starch and disaccharides Oligosaccharides and disaccharides Lactose Maltose Sucrose Galactose Glucose Fructose Copyright © 2010 Pearson Education, Inc. Salivary amylase Pancreatic amylase Brush border enzymes in small intestine (dextrinase, glucoamylase, lactase, maltase, and sucrase) Mouth Small intestine Small intestine Path of absorption • Glucose and galactose are absorbed via cotransport with sodium ions. • Fructose passes via facilitated diffusion. • All monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Figure 23.32 Flowchart of chemical digestion and absorption of foodstuffs (2 of 4). Protein digestion Foodstuff Protein Large polypeptides Small polypeptides, small peptides Amino acids (some dipeptides and tripeptides) Copyright © 2010 Pearson Education, Inc. Enzyme(s) and source Pepsin (stomach glands) in presence of HCl Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) Brush border enzymes (aminopeptidase, carboxypeptidase, and dipeptidase) Site of action Path of absorption • Amino acids are absorbed by cotransport with Stomach sodium ions. • Some dipeptides and tripeptides are absorbed via cotransport with H++ Small and hydrolyzed to amino intestine acids within the cells. • Amino acids leave the epithelial cells by Small facilitated diffusion, enter intestine the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Figure 23.32 Flowchart of chemical digestion and absorption of foodstuffs (3 of 4). Fat digestion Foodstuff Enzyme(s) and source Unemulsified fats Emulsification by the detergent action of bile salts ducted in from the liver Pancreatic lipases Monoglycerides Glycerol and fatty acids and fatty acids Copyright © 2010 Pearson Education, Inc. Site of action Path of absorption • Fatty acids and monoglycerides enter the intestinal cells via diffusion. Small intestine • Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells, and the resulting chylomicrons are Small extruded by exocytosis. intestine • The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct. • Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein. Figure 23.32 Flowchart of chemical digestion and absorption of foodstuffs (4 of 4). Nucleic acid digestion Foodstuff Enzyme(s) and source Nucleic acids Pentose sugars, N-containing bases, phosphate ions Copyright © 2010 Pearson Education, Inc. Pancreatic ribonuclease and deoxyribonuclease Brush border enzymes (nucleosidases and phosphatases) Site of action Path of absorption • Units enter intestinal cells by active transport via Small intestine membrane carriers. • Units are absorbed into capillary blood in the villi Small and transported to the intestine liver via the hepatic portal vein. Figure 23.33 Protein digestion and absorption in the small intestine. Amino acids of protein fragments Brush border enzymes Apical membrane (microvilli) Lumen of intestine Pancreatic proteases 1 Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxypeptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase) of mucosal cells. Na+ Na+ Absorptive epithelial cell 2 The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side (transcytosis). Amino acid carrier 3 The amino acids leave the Active transport Passive transport Copyright © 2010 Pearson Education, Inc. Capillary villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts. Figure 23.34 Emulsification, digestion, and absorption of fats. Fat globule 1 Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum. Bile salts Fat droplets coated with bile salts 2 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa. Micelles made up of fatty acids, monoglycerides, and bile salts 3 Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other lipoid substances and proteins to form chylomicrons. 4 Chylomicrons are extruded from the Epithelial cells of small intestine Copyright © 2010 Pearson Education, Inc. Lacteal epithelial cells by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph. Figure 23.35 Embryonic development of the digestive system. Lung bud Brain Oral membrane Heart Yolk sac Cloacal membrane Body stalk Stomodeum Foregut Site of liver development Midgut Spinal cord Bile duct Gallbladder Hindgut Cystic duct Ventral pancreatic bud Proctodeum Endoderm (a) Copyright © 2010 Pearson Education, Inc. Stomach Liver (b) Dorsal pancreatic bud Duodenum Figure 23.35a Embryonic development of the digestive system. Brain Oral membrane Heart Yolk sac Cloacal membrane Body stalk (a) Copyright © 2010 Pearson Education, Inc. Stomodeum Foregut Site of liver development Midgut Spinal cord Hindgut Proctodeum Endoderm Figure 23.35b Embryonic development of the digestive system. Lung bud Liver Stomach Bile duct Stomodeum Gallbladder Cystic duct Ventral pancreatic bud (b) Copyright © 2010 Pearson Education, Inc. Dorsal pancreatic bud Duodenum Table 23.1 Hormones and Paracrines that Act in Digestion (1 of 2) Copyright © 2010 Pearson Education, Inc. Table 23.1 Hormones and Paracrines that Act in Digestion (2 of 2) Copyright © 2010 Pearson Education, Inc. Table 23.2 Overview of the Functions of the Gastrointestinal Organs (1 of 2) Copyright © 2010 Pearson Education, Inc. Table 23.2 Overview of the Functions of the Gastrointestinal Organs (2 of 2) Copyright © 2010 Pearson Education, Inc. Table 23.3 Control of Small Intestinal Motility Copyright © 2010 Pearson Education, Inc. Making Connections 23.1 Homeostatic Interrelationships Between the Digestive System and Other Body Systems Copyright © 2010 Pearson Education, Inc.