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Digestive System Dr. Spandana Charles
Digestive System The system by which ingested food is acted upon by physical and chemical means to provide the body with absorbable nutrients and to excrete waste products. This system includes The alimentary canal extending from the mouth to the anus, and the accessory digestive organs like the teeth, tongue, salivary glands, gallbladder, liver and pancreas.
Figure 23. 1 Alimentary canal and related accessory digestive organs. Mouth (oral cavity) Tongue* Parotid gland Sublingual gland Submandibular gland Salivary glands* Pharynx Esophagus Stomach Pancreas* (Spleen) Liver* Gallbladder* Transverse colon Small intestine Anus Duodenum Jejunum Ileum Descending colon Ascending colon Cecum Sigmoid colon Rectum Appendix © 2013 Pearson Education, Inc. Anal canal Large intestine 3
Digestive Processes Six essential activities 1. Ingestion 2. Mechanical break down 3. Propulsion 4. Digestion 5. Absorption 6. Defecation © 2013 Pearson Education, Inc. 4
Figure 23. 2 Gastrointestinal tract activities. Ingestion Food Mechanical breakdown Pharynx • Chewing (mouth) • Churning (stomach) • Segmentation (small intestine) Esophagus Propulsion • Swallowing (oropharynx) • Peristalsis (esophagus, stomach, small intestine, large intestine) Digestion Stomach Absorption Lymph vessel Small intestine Large intestine Blood vessel Mainly H 2 O Feces Defecation Anus © 2013 Pearson Education, Inc. 5
Peritoneum and Peritoneal Cavity Peritoneum - serous membrane of abdominal cavity Visceral peritoneum on external surface of most digestive organs Parietal peritoneum lines body wall Peritoneal cavity Between two peritoneums Fluid lubricates mobile organs © 2013 Pearson Education, Inc. 6
Figure 23. 5 b The peritoneum and the peritoneal cavity. Mesentery resorbed and lost Alimentary canal organ in a retroperitoneal position Some organs lose their mesentery and move, becoming retroperitoneal, during development. © 2013 Pearson Education, Inc. 7
Blood Supply: Splanchnic Circulation Branches of aorta serving digestive organs Hepatic, splenic, and left gastric arteries Inferior and superior mesenteric arteries Hepatic portal circulation Drains nutrient-rich blood from digestive organs Delivers it to the liver for processing © 2013 Pearson Education, Inc. 8
Histology of the Alimentary Canal Four basic layers (tunics) Mucosa Secretes mucus, digestive enzymes, and hormones Absorbs end products of digestion Submucosa Contains blood and lymphatic vessels, lymphoid follicles, and submucosal nerve plexus Muscularis externa Responsible for peristalsis and segmentation movements. Contains inner circular and outer longitudinal muscle layers. Myentric nerve plexus exists between the two muscle layers. Serosa © 2013 Pearson Education, Inc. Serosa is the visceral peritoneum. 9
Enteric Nervous System Intrinsic nerve supply of alimentary canal – enteric neurons. Major nerve supply to GI tract wall; control motility Submucosal nerve plexus Regulates glands and smooth muscle in the mucosa Myenteric nerve plexus Controls GI tract motility Long ANS fibers synapse with enteric plexuses Sympathetic impulses inhibit digestive activities Parasympathetic impulses stimulate digestive activities © 2013 Pearson Education, Inc. 10
Mouth Oral (buccal) cavity Bounded by lips, cheeks, palate, and tongue The lips and cheek contain orbicularis oris and buccinator muscles. Hard palate – formed by palatine bones and palatine processes of maxillae. Slightly corrugated to help create friction against tongue. Soft palate - formed mostly of skeletal muscle. Closes off nasopharynx during swallowing. Uvula projects downward from its free edge
Figure 23. 7 b Anatomy of the oral cavity (mouth). Upper lip Gingivae (gums) Superior labial frenulum Palatine raphe Palatoglossal arch Hard palate Soft palate Uvula Palatopharyngeal arch Posterior wall of oropharynx Palatine tonsil Tongue Lingual frenulum Sublingual fold with openings of sublingual ducts Opening of Submandibular duct Gingivae (gums) Oral vestibule Inferior labial frenulum Lower lip Anterior view © 2013 Pearson Education, Inc. 12
Tongue Skeletal muscle Functions include Repositioning and mixing food during chewing Formation of bolus Initiation of swallowing, speech, and taste Lingual frenulum: attachment to floor of mouth Surface bears filiform, foliate, fungiform and circumvallate papillae(also known as taste buds, help in identifying taste) © 2013 Pearson Education, Inc. 13
Teeth Tear and grind food for digestion Primary and permanent dentitions formed by age 21 20 deciduous teeth erupt (6– 24 months of age) Roots resorbed, teeth fall out (6– 12 years of age) as permanent teeth develop 32 permanent teeth All but third molars in by end of adolescence Third molars at 17– 25, or may not erupt © 2013 Pearson Education, Inc. 14
Classes of Teeth Incisors Canines Fanglike teeth that tear or pierce Premolars (bicuspids) Chisel shaped for cutting Broad crowns, rounded cusps – grind/crush Molars Broad crowns, rounded cusps – best grinders © 2013 Pearson Education, Inc. 15
Figure 23. 11 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 © 2013 Pearson Education, Inc. 16 Bone
Salivary Glands Produce most saliva; lie outside oral cavity Parotid- Anterior to ear; external to masseter muscle. Duct opens next to second upper molar. Mumps is inflammation of parotid. Submandibular- Medial to body of mandible. Duct opens at base of tongue. Sublingual- Present under the tongue. Opens into the floor of the mouth. © 2013 Pearson Education, Inc. 17
Saliva Function of saliva: Cleanses mouth Dissolves food chemicals for taste Moistens food; compacts into bolus Begins breakdown of starch with enzymes Composition of Saliva: 97– 99. 5% water, slightly acidic + + – 2–, HCO Electrolytes—Na , K , Cl , PO 4 3– Salivary amylase -Breaks down starch to oligosaccharides © 2013 Pearson Education, Inc. Mucin, Ig. A, Lysosyme, Defensins 18
Control of Salivation 1500 ml/day Extrinsic 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. 19
Mastication/Chewing Cheeks and closed lips hold food between teeth Tongue mixes food with saliva; compacts food into bolus Teeth cut and grind Mastication is partly voluntary and partly reflexive. pressure receptors in cheeks, gums and tongue initiate a stretch reflex which is responsible for reflexive chewing. © 2013 Pearson Education, Inc. 20
Pharynx Food passes from mouth oropharynx laryngopharynx Allows passage of food, fluids, and air Epiglottis closes the laryngeal inlet during swallowing Soft palate prevents ingested food from entering the nasal cavity. Skeletal muscle layers: inner longitudinal, outer pharyngeal constrictors © 2013 Pearson Education, Inc. 21
Esophagus Flat muscular tube from laryngopharynx to stomach. Upper esophageal sphincter separates the pharynx from the esophagus. Pierces diaphragm at esophageal hiatus Joins stomach at cardial orifice Lower esophageal sphincter(LES) surrounds the junction of esophagus with the stomach. Heart burn is when the stomach acid regurgitates into esophagus © 2013 Pearson Education, Inc. 22
Deglutition Involves tongue, soft palate, pharynx, esophagus 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. 23
Stomach: Gross Anatomy In upper left quadrant; temporary storage; chemical digestion of bolus to chyme Cardial part (cardia) -Surrounds cardial orifice Fundus-Dome-shaped region beneath diaphragm Body-Midportion Pyloric part - continuous with duodenum through pyloric valve (sphincter controlling stomach © 2013 Pearson Education, Inc. emptying). 24
Stomach: Gross Anatomy In upper left quadrant; temporary storage; chemical digestion of bolus to chyme Cardial part (cardia) -Surrounds cardial orifice Fundus-Dome-shaped region beneath diaphragm Body-Midportion Pyloric part - continuous with duodenum through pyloric valve (sphincter controlling stomach © 2013 Pearson Education, Inc. emptying). 25
Figure 23. 14 a Anatomy of the stomach. Cardia Fundus Esophagus Muscularis externa • Longitudinal layer • Circular layer • Oblique layer Serosa Body Lumen Lesser curvature Rugae of mucosa Greater curvature Duodenum Pyloric sphincter (valve) at pylorus Pyloric canal © 2013 Pearson Education, Inc. Pyloric antrum 26
Gastric Glands Cell types Mucous neck cells- Secrete thin, acidic mucus Parietal cells- Secrete Hcl and intrinsic factor. Chief cells -Secrete Pepsinogen and Lipases Hcl in stomach converts pepsinogen to pepsin Pepsin breaks down proteins to proteases and peptones Enteroendocrine cells – Secrete chemical messengers/hormones like Histamine, serotonin somatostatin and gastrin. © 2013 Pearson Education, Inc. 27
Figure 23. 15 c Microscopic anatomy of the stomach. Pepsinogen HCI Pepsin Mitochondria Parietal cell Chief cell Enteroendocrine cell Location of the HCl-producing parietal cells © 2013 Pearson Education, Inc. in a gastric and pepsin-secreting chief cells gland 28
Mucosal Barrier 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 Gastritis-Inflammation caused by anything that breaches mucosal barrier Peptic or gastric ulcers- Erosions of stomach wall Most caused by Helicobacter pylori bacteria © 2013 Pearson Education, Inc. 29
Digestive Processes in the Stomach Physical digestion Denaturation of proteins by HCl Enzymatic digestion of proteins by pepsin (and milk protein by rennin in infants) Lingual lipase digests some triglycerides before digested as any other protein Delivers chyme to small intestine © 2013 Pearson Education, Inc. 30
Regulation of Gastric Secretion About 3 litres of gastric juice is produced per day. Three phases of gastric secretion Cephalic (reflex) phase – conditioned reflex triggered by aroma, taste, sight, thought Gastric phase – lasts 3– 4 hours; ⅔ gastric juice released Stimulated by distension, peptides, low acidity, gastrin (major stimulus) Enteroendocrine G cells stimulated by peptides, rising ph release gastrin Gastrin facilitates release of enzyme and HCl release Stimulation of vagus nerve also increases Hcl secretion © 2013 Pearson Education, Inc. 31
Regulation of Gastric Secretion Intestinal phase Stimulatory component Partially digested food enters small intestine brief intestinal gastrin release Inhibitory effects (enterogastric reflex) Chyme with H+, fats, peptides, irritating substances - inhibition-tightening of pyloric sphincter - no more food entry to small intestine. If small intestine is forced to accept more chyme it causes dumping syndrome-(nausea and vomiting). Hormones such as secretin, cholecystokinin and VIP secreted by small intestine also inhibit gastric juice secretion. © 2013 Pearson Education, Inc. 32
Response of the Stomach to Filling Stretches to accommodate incoming food Pressure constant until 1. 5 L food ingested Gastric accommodation Plasticity (stress-relaxation response) of smooth muscle Peristaltic waves move toward pylorus at rate of 3 per minute Peristaltic waves are initiated by Enteric pace maker cells also called interstitial cells of Cajal. Peristaltic waves most vigorous near pylorus. Chyme is either delivered in ~3 ml spurts to duodenum, or is forced backward into stomach © 2013 Pearson Education, Inc. 33
Homeostatic Imbalance Vomiting (emesis) caused by Extreme stretching Intestinal irritants, e. g. , bacterial toxins, excessive alcohol, spicy food, certain drugs Chemicals/sensory impulses emetic center of medulla Excessive vomiting dehydration, electrolyte and acid-base imbalances (alkalosis) © 2013 Pearson Education, Inc. 34
Small Intestine: Gross Anatomy Major organ of digestion and absorption 2 -4 m long; from pyloric sphincter to ileocecal valve Subdivisions Duodenum -curves around head of pancreas Shortest part( about 25 cm) Bile duct from liver and pancreatic duct from pancreas drain into it. Jejunum – about 2. 5 meters long Ileum – about 3. 6 meters long © 2013 Pearson Education, Inc. 35
Structural Modifications Increase surface area of proximal part for nutrient absorption Circular folds (plicae circulares)- force chyme to slowly spiral through the lumen. Villi- Extensions (~1 mm high) of mucosa with capillary bed and lacteal for absorption Microvilli- contain enzymes for carbohydrate and protein digestion © 2013 Pearson Education, Inc. 36
Figure 23. 22 a 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. 37
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. 38
Liver Largest gland in body Four lobes—right, left, caudate, and quadrate lobe Liver lobules Hexagonal structural and functional units Composed of plates of hepatocytes (liver cells) Central vein in longitudinal axis Portal triad at each corner of lobule. Branch of hepatic artery supplies oxygen Branch of hepatic portal vein brings nutrientrich blood Bile duct receives bile from bile canaliculi © 2013 Pearson Education, Inc. 39
Figure 23. 25 c 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 Stellate macrophages in sinusoid walls Bile duct Portal venule Portal arteriole Portal triad Portal vein © 2013 Pearson Education, Inc. 40
Liver produces bile which helps in digestion. Bile leaves the liver through the common hepatic duct. Bile is stored in the gallbladder till the need arises. The cystic duct drains bile from the gallbladder. Composition of 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 bilirubin in intestine to form stercobilin - brown color of feces © 2013 Pearson Education, Inc. 41
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 Tail of pancreas Pancreas Gallbladder Jejunum Major duodenal papilla Hepatopancreatic ampulla and sphincter Main pancreatic duct and sphincter Duodenum of pancreas © 2013 Pearson Head Education, Inc. 42
Pancreas Location Mostly retroperitoneal, deep to greater curvature of stomach Head encircled by duodenum; tail abuts spleen Endocrine function-Pancreatic islets secrete insulin and glucagon Exocrine function- Acini (clusters of secretory cells) secrete pancreatic juice To duodenum via main pancreatic duct © 2013 Pearson Education, Inc.
Pancreatic Juice 1200 – 1500 ml/day Watery alkaline solution (p. H 8) neutralizes chyme Electrolytes (primarily HCO 3–) Enzymes Amylase, lipases, nucleases secreted in active form but require ions or bile for optimal activity Proteases secreted in inactive form, activated in the duodenum. Trypsinogen Chymotrypsinogen © 2013 Pearson Education, Inc.
Digestion in the Small Intestine Chyme from stomach contains Partially digested carbohydrates and proteins Undigested fats Entry of chyme into the stomach causes duodenum to release CCK and secretin. CCK causes gall bladder to contract and the hepatopancreatic sphincter to relax , so that the bile and the pancreatic juices enter the duodenum. Secretin stimulates the pancreas to increase production of pancreatic juice. © 2013 Pearson Education, Inc.
Digestion in small intestine Starch digestion Pancreatic amylase (small intestine) breaks down carbohydrates that escaped salivary amylase oligosaccharides Brush border enzymes (dextrinase, glucoamylase, lactase, maltase, sucrase) oligosaccharides monosaccharides Protein digestion Pancreatic proteases-Trypsin, chymotrypsin, and carboxypeptidase Brush border enzymes-Aminopeptidases, carboxypeptidases, and dipeptidases © 2013 Pearson Education, Inc.
Digestion of Lipids Pre-treatment—emulsification by bile salts Broken down into smaller particles Enzymes—pancreatic lipases � Fatty acids and monoglycerides © 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 Migrating motor complex/Hunger pangs Initiated by the hormone motilin Meal remanants, bacteria and debris are moved to © 2013 Pearson Education, Inc. the large intestine.
To be continued. . © 2013 Pearson Education, Inc.