23

1. 23 The Digestive System: Part B

2. Pharynx • Oropharynx and laryngopharynx • Allow passage of food, fluids, and air • Stratified squamous epithelium lining • Skeletal muscle layers: inner longitudinal, outer pharyngeal constrictors

3. Esophagus • Flat muscular tube from laryngopharynx to stomach • Pierces diaphragm at esophageal hiatus • Joins stomach at the cardiac orifice

4. Esophagus • Esophageal mucosa contains stratified squamous epithelium • Changes to simple columnar at the stomach • Esophageal glands in submucosa secrete mucus to aid in bolus movement • Muscularis: skeletal superiorly; smooth inferiorly • Adventitia instead of serosa

5. Mucosa (contains a stratified squamous epithelium) Submucosa (areolar connective tissue) Lumen Muscularis externa • Longitudinal layer • Circular layer Adventitia (fibrous connective tissue) (a) Figure 23.12a

6. Mucosa (contains a stratified squamous epithelium) (b) Figure 23.12b

7. Go to GI Diseases (Esophagus)

8. Digestive Processes: Mouth • Ingestion • Mechanical digestion • Mastication is partly voluntary, partly reflexive • Chemical digestion (salivary amylase and lingual lipase) • Propulsion • Deglutition (swallowing)

9. Deglutition • Involves the tongue, soft palate, pharynx, esophagus, and 22 muscle groups • Buccal phase • Voluntary contraction of the tongue • Pharyngeal-esophageal phase • Involuntary • Control center in the medulla and lower pons

10. Bolus of food Tongue Uvula Pharynx Bolus Epiglottis Epiglottis Glottis Trachea Bolus Esophagus 1 2 3 Upper esophageal sphincter iscontracted. During the buccal phase, thetongue presses against the hard palate,forcing the food bolus into the oropharynxwhere the involuntary phase begins. The uvula and larynx rise to prevent foodfrom entering respiratory passageways. Thetongue blocks off the mouth. The upperesophageal sphincter relaxes, allowing foodto enter the esophagus. The constrictor muscles of thepharynx contract, forcing foodinto the esophagus inferiorly. Theupper esophageal sphinctercontracts (closes) after entry. Relaxed muscles 4 5 Food is movedthrough the esophagusto the stomach byperistalsis. The gastroesophagealsphincter opens, and foodenters the stomach. Relaxedmuscles Circular musclescontract Bolus of food Longitudinal musclescontract Gastroesophagealsphincter closed Gastroesophagealsphincter opens Stomach Figure 23.13

11. Bolus of food Tongue Pharynx Epiglottis Glottis Trachea 1 Upper esophageal sphincter is contracted. Duringthe buccal phase, the tongue presses against the hardpalate, forcing the food bolus into the oropharynxwhere the involuntary phase begins. Figure 23.13, step 1

12. Bolus 3 The constrictor muscles of the pharynx contract,forcing food into the esophagus inferiorly. The upperesophageal sphincter contracts (closes) after entry. Figure 23.13, step 3

13. Uvula Bolus Epiglottis Esophagus 2 The uvula and larynx rise to prevent food fromentering respiratory passageways. The tongue blocksoff the mouth. The upper esophageal sphincterrelaxes, allowing food to enter the esophagus. Figure 23.13, step 2

14. Relaxed muscles 4 Food is moved throughthe esophagus to thestomach by peristalsis. Circular musclescontract Bolus of food Longitudinal musclescontract Gastroesophagealsphincter closed Stomach Figure 23.13, step 4

15. 5 The gastroesophagealsphincter opens, and foodenters the stomach. Relaxedmuscles Gastroesophagealsphincter opens Figure 23.13, step 5

16. Bolus of food Tongue Uvula Pharynx Bolus Epiglottis Epiglottis Glottis Trachea Bolus Esophagus 1 2 3 Upper esophageal sphincter iscontracted. During the buccal phase, thetongue presses against the hard palate,forcing the food bolus into the oropharynxwhere the involuntary phase begins. The uvula and larynx rise to prevent foodfrom entering respiratory passageways. Thetongue blocks off the mouth. The upperesophageal sphincter relaxes, allowing foodto enter the esophagus. The constrictor muscles of thepharynx contract, forcing foodinto the esophagus inferiorly. Theupper esophageal sphinctercontracts (closes) after entry. Relaxed muscles 4 5 Food is movedthrough the esophagusto the stomach byperistalsis. The gastroesophagealsphincter opens, and foodenters the stomach. Relaxedmuscles Circular musclescontract Bolus of food Longitudinal musclescontract Gastroesophagealsphincter closed Gastroesophagealsphincter opens Stomach Figure 23.13

17. Stomach: Gross Anatomy • Cardiac region (cardia) • Surrounds the cardiac orifice • Fundus • Dome-shaped region beneath the diaphragm • Body • Midportion

18. Stomach: Gross Anatomy • Cardiac region (cardia) • Surrounds the cardiac orifice • Fundus • Dome-shaped region beneath the diaphragm • Body • Midportion

19. Stomach: Gross Anatomy • Pyloric region: antrum, pyloric canal, and pylorus • Pylorus is continuous with the duodenum through the pyloric valve (sphincter) • Greater curvature • Convex lateral surface • Lesser curvature • Concave medial surface

20. Cardia Fundus Esophagus Muscularis externa Serosa • Longitudinal layer • Circular layer Body • Oblique layer Lumen Lesser curvature Rugae of mucosa Greater curvature Pyloric canal Pyloric antrum Duodenum Pyloric sphincter (valve) at pylorus (a) Figure 23.14a

21. Stomach: Gross Anatomy • Lesser omentum • From the liver to the lesser curvature • Greater omentum • Drapes from greater curvature • Anterior to the small intestine • The omenta have fat deposits and lots of lymph nodes. The immune cells and macrophages in the omenta police the peritoneal cavity. The omenta can wall off peritoneal infections.

22. Greater and Lesser Omentums

23. ANS nerve supply to stomach • Sympathetic via splanchnic nerves and celiac plexus • Parasympathetic via vagus nerve

24. Blood supply to Stomach • Celiac trunk – branches go to liver, stomach, spleen, pancreas • Veins of the hepatic portal system

25. Inferior vena cava (not part of hepatic portal system) Gastric veins Hepatic veins Spleen Inferior vena cava Liver Splenic vein Right gastroepiploic vein Hepatic portal vein Inferior mesenteric vein Superior mesenteric vein Small intestine Large intestine Rectum (c) The hepatic portal circulation. Figure 19.29c

26. Diaphragm Abdominal aorta L. gastric artery Inferior phrenic arteries R. gastric artery Common hepatic artery Hepatic artery proper L Celiac Trunk Celiac trunk Gastro- duodenal artery Splenic artery R R. gastroepiploic artery Middle suprarenal arteries L. gastroepiploic artery Intestinal arteries Middle colic artery Superior mesenteric artery R. colic artery Renal arteries Ileocolic artery Gonadal arteries Sigmoidal arteries Inferior mesenteric artery L. colic artery Superior rectal artery Lumbar arteries Median sacral artery Common iliac arteries (a) Schematic flowchart. Figure 19.24a

27. Liver (cut) Diaphragm Inferior vena cava Esophagus Celiac trunk Left gastric artery Common hepatic artery Stomach Hepatic artery proper Splenic artery Gastroduodenal artery Left gastroepiploic artery Right gastric artery Gallbladder Spleen Pancreas (major portion lies posterior to stomach) Right gastroepiploic artery Superior mesenteric mesenteric Duodenum Abdominal aorta (b) The celiac trunk and its major branches. The left half of the liver has been removed. Figure 19.24b

28. Falciform ligament Liver Gallbladder Spleen Stomach Ligamentum teres Greater omentum Small intestine Cecum (a) The Ligamentum Teres Hepatis is the remnant of the umbilical vein Figure 23.30a

29. Stomach: Microscopic Anatomy • Four tunics • Muscularis and mucosa are modified • Muscularis externa • Three layers of smooth muscle • Inner oblique layer allows stomach to churn, mix, move, and physically break down food

30. Liver Gallbladder Lesser omentum Stomach Duodenum Transverse colon Small intestine Cecum Urinary bladder (b) Figure 23.30b

31. 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 (a) Layers of the stomach wall (l.s.) Figure 23.15a

32. Stomach: Microscopic Anatomy • Mucosa • Simple columnar epithelium composed of mucous cells – they produce a cloudy two layer coat of alkaline mucus which the surface layer consists of a viscous-insoluble mucus that traps bicarbonate-rich fluid beneath it • The smooth lining is lined with dotted Gastric pits that lead into gastric glands that produce the various gastric juices

33. The cells forming the walls of the gastric pits are primarily mucous cells – but the gastric gland cells differ in the different regions of the stomach. • Cardia (entrance) and pylorus (exit) are primarily mucus secreting cells • Pyloric Antrum produce mucus and hormones (enteroendocrine cells) • Fundus and body – where most chemical digestion occurs produce the majority of stomach secretions

34. 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 Figure 23.15b

35. Gastric Glands • Cell types • Mucous neck cells (secrete thin, acidic mucus) • Parietal cells • Chief cells • Enteroendocrine cells

36. Pepsinogen Pepsin HCl Mitochondria Parietal cell Chief cell Enteroendocrine cell (c) Location of the HCl-producing parietal cells and pepsin-secreting chief cells in a gastric gland Figure 23.15c

37. Gastric Gland Secretions • Glands in the fundus and body produce most of the gastric juice • Parietal cell secretions • HCl •  pH 1.5–3.5 denatures protein in food, activates pepsin, and kills many bacteria • Intrinsic factor • Glycoprotein required for absorption of vitamin B12 in small intestine

38. Gastric Gland Secretions • Chief cell secretions • Inactive enzyme pepsinogen • Activated to pepsin by HCl and by pepsin itself (a positive feedback mechanism) • Chief cells also secrete insignificant amounts of gastric lipase

39. Gastric Gland Secretions • Enteroendocrine cells • Secrete chemical messengers into the lamina propria • Paracrines • Serotonin and histamine • Hormones • Somatostatin and gastrin

40. Mucosal Barrier • Layer of bicarbonate-rich mucus • Tight junctions between epithelial cells • Damaged epithelial cells are quickly replaced by division of stem cells – that reside where the gastric pits join the gastric glands. • The surface epithelia are replaced every three to six days

41. Homeostatic Imbalance • Gastritis: inflammation caused by anything that breaches the mucosal barrier • Peptic or gastric ulcers: erosion of the stomach wall • Most are caused by Helicobacter pylori bacteria • Go to GI Diseases PowerPoint

42. Bacteria Mucosa layer of stomach (b) H. pylori bacteria (a) A gastric ulcer lesion Figure 23.16

43. Digestive Processes in the Stomach • Physical digestion • Denaturation of proteins • Enzymatic digestion of proteins by pepsin (and rennin in infants) • Secretes intrinsic factor required for absorption of vitamin B12 • Lack of intrinsic factor  pernicious anemia • Delivers chyme to the small intestine

44. Regulation of Gastric Secretion • Gastric Secretion has three phases – (1) Cephalic (2) Gastric and (3) Intestinal. • Some are more stimulatory – Cephalic and Gastric and one is more inhibitory – Intestinal Phase • Neural (vagus and enteric plexus) and hormonal mechanisms control the secretions • Cephalic (reflex) phase: last just a few minutes prior to food entry into the stomach. It occurs even if you don’t actually get the food – if you desire the food and are not depressed or have a lack of appetite

45. Gastric Phase • Lasts approximately 3–4 hours after food enters the stomach • Stimuli for this phase is gastric distention, peptides, and low acidity • Gastric Distention activates stretch receptors and initiates both local (myenteric) reflexes and vagovagal – both stimulate acetylcholine release

46. Gastrin (1) • Gastrin is secreted by G-cells in the Pyloric Antrum in accordance with chemical stimuli and neural stimuli • The chemical stimuli for Gastrin secretion are partially digested proteins, caffeine, and rising alkaline pH. High acidity less than a pH of 2 inhibits Gastrin secretion • Gastric stimulates release of enzymes, also Histamine from the enterochromaffin cells – but its main targets are the Parietal cells in body of the stomach that secrete HCl- prodding them to secrete increased amounts of HCl

47. Gastrin (2) • When protein products enter the stomach, the pH generally rises due to the proteins buffering H+. • The rising pH stimulates Gastrin which causes HCl to spew out thus denaturing the proteins. The more proteins the more Gastrin. • As proteins are decomposed the acidity rises and Gastrin is inhibited

48. Gastrin (3) • In addition to G-cells being stimulated chemically – they are also stimulated neurally. The parasympathetic turns on secretion via acetylcholine from the Vagus and Sympathetic turns it off • The vagus was activated in the Cephalic Phase and Gastric Phase due to stomach distention • Emotional upset, fear, anxiety, and anything that triggers the fight and flight response turns off Gastric secretion.

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

50. Regulation and Mechanism of HCl Secretion • Three chemicals (ACh, histamine, and gastrin) stimulate parietal cells through second-messenger systems • All three are necessary for maximum HCl secretion • Antihistamines block H2 receptors and decrease HCl release