Digestive System
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Gross Structure of Digestive System Can you name all of the organs of the digestive system?
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Gross Structure of Digestive System contd. •
Two groups of organs 1.
Alimentary canal (gastrointestinal or GI tract)
•
Digests and absorbs food
•
Mouth, pharynx, esophagus, stomach, small intestine, and large intestine
2.
Accessory digestive organs
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•
Teeth, tongue, gallbladder
•
Digestive glands •
Salivary glands
•
Liver
•
pancreas
Gross Structure of Digestive System contd. 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 23.1
Gross Structure of Digestive System contd.
What is the name of the cavity that holds most of the digestive system organs and What is the name of the membrane that surrounds this cavity?
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Gross Structure of Digestive System contd. Abdominopelvic cavity Vertebra
Mesentery
Parietal peritoneum Visceral peritoneum Peritoneal cavity
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Organs (GI tract, liver, etc.)
Figure 23.5a
Peritoneum and Peritoneal Cavity • Visceral peritoneum on external surface of most digestive organs • Parietal peritoneum lines the body wall • Fluid lubricates mobile organs •
Mesentery is a double layer of peritoneum and serves as routes for blood vessels, lymphatics, and nerves AND holds organs in place and stores fat
• Retroperitoneal organs lie posterior to the peritoneum
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General Function of Digestive
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General Function of Digestive •
Six essential activities 1. Ingestion 2. Propulsion 3. Mechanical digestion 4. Chemical digestion 5. Absorption 6. Defecation
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General Function of Digestive contd. Ingestion Mechanical digestion
•
Iron and calcium are absorbed in duodenum
• Chewing (mouth) • Churning (stomach) • Segmentation (small intestine) Chemical digestion
Pancreatic NZs (and bile salts for fats and pepsin and HCL for protein and saliva for carbs) break down carbs, nucleic acids, fats and proteins
• In large intestine • Vitamin K and B vitamins from bacterial metabolism are absorbed
Small intestine Large intestine
Defecation Copyright © 2010 Pearson Education, Inc.
Food
In small intestine
Pharynx Esophagus Propulsion • Swallowing (oropharynx) • Peristalsis Stomach (esophagus, stomach, small intestine, large intestine) Absorption Lymph vessel
Blood vessel Mainly H2O Feces
Fat-soluble vitamins (A, D, E, and K) are carried by micelles and then diffuse into absorptive cells Water-soluble vitamins (vitamin C and B vitamins) are absorbed by diffusion or by passive or active transporters. Vitamin B12 binds with intrinsic factor, and is absorbed by endocytosis 95% of water is absorbed in the small intestine by osmosis
Ca2+ absorption is regulated by vitamin D and parathyroid hormone (PTH) Ionic iron is stored in mucosal cells with ferritin Na+ is coupled with absorption of glucose and amino acids
Anus Figure 23.2
How does food move along the GI tract?
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Figure 23.3
How does food move along the GI tract? From mouth
(a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally.
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Figure 23.3
GI tract regulatory mechanisms
What regulates digestion?
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GI tract regulatory mechanisms Nervous System and Hormones provide intrinsic and extrinsic controls •
Enteric nerve plexuses in the digestive tract (gut brain) initiate local reflexes in response to stimuli in the GI tract
•
The brain and spinal cord also respond to stimuli inside and outside the GI tract
•
Hormones from cells in the stomach and small intestine stimulate target cells in the same or different organs
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GI tract regulatory mechanisms contd. Receptors located in the GI tract also provide control •
Mechanoreceptors and chemoreceptors respond to stretch, changes in osmolarity and pH, and presence of substrate and end products of digestion
•
Initiate reflexes that activate or inhibit digestive glands and stimulate smooth muscle to mix and move lumen contents
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Stomach: Gross Anatomy contd.
• The autonomic nervous system (ANS) sends nerves to the stomach (sympathetic and parasympathetic divisions) to control the stomach activity The ANS will be studied next semester.
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Motility of the Large Intestine • Gastrocolic reflex • Initiated by presence of food in the stomach • Activates three to four slow powerful peristaltic waves per day in the colon (mass movements)
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A More Detailed Study of the Gross Structures of Digestive System
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Soft palate Palatoglossal arch Hard palate
Uvula
Mouth or Oral (buccal) cavity is lined with stratified squamous epithelium
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 23.7a
Gingivae (gums) Palatine raphe Hard palate Soft palate Uvula Palatine tonsil Sublingual fold with openings of sublingual ducts Vestibule Lower lip
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
(b) Anterior view Copyright © 2010 Pearson Education, Inc.
Figure 23.7b
Palate Hard palate: palatine bones and palatine processes of the maxillae Soft palate: fold formed mostly of skeletal muscle; closes off the nasopharynx during swallowing
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Epiglottis Palatopharyngeal arch Palatine tonsil Lingual tonsil Palatoglossal arch Terminal sulcus
have taste buds
Foliate papillae Circumvallate papilla Midline groove of tongue Dorsum of tongue Fungiform papilla Filiform papilla
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Figure 23.8
Salivary Glands Extrinsic salivary glands (parotid, submandibular, and sublingual) and intrinsic salivary glands scattered in oral mucosa Saliva cleanses the mouth, moistens and dissolves food chemicals, aids in bolus formation, and contains enzymes that begin the breakdown of starch Opens via 10–12 ducts into the floor of the mouth
Parotid duct opens into the vestibule next to second upper molar
Tongue Teeth
Parotid gland
Ducts of sublingual gland Frenulum of tongue Sublingual gland Mylohyoid muscle (cut) Anterior belly of digastric muscle (a)
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Submandibular gland Duct opens at the base of the lingual frenulum
Parotid duct Masseter muscle Body of mandible (cut) Posterior belly of digastric muscle Submandibular duct
Mucous cells (b)
Serous cells forming demilunes Figure 23.9
Composition of Saliva • 97–99.5% water, slightly acidic solution containing • 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
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Teeth • Primary and permanent dentitions are formed by age 21 • 20 deciduous teeth erupt (6–24 months of age) • Roots are resorbed, teeth fall out (6–12 years of age) as permanent teeth develop
• 32 permanent teeth • All except third molars (wisdom teeth)erupt by the end of adolescence Copyright © 2010 Pearson Education, Inc.
(b)
Deciduous teeth
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Permanent teeth Figure 23.10b
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)
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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 23.10a
Crown
Neck
Enamel Dentin Dentinal tubules Pulp cavity (contains blood vessels and nerves) Gingiva (gum) Cementum
Root
Root canal Periodontal ligament Apical foramen Bone
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Figure 23.11
Esophagus • The esophagus is muscular tube from laryngopharynx to stomach • It pierces diaphragm at esophageal hiatus (a hole in the diaphragm) • The esophagus attaches to the cardiac portion of the stomach and this point where it opens into the stomach is called the cardiac orifice • The esophagus is linned with stratified squamous epithelium like your skin and your mouth whereas the remainder of the GI tract beginning at the stomach is lined with columnar epithelium • Glands in the esophagus secrete mucus to aid in bolus (ball of food) movement • The muscle wall in the esophagus is skeletal muscle for the upper half and that means you can control it such as when you swallow, but then the lower half of the esophagus is smooth muscle which you cannot voluntarily control, it is an automatic reflex
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Stomach: Gross Anatomy Cardia Esophagus Muscularis externa • Longitudinal layer • Circular layer • Oblique layer Lesser curvature
Fundus
Body Lumen
Greater curvature Duodenum (a) Copyright © 2010 Pearson Education, Inc.
Pyloric Pyloric canal antrum Pyloric sphincter (valve) at pylorus Figure 23.14a
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 23.15c
Mucosal Barrier • Layer of bicarbonate-rich mucus • Tight junctions between epithelial cells • Damaged epithelial cells are quickly replaced by division of stem cells
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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 Copyright © 2010 Pearson Education, Inc.
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
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Duodenum, Pancreas and Liver Pancreatic juice - watery alkaline solution (pH 8) that neutralizes chyme from stomach (chyme is partially digested food). Pancreatic juice also contains electrolytes (primarily HCO3–) and enzymes (amylase, lipases, and nucleases and proteases)
Cystic duct Common hepatic duct Bile duct and sphincter Accessory pancreatic duct
Liver liver and gallbladder secrete bile essential for fat absorption
Tail of pancreas Pancreas Jejunum
Gallbladder
Hepatopancreatic sphincter Copyright © 2010 Pearson Education, Inc.
Duodenum
Main pancreatic duct and sphincter Head of pancreas Figure 23.21
Duodenum, Pancreas and Liver • The bile duct and main pancreatic duct • Join at the hepatopancreatic ampulla • Enter the duodenum at the major duodenal papilla • Are controlled by the hepatopancreatic sphincter
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Liver • Largest gland in the body • Four lobes—right, left, caudate, and quadrate
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Liver Function • Hepatocyte functions • Process bloodborne nutrients • Store fat-soluble vitamins • Perform detoxification • Produce ~900 ml bile per day
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Liver: Microscopic Anatomy • Liver lobules • Hexagonal structural and functional units • Filter and process nutrient-rich blood • Composed of plates of hepatocytes (liver cells) • Longitudinal central vein
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(a)
Lobule
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(b)
Central vein
Connective tissue septum Figure 23.25a, b
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
Portal vein
Hepatic macrophages in sinusoid walls
Bile duct Portal venule Portal arteriole
Portal triad
(c) Copyright © 2010 Pearson Education, Inc.
Figure 23.25c
Bile • Yellow-green, alkaline solution containing • Bile salts: cholesterol derivatives that function in fat emulsification and absorption • Bilirubin: pigment formed from heme • Cholesterol, neutral fats, phospholipids, and electrolytes
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Small Intestine: Gross Anatomy •
Major organ of digestion and absorption
•
2–4 m long; from pyloric sphincter to ileocecal valve
•
Subdivisions 1. Duodenum (retroperitoneal) 2. Jejunum (attached posteriorly by mesentery) 3. Ileum (attached posteriorly by mesentery)
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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 23.1
Requirements for Digestion and Absorption in the Small Intestine • Slow delivery of hypertonic chyme • Delivery of bile, enzymes, and bicarbonate from the liver and pancreas • Mixing
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Duodenum • The bile duct and main pancreatic duct • Join at the hepatopancreatic ampulla • Enter the duodenum at the major duodenal papilla • Are controlled by the hepatopancreatic sphincter
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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
Sigmoid colon
Vermiform appendix
Rectum
Anal canal (a) Copyright © 2010 Pearson Education, Inc.
External anal sphincter Figure 23.29a
Clinical Correlations
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Tooth and Gum Disease Dental caries (cavities) is the gradual demineralization of enamel and dentin which is caused by dental plaques which is sugar, bacteria, and debris that has adhered to your teeth. Acid from the bacteria (that is stuck to your teeth) dissolves calcium salts and proteolytic enzymes from the bacteria digest the organic matter (collagen, bone cells, etc.) of your teeth. This can be prevented by daily flossing and brushing.
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Tooth and Gum Disease • Gingivitis • Plaque calcifies to form calculus (tartar) • Calculus disrupts the seal between the gingivae and the teeth • Anaerobic bacteria infect gums • Infection reversible if calculus removed
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Tooth and Gum Disease • Periodontitis • Immune cells attack intruders and body tissues • Destroy periodontal ligament • Activate osteoclasts • Consequences • Possible tooth loss, promotion of atherosclerosis and clot formation in coronary and cerebral arteries Copyright © 2010 Pearson Education, Inc.
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
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Bacteria
Mucosa layer of stomach
(a) A gastric ulcer lesion
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(b) H. pylori bacteria
Figure 23.16
Histology of the GI Tract (Alimentary Canal)
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Histology of the GI tract contd. Four basic layers (tunics) for the entire tract including the stomach and intestines • Mucosa – lines the inside of the tract (lumen) with simple columnar epithelium and mucus-secreting cells, secretes mucus, secretes digestive enzymes and secretes hormones that control digestion; absorbs end products of digestion and protects against infectious disease and contains smooth muscle that produces local movement • Submucosa – contains dense connective tissue, blood and lymphatic vessels, lymphoid follicles (like tonsils), and nerve plexus • Muscularis externa - responsible for peristalsis, has an inner circular and outer longitudinal layer of smooth muscle, contains the myenteric nerve plexus, and in some regions the muscle layers for sphincters • Serosa – is essentially the peritoneum which is called visceral peritoneum because it is the peritoneum that surrounds the organs (stomach, intestines, etc.)
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Intrinsic nerve plexuses (such as the Myenteric plexus which controls GI tract motility)
Four basic layers (tunics) for the entire tract including the stomach and intestines Villi and microvilli and goblet cells are special features of small intestine
Mucosa • Epithelium • Lamina propria • Muscularis mucosae Submucosa Muscularis externa
• Longitudinal muscle • Circular muscle Nerve Artery Vein
Serosa
Lymphatic vessel
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Duct of gland outside alimentary canal such as the pancreas
Lumen Figure 23.6
Microvilli
(b) Copyright © 2010 Pearson Education, Inc.
Absorptive cell Figure 23.3b
Glands in submucosa
Lumen Nerve Artery Vein Gland in mucosa Lymphatic vessel Copyright © 2010 Pearson Education, Inc.
Mucosa-associated lymphoid tissue (like a lymph node or tonsil) Figure 23.6
Structural Modifications • Villi • Motile fingerlike extensions (~1 mm high) of the mucosa • Villus epithelium • Simple columnar absorptive cells (enterocytes) • Goblet cells
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Structural Modifications • Microvilli • Projections (brush border) of absorptive cells • Bear brush border enzymes
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Surface epithelium
Mucosa
Lamina propria
Submucosa (contains submucosal plexus) Muscularis externa (contains myenteric plexus)
Muscularis mucosae Oblique layer Circular layer Longitudinal layer
Serosa
Stomach wall
(a) Layers of the stomach wall (l.s.) Copyright © 2010 Pearson Education, Inc.
Figure 23.15a
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 23.22b
