BIOL1410 Lecture Notes - Digestive System (2024) PDF

BIOL1410 Lecture Notes – Fall 2024 Unit 15: Digestive System UNIT 15: DIGESTIVE SYSTEM A) Digestive System: Overview — includes: 1) Gastrointestinal (GI) tract — tube from...

BIOL1410 Lecture Notes – Fall 2024 Unit 15: Digestive System UNIT 15: DIGESTIVE SYSTEM A) Digestive System: Overview — includes: 1) Gastrointestinal (GI) tract — tube from mouth to anus 2) Accessory organs — teeth, tongue, salivary glands — pancreas — liver, gall bladder — digestive system processes: 1) Ingestion — food into oral cavity 2) Digestion — large molecules broken down into smaller molecules — 2 types: a) mechanical — physical breakdown & motility — e.g. chewing b) chemical — enzymes and acid secretions 3) Absorption — end products of digestion enter blood or lymph 4) Defecation — elimination of undigested material B) Digestive System: Oral Cavity & Pharynx 1) Oral Cavity: — lined by a mucosa (mucous membrane) made of a stratified squamous epithelium and lamina propria — includes: a) lips b) cheeks c) palate i. hard palate — 2 maxillae and 2 palatine bones ii. soft palate (posterior to hard palate) — skeletal muscle — posterior projection is uvula which rises to close the nasopharynx when swallowing 1  BIOL1410 Lecture Notes – Fall 2024 Unit 15: Digestive System d) tongue — attached to hyoid bone — skeletal muscle — projections of mucosa = papillae (taste buds) 2) Salivary Glands: — 3 pairs: a) parotid — inferior and anterior to ears — mumps = inflammation of 1 or both parotids b) submandibular — floor of mouth c) sublingual — below tongue on floor of mouth — saliva o 99.5% water o 0.5% solutes (e.g. enzymes) 3) Dentition (teeth): — in maxillae and mandible — child dentition 1) primary dentition – deciduous (“baby”) teeth — adult dentition 1) secondary dentition – permanent teeth Classification Child Adult (in each quadrant) (in each quadrant) Central Incisor 1 1 Lateral Incisor 1 1 Canine 1 1 Premolars 0 2 Molars 2 3 Total Teeth 20 32 (= # in each quadrant x 4) — Tooth Structure: a) crown — above the gum — dentin forms majority of tooth — enamel overlay is acellular, highly calcified – hard! b) root — dentin with a cementum overlay NOTE: dentin, enamel, & cementum is similar to bone, but avascular c) neck — enamel and cementum boundary (gums) d) periodontal ligaments — attach root to bones e) root canal extends to pulp cavity — contains connective tissue, blood/lymph vessels, and nerves 2  BIOL1410 Lecture Notes – Fall 2024 Unit 15: Digestive System 4) Oropharynx and Laryngopharynx: — only muscularis externa (skeletal muscle) and stratified squamous epithelium C) Gastrointestinal Tract Histology (Esophagus to Rectum) — 4 basic layers: 1) Mucosa — 3 layers: a) epithelium with numerous goblet cells — stratified squamous: esophagus, anal canal — simple columnar: stomach, small and large intestines, rectum b) lamina propria (areolar connective tissue) — contains blood, lymph vessels, lymph nodules/tissues (immune) c) muscularis mucosa — smooth muscle - allows movement of mucosa 2) Submucosa — areolar CT — contains: blood, lymphatic vessels, and a network of nerve cells 3) Muscularis externa — two layers of smooth muscle separated by a second network of nerve cells o inner circular layer (contraction constricts the lumen) o outer longitudinal layer (contraction shortens gut length) o contractions controlled by the nerve network cause motility (mixing and movement) 4) Serosa or adventitia Serosa is a double walled membrane — cavity = lumen D) Peritoneum — serous membrane that lines the abdominopelvic cavity and most of the abdominal organs — structure of serosa: 1) visceral peritoneum (against organ wall) 2) parietal peritoneum (against abdominal cavity wall) 3) peritoneal cavity (space between parietal and visceral peritoneum) filled with serous fluid that minimizes friction between parts of the digestive tract during motility — specializations of the peritoneum: 1) omenta — folds of serosa between organs made of a sheet of 2 fused layers of visceral peritoneum — contains blood/lymph vessels and nerves — examples: a) greater omentum (“fatty apron”) — connects stomach to transverse colon 3  BIOL1410 Lecture Notes – Fall 2024 Unit 15: Digestive System — forms large fold that hangs down over transverse colon and small intestine b) lesser omentum — connects liver to stomach 2) mesentery — fold of serosa between the posterior abdominal cavity wall and the small/large intestine made of a sheet of 2 fused layers of parietal peritoneum — entry and exit point for blood vessels, nerves, and lymphatic vessels supplying digestive organs — Retroperitoneal organs — located posterior to the parietal peritoneum — peritoneum lines only one side of the organ — e.g. pancreas, duodenum o anterior surface is covered by parietal peritoneum o posterior surface is covered by adventitia that connects the organ to the body wall D) Digestive System: Esophagus to Anus 1. Esophagus: — posterior to trachea — passes through the diaphragm entering into the abdominal cavity — histology of esophagus transitions from superior to inferior: a) muscularis externa — upper 1/3 = skeletal muscle — middle 1/3 = skeletal and smooth muscle — lower 1/3 = smooth muscle b) the outer layer is adventitia within thoracic cavity and serosa within the abdominal cavity 2. Stomach: — Stores, partially digests, and regulates emptying of chyme (food and gastric juices) into the small intestine — has 4 regions: a) cardiac region (cardia): — attached to inferior esophagus b) fundus — superior to esophageal entrance c) body middle portion d) pyloric region (pylorus) — inferior portion of the stomach — the pyloric sphincter regulates release of stomach contents into small intestine — unique histological features of the stomach: 1) mucosal surface: a) formed exclusively by mucous cells b) has millions of gastric pits (invaginations of the epithelium) that are connected to underlying gastric glands — gastric glands are exocrine glands that secrete gastric juice into the gastric pits where it then enters the lumen of the stomach — gastric glands contain the following cell types: 4  BIOL1410 Lecture Notes – Fall 2024 Unit 15: Digestive System a) goblet cells b) chief cells — secrete enzymes for protein and fat digestion c) parietal cells — secrete hydrochloric acid (HCl) which lowers the pH of the stomach d) G cells (enteroendocrine cells) — secrete a hormone into the blood that regulates activity of parietal cells and other digestive processes. — rugae: are folds of the mucosa and submucosa due to contraction of muscularis mucosa o visible when stomach is empty o allows expansion of stomach without tearing — muscularis externa o function = churning chyme o 3 layers instead of 2: i. inner oblique ii. middle circular iii. outer longitudinal 3. Small Intestine: — pyloric sphincter to ileocaecal valve — where most food digestion/absorption occurs — histology: simple columnar epithelium 3 main cell types: a) enterocytes: simple columnar cells (absorptive cells that form most of the mucosa) b) goblet cells: secrete mucus c) enteroendocrine cells: a) located within intestinal glands b) secrete hormones into the blood that help regulate digestive processes — 3 segments: a) duodenum — first fold (short) – is retroperitoneal — extra glands here secrete alkaline mucous to protect against stomach acid — ducts of accessory organs (liver, gall bladder, pancreas) enter the digestive system here b) jejunum — middle section c) ileum — attached to caecum (part of large intestine) — has groups of lymph nodules called Peyer’s patches o prevent infection of small intestine and prevent bacteria from entering blood — segments specialized to increase absorption surface area: a) plicae circulares – submucosa thrown into large folds b) villi – projections of mucosa into lumen of small intestine — contains: o blood capillaries o lacteals (are lymphatic capillaries that absorb fats) c) microvilli – projections of the enterocyte cell membranes that extend into the lumen of the small intestine, forming a fuzzy “brush border” on the surface of the mucosa 5  BIOL1410 Lecture Notes – Fall 2024 Unit 15: Digestive System 4. Large Intestine: — ileocaecal valve to anus — basic functions: a) absorption of water, electrolytes, vitamins b) formation and temporary storage of feces — histology: mucosa has a smooth surface with no folds or villi — consists of: a) caecum – connected to ileum by ileocaecal valve b) appendix c) colon — parts: i. ascending colon (right side of abdominal cavity) ii. hepatic flexure iii. transverse colon iv. splenic flexure v. descending colon (left side of abdominal cavity) vi. sigmoid colon — longitudinal layer of muscularis externa is reduced to three bands of muscle called teniae coli o contraction of the teniae coli forms pouches called haustra d) rectum — no taeniae coli e) anal canal — last segment of large intestine, but external to abdominopelvic cavity — histology: mucosa transitions to stratified squamous epithelium — opening and closing of the inferior anal canal during defecation is controlled by two anal sphincters: i. internal anal sphincter (smooth muscle) ii. external anal sphincter (skeletal muscle – voluntary control) — opening at inferior end of anal canal is the anus D) Accessory Organs of the Digestive Tract — 3 organs that produce exocrine and/or endocrine secretions involved in digestive processes: a) Pancreas – retroperitoneal — parts: head, body, tail — contains: i. exocrine glands that secrete pancreatic juice (digestive enzymes and alkaline fluid) into the duodenum 1 acinar cells (most of pancreas) — secrete digestive enzymes into ducts 2 duct cells — secrete alkaline fluid to neutralize stomach acid ii. endocrine glands — are formed by the Islets of Langerhans (in between clusters of acinar cells) — secrete hormones: insulin (from beta cells) and glucagon (from alpha cells) that regulate blood sugar levels 6  BIOL1410 Lecture Notes – Fall 2024 Unit 15: Digestive System b) Liver — 4 lobes: o right, left, quadrate, caudate — liver cells are called hepatocytes — performs many metabolic and digestive roles, e.g.: i. processes, modifies and detoxifies absorbed material from GI tract before being transported to the rest of the body ii. produces bile which aides in fat digestion c) Gall Bladder — muscular sac attached to inferior surface of liver — general histology similar to GI tract histology, but no submucosa and has rugae (when empty) for expansion — stores and concentrates bile — contraction releases bile into duodenum E) Duct System of Accessory Organs — accessory organs (pancreas, liver, gall bladder) are connected to the duodenum by a series of ducts F) Abdominopelvic Quadrants 1. Right Upper Quadrant includes a. liver b. gall bladder c. duodenum d. head of pancreas e. right kidney and right adrenal gland f. hepatic flexure of colon 2. Right Lower Quadrant a. caecum b. appendix c. ascending colon d. right ovary/uterine tube e. right ureter 7  BIOL1410 Lecture Notes – Fall 2024 Unit 15: Digestive System 3. Left Upper Quadrant a. stomach b. spleen c. left lobe of liver d. body of pancreas e. left kidney and left adrenal gland f. splenic flexure of colon g. parts of transverse and descending colon 4. Left Lower Quadrant a. descending colon b. sigmoid colon c. ovary/uterine tube d. left ureter G) Lower GI Tract Blood Circulation — Portal System = blood vessels that are connected to capillary beds on both ends (i.e. blood does not return to the heart before moving to a second capillary bed) G) Related Medical Conditions 1. Gastroesophageal reflux disease (GERD) – stomach contents flow into esophagus 2. Appendicitis – inflammation of the appendix, causes pain in right lower quadrant 3. Cholecystitis – inflammation of gall bladder often due to blockage of cystic duct with gall stone; pain in right upper quadrant 8