Digestive System

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  • 1. DIGESTIVE SYSTEM
  • 2. BASIC DIGESTIVE PROCESSES
    • Ingestion of food via the mouth
    • Movement of food via the pharynx & esophagus
    • Chemical & mechanical preparation of food in the stomach
    • Completion of chemical digestion and absorption of food in the small intestine
    • Elimination of wastes via the large intestine and rectum
  • 3. Entire digestive tract lined with mucous membrane
  • 4. GASTROINTESTINAL WALL-4 LAYERS
    • Tunica mucosa-innermost-itself composed of 3 layers: a. epithelial layer b. lamina propria-areolar CT, blood vessels, lymphatic nodules, small glands
    • c. muscularis mucosae-thin layer of smooth muscle
    • Tunica submucosa-thick layer of CT  blood & lymph vessels, nerves, glands
    • Tunica muscularis-double layer of smooth muscle tissue, with sphincters forming at several points along the tract
    • Tunica serosa (adventitia in esophagous)-outermost Ct, covered by visceral peritoneum everywhere but esophagous & rectum
  • 5. Plexuses innervate muscle & secretory cells of the gi tract
  • 6. MOUTH=ORAL CAVITY
    • Lined with stratified squamous epithelium
    • Lips=outer surface
    • Vestibule=space between teeth & lips or cheeks (all aid in mastication)
    • Roof of the mouth: anterior hard palate & posterior soft palate
    • Hard palate-bordered by upper teeth, formed by portions of palatine bones & maxillae. Provides hard surface against which tongue crushes & softens food
  • 7.
    • Soft palate-extends between oral & nasal portions of the pharynx. Functions to close off the nasopharynx during swallowing, preventing food from entering into the nasal cavity
    • Tongue=floor of the mouth. Skeletal muscle covered with mucous membrane. Used in manipulating food, swallowing, speaking. Contains papillae in which taste buds are present
  • 8. TEETH
    • Protrude from alveoli (sockets) located along the mandible and maxillae. Surrounded by gums (=gingivae)  stratified squamous + dense fibrous CT
    • 2 sets during human lifespan:
    • Deciduous (milk)teeth-20
    • Permanent-32
    • Heterodont dentition-4 types of teeth:
    • Incisors-chisel-shaped, for cutting
    • Canines(cuspids)-tearing
    • Premolars (bicuspids)-for crushing/grinding
    • molars-- " " "
  • 9.  
  • 10.  
  • 11. SALIVARY GLANDS
    • Collectively secrete 1000-2000 ml saliva into the mouth daily. 3 distinct glands:
    • Parotid-predominantly serous acini
    • Submandibular-mixed acini
    • Sublingual-predominantly mucous acini
  • 12.  
  • 13.  
  • 14. OMENTA
    • Coelom effectively subdivided into greater & lesser sacs by the stomach & 2 special mesenteries: greater & lesser omenta
    • Greater omentum-extensive folded membrane, extending from the greater curvature of the stomach to the back wall & dowm to the pelvic cavity;  considerable fat ("pot belly" with excess)  hangs down like a fatty apron over abdominal organs to protect & insulate.
    • Filled with plasma cells & other WBC. Can wraps around site of inflammation to wall off infectious organisms as with ruptured appendix, peptic ulcer, diverticulitis, etc
  • 15.  
  • 16.  
  • 17. ESOPHAGUS
    • Superior esophageal sphincter-closed by passive elastic tension in the wall of the esophagus when esophageal muscles relaxed
    • Lower esophageal sphincter=cardiac sphincter  Last 4 cm of esophagus. Relaxes only long enough to allow food & liquids to pass into stomach-remains contracted otherwise to prevent food & HCl from being forced back into esophagus with  pressure in abdomen (breathing cycle, stomach contractions, late pregnancy)
    • If lower esophageal sphincter doesn't close  heartburn
  • 18.  
  • 19. STOMACH
    • J-shaped sac, 25 cm long, 1.5 l capacity
    • Bolus enters through cardiac orifice, exits through pyloric orifice-both controlled by sphincters
    • Empty stomach inner mucous membrane
    • Branching wrinkles=rugae-flatten as the stomach fills
    • Stomach lined with simple columnar epit indented by 3.5 x 10 6 gastric pits, 3-8 tubular gastric glands extend from each pit
  • 20. 4 regions of the stomach 1 3 3 4 1 2
  • 21. CELL TYPES IN THE STOMACH
    • Surface mucous cells-line the lumen & gastric pits  secrete alkaline mucus. Shed into the lumen
    • Neck mucous cells-line the cardiac, pyloric, & fundic glands  secrete a more neutral mucus than at the surface. Replace lost surface cells
    • Parietal (oxyntic) cells-eosinophilic, oval
    • Secrete HCl & gastric intrinsic factor (Vit B 12 absorption)
    • Chief cells-Lg, pyrimidal. Secrete pepsinogen
    • Enteroendocrine cells-secrete hormones
    • Undifferentiated cells-Replace others when they die (500,000 cells shed/d  renewed every 3 d
  • 22.  
  • 23. SMALL INTESTINE
    • 6 m (20')long-completion of digestion & absorption of digestion products (EtOH absorbed in the stomach)
    • Intestinal mucosa adapted for absorption:
    • Plicae circulares-circular folds of the inner wall to  absorptive surface area
    • Villi-fingerlike projections at the mucosal surface to  surface area 6000X
    • Microvilli-projections from epit cells of the villi
  • 24. 1 2 3 3 intestinal regions
  • 25. INTESTINAL FEATURES
    • See infolding of epit between bases of villi forming tubular intestinal glands=crypts of Lieberk ühn-extend into lamina propria
    • Ea villus  blood capillaries & a lymph capillary=lacteal
    • Brunner's glands=duodenal submucosal glands-secrete viscid, alkaline mucus to neutralize acidic chyme
    • Peyer's patches=aggregated lmph nodules along the gi tract, esp in ileum
  • 26.  
  • 27.  
  • 28. CELL TYPES IN THE SMALL INTESTINE
    • Columnar absorptive cells-produce enzymes for terminal digestion of CHO & protein. Involved in the absorption of CHO, proteins, & fats from the intestinal lumen
    • Undifferentiated cells-in depths of crypts  differentiate to replace other cell types as needed. Replaced cells disintegrate into intestinal lumen  discharge digestive enzymes
  • 29.
    • 3. Mucous goblet cells-indepths ofcrypts & migrate upward  accumulate mucous until swelling into goblet shape  release mucus & die, esp abundant in duodenum
    • (NOTE:mucous glands inhibited by sympathetic stimulation  superior portion of duodenum prone to peptic ulcers caused by nervous stress
    • 4. Paneth cells-deep within intestinal crypts
    • Secrete peptidases, lysozyme
    • 5. Enteroendocrine cells-same as in stomach. Synth of > 20 gastrointestinal hormones
  • 30. LARGE INTESTINE
    • Aka colon=large bowel-Forms a rectangle that frames the tightly-packed sm intestine
    • Cecum-cul-de-sac pouch, 6cm long
    • Veriform appendix -opens into cecum, 2cm below ileocecal valve  bacteria & indigestible material easily trapped  inflammation (appendicitis)
    • 2. Ascending colon-upward from cecum, bends @ R angle under liver @ R colic (hepatic) flexure
  • 31.
    • 3. Transverse colon-extends across abdominal cavity R  L, makes R angle downward turn @ spleen=L colic (splenic) flexure
    • 4. Descending colon-descends down to the rim of the pelvis
    • 5. Sigmoid colon-S-shaped, travels transversely across the pelvis to the middle of the sacrum, continues to the rectum
  • 32.  
  • 33.  
  • 34. DISTINCTIVE FEATURES OF THE LARGE INTESTINE
    • An incomplete layer of longitudinal muscle forms 3 separate bands of muscle=taeniae coli along the full length of the colon
    • Since taeniae coli don't cover all of the intestinal wall, the wall becomes puckered with bulges=haustra
    • See epiploic appendages=fat-filled pouches which form at points where visceral peritoneum attached to taeniae in the serous layer
  • 35.  
  • 36. MICROSCOPIC ANATOMY OF THE LARGE INTESTINE
    • Numerous goblet cells in the mucosal layer
    • No villi or plicae circulares  smooth absorptive surface
    • Lamina propria & submucosa  lymphoid nodules=gut-associated lymphoid tissue
  • 37. RECTUM
    • 15 cm, extends from the sigmoid colon to the anus. Retroperitoneal-no mesentery
    • Mucosa & muscularis form shelves=plicae transversalis ,  vein network=hemorrhoidal plexus (may develop hemorrhoids)
    • Anal canal-4cm, compressed by anal muscles. Upper part  5-10 anal (rectal) columns of mucous membranes united by folds=anal valves. Anus=slit opening to outside
    • Anus & anal canal open only during defecation- otherwise closed by involuntary internal anal sphincter & voluntary external anal sphincter
  • 38. ACCESSORY DIGESTIVE ORGANS
    • Pancreas-12-15 cm, lies transversely across posterior abdominal wall. Retroperitoneal.
    • 3 parts: Head, body, tail
    • Exocrine cells arranged in groups=acini, ea with central lumen connecting to the main pancreatic duct
  • 39.  
  • 40.  
  • 41. LIVER
    • Lg, compound tubular gland. Ave 3 lb. Red
    • Covered by network of CT=Glisson's capsule. Located under diaphragm, mostly on R side. Held in place by peritoneal attachments & by intra-abdominal pressure created by tonus of abdominal wall musculature
    • Divided into 2 main lobes by falciform ligament, R lobe 6X larger, further subdivided into quadrate & caudate lobes
  • 42.  
  • 43.
    • Bile ducts=bile canaliculi formed by bile capillaries that unite after collecting bile from liver cells  drain into R & L hepatic ducts  converge with cystic duct from the gallbladder to form the common bile duct
    • Common bile duct joins with main pancreatic duct  enlarges into hepatopancreatic ampulla  joins duodenal papilla which opens into the 2 nd part of the duodenum
  • 44.  
  • 45. MICROSCOPIC ANATOMY OF THE LIVER
    • Lobules=functional liver units, ea  branch of the hepatic vein. Hepatocytes within lobules  arranged in platelike layers, one cell thick
    • Sinusoids-between radiating rows of hepatocytes  transport blood from the portal vein & hepatic artery. Walls lined with endothelial cells and Kupffer cells
  • 46.  
  • 47. GALLBLADDER
    • Small, pear-shaped, saclike. In a depression under quadrate lobe of liver
    • Layers of the gallbladder wall:
    • outer, serous peritoneal coat
    • middle muscular layer
    • inner mucous membrane, continuous with the linings of the ducts-secretes mucin & actively transports H 2 O and salts out  bile more conc than in liver
  • 48.  
  • 49. PHYSIOLOGY OF DIGESTION
    • Saliva: > 1 liter secreted/d
    • 25-35% from parotid glands
    • 60-70% from submandibular glands
    • 3-5% from sublingual glands
    • 99% H 2 O, 1% electrolytes & proteins, incl mucin & salivary amylase (breaks starch down into maltose & dextrin  works better on cooked starch because cooking disrupts cellulose of plant cell walls
  • 50.
    • Saliva also : -cleanses the mouth &  cellular & food debris on teeth
    • -keeps soft parts of the mouth supple
    • -buffers acidity in the oral cavity
    • (high HCO 3 - , pH 6.35-6.85)
    • -  IgA & lysozyme
  • 51.
    • Continuous secretion of saliva stimulated by parasympathetic nerve endings terminating in salivary glands
    • Other factors stimulating saliva secretion: food in mouth & stomach, chewing, smell/taste/sight/thought of food
    • Unpleasant stimuli related to food (eg rotten smells) inhibit parasympathetic system. Sympathetic stimulation (eg stress)  mouth & throat become dry
  • 52. DIGESTIVE MOVEMENTS WITHIN THE STOMACH
    • Food enters the cardiac orifice  slow, peristaltic mixing waves start in the smooth muscle pacemaker cells in the fundus and body= basic electrical rhythm
    • As pyloric region fills, strong peristaltic waves churn chyme & propel it toward the pyloric orifice= pyloric pump
    • As the stomach empties, peristaltic waves move farther up the body, ensuring that all chyme enters the pyloric region
  • 53.  
  • 54. REGULATION OF GASTRIC EMPTYING
    • Fuller stomach  stronger force of pyloric pump.
    • Distension of stomach  vagus nerve stimulated  strengthens peristaltic waves & causes gastrin secretion (promotes gastric secretion & motility). Pyloric sphincter relaxes  chyme enters duodenum.
    • Secretagogues (EtOH, caffeine, partially digested protein) all stimulae gastric emptying
  • 55.
    • Gastric emptying inhibited by duodenum
    • Neural response-enterogastric reflex-mediated by intrinsic nerve plexuses & autonomic nerves   stomach motility & gastric secretions
    • Hormonal response-release of enterogastrones   peristaltic contractions & gastric motility
  • 56. GASTRIC JUICE
    • Colorless fluid, > 1.5 l secreted/d,  HCl, mucus, enzymes (gastric lipase, pepsin (for proteins  peptones), rennin (in children, caseinogen  casein)
  • 57. DIGESTIVE MOVEMENTS IN THE SMALL INTESTINE
    • Segmenting contractions-divide intestine into segments by sharp contractions of areas of circular smooth muscle in the intestinal wall. Duodenal segmentation caused by distension, ileal segmentation caused by gastrin
    • Peristaltic contractions-propulsive  weak, repetitive waves which propel chyme through the sm intestine into colon. Regulated by motilin. Slow: ea wave takes 100-150 min from beginning to end of sm intestine
  • 58. DIGESTIVE ENZYMES OF THE SMALL INTESTINE
    • 1.5 l intestinal juice secreted/d  just dilute salt & mucus (enzymes from disintegration of brush border)
    • Digestion in lumen via action of pancreatic enzymes & bile entering duodenum
    • Bile: emulsifies fat & activates lipase
    • Pancreatic enzymes: amylase-digests starch into disaccharides & monosaccharides
    • proteases: trypsin, chymotrypsin, carboxypeptidases, aminopeptidases, etc
  • 59.
    • BILE-secreted by the liver. Alkaline,  H 2 O, HCO 3 - , bile salts & pigments, cholesterol, mucin, lecithin, bilirubin.
    • 1 liter bile secreted/d. Most reabsorbed by a special active transport mechanism in the terminal ileum  returned to the liver
  • 60. INTESTINAL MICROVILLI
    • Have special actin-stiffened hairlike projections=brush border  3 categories of enzymes:
    • Enterokinases: trypsinogen  trypsin
    • Disaccharidases: Disaccs into monosaccs (eg lactase, sucrase, maltase)
    • Aminopeptidases-aid enterokinase by breaking peptones into amino acids
  • 61. ABSORPTION OF NUTRIENTS
    • Monosaccharides like glucose & galactose taken up into intestinal epit cells by Na + cotransport (secondary active transport), fructose by passive transport.
    • Most amino acids taken up by active transport mech similar to that for glucose (different carriers for basic, acidic, & neutral amino acids-basic come in by passive transport). Di- & tripeptides split into constituent amino acids once inside the intestinal epit cells
  • 62.  
  • 63.  
  • 64. LIPID ABSORPTION & TRANSPORT
    • Bile salts surround fatty acids & glycerol to form micelles (emulsification)
    • Micelles attach to intestinal epit cell membrane  fatty acids & glycerol diffuse into the cell
    • Once in, triglycerides resynthesized & are then coated with proteins forming chylomicrons
    • Chylomicrons enter the lacteals & are transported through lymph
  • 65.  
  • 66. ABSORPTION OF WATER
    •  9 l H 2 O enters gi tract/d. 92% absorbed in sm int, 6-7% absorbed in lg int. Water can move either way in sm int: when chyme is dilute  H 2 O absorbed across intestinal wall into the blood; when chyme is conc  H 2 O moves into the lumen of the sm int.
    • Osmotic P in sm int  as nutrients absorbed & H 2 O moves from the intestine into the circulation
  • 67.  
  • 68. ABSORPTION OF VITAMINS
    • H 2 O-soluble vits (most B vits, vit C) absorbed by facilitated transport
    • Vit B 12 requires gastric intrinsic factor for absorption in terminal ileum
    • Fat-soluble vits (A,D,E, &K) absorbed with fat
  • 69. FUNCTIONS OF THE LARGE INTESTINE
    • 100-500 ml chyme enters/d, only 1/3 excreted as feces  remainder (mostly H 2 O) absorbed in ascending or transverse colons  avoids dehydration
    • Bacteria (living & dead) account for 25-50% dry weight of feces.
    • Cells in crypts of Lieberk ühn secrete alkaline mucus  neutralizes H + produced by bacteria & lubricates lumen for the passage of feces
  • 70. FORMATION OF FECES
    • 150 g feces/d (100 g H 2 O + 50 g solids)
    • Brown due to bilirubin; dark if  blood or from foods high in Fe, pale if high fat content
    • Odor due to indole, skatole, H 2 S  result from decomposition of undigested food residue, unabsorbed amino acids, dead bacteria, cell debris
  • 71. MOVEMENTS OF THE LARGE INTESTINE
    • 1 ° motility  haustral contractions  depend on the slow rhythmicity of smooth muscle cells
    • 3-4 X/d (following meals) motility  markedly=mass movements  drive feces into the descending colon-produced by the gastrocolic reflex
    • When feces pushed into the rectum  the defecation reflex triggered
  • 72. METABOLIC FUNCTIONS OF THE LIVER
    • Deamination
    • Formation of urea
    • Synth of plasma proteins, fetal RBC, fibrinogen. Removal of bilirubin
    • Amino acid synth
    • Conversion of fructose & galactose into glucose
    • Fatty acid oxidation
    • Cholesterol & lipoprotein synth
    • Detoxification rxns
    • Vitamin A synth
    • Production of metabolic heat
  • 73.  
  • 74.  
  • 75.  
  • 76.