Digestive System

Chapter 23

Overview• Alimentary canal or gastrointestinal (GI)

tract

• Accessory digestive organs

• Food moved by peristalsis

• Regulated by sphincters

• Processing time varies

Digestive Processes

• Ingestion: act of eating• Propulsion: moving food through GI tract– Deglutition and peristalsis

• Digestion: break down of food– Mechanical uses chewing, mixing, and churning– Chemical uses enzymes

• Absorption: nutrients, vitamins, minerals, and H2O into blood

• Defecation: elimination of undigested materials

Peritoneum

• Serous membrane of the abdominal cavity– Review: what are components?– Mesenteries fuse parietal to visceral

• Routes for vascular, lymph, and nervous supply to viscera• Holds organs in place and stores fat

• Retroperitoneal organs lie outside peritoneum– Includes pancreas, kidneys, parts of large intestine

• Intraperitoneal organs stay w/i cavity– Includes stomach, small and parts of large intestine, liver

• Blood supply– Celiac trunk and mesenteric arteries to digestive viscera– Hepatic portal to liver for storage and processing

http://www.vivo.colostate.edu/hbooks/pathphys/misc_topics/peritoneum.html

Basic Histology• 4 tunics line entire GI tract

– Mucosa from mouth to anus• Epithelium is simple columnar w/goblet cells• Lamina propria is areolar CT a nd MALT (chpt 20 review)• Muscularis mucosae is smooth muscle

– Submucosa• Areolar CT, blood, lymph, and nerves

– Muscularis externa• Smooth muscle in circular (inner) and longitudinal (outer) layers

– Serosa (visceral peritoneum) • Areolar CT w/ simple squamous layer

• Predominant tissue type facilitates processing role• Fig 23.6

Oral Cavity: Mouth• Stratified squamous epithelium• Lips (orbicularis oris) and cheeks (buccinators)

– Mastication and forming words/pronunciation• Palate

– Hard (palatine process of maxilla and palatine bones)– Soft (skeletal muscle) with hanging uvula

• Tongue (skeletal muscle)– Food manipulation (bolus), taste, and speech production– Tastebuds

• Filiform papillae: not tastebuds, roughen tongue surface• Fungiform papillae: mushroom shaped w/ tastebuds on top• Circumvallate papillae: circular w/tastebuds on sides

Oral Cavity: Salivary Glands• Parotid, submandibular, sublingual, and buccal

– Serous and mucus cells produce saliva components• Saliva

– Lysozymes, IgA, and defensins cleanse and protects mouth from microorganisms

– Mucin and water (97 – 99%) to moisten and dissolve food– Salivary amylase breaks down carbs

• Control– Intrinsic continuously secrete to keep mouth moist– Extrinsic in response to food; sight, smell & thought too– SNS inhibits causing dry mouth, xerostomia

• Halitosis and caries from food particles accumulating and bacteria flourishing

Oral Cavity: Teeth

• Chew, tear and grind food to initiate breakdown• Deciduous, or milk teeth, form 1° dentition (20)• Permanent teeth from permanent dentition (32*)– Types

• Incisors cut and nip (8, 2 pairs top and bottom)• Canines tear and pierce (4, 1 pair top and bottom)• Premolars grinding and crushing (8, 2 pairs top and bottom)• Molars see premolars (12, 3* pairs top and bottom)

– Dental formula • (__I x __C x __M / __I x __C x __M) x 2 =

• Impacted teeth when remain in jaw

Oral Cavity: Teeth (cont.)• Crown exposed above gingiva– Covered in enamel, hardest material in body

• No mitotic activity to fix cracks and decay• Overlays bone-like dentin

• Root anchored in jaw by periodontal ligament– Gomphosis in alveolar margin– Number varies by tooth type and location– Covered in cementum, calcified CT

• Pulp cavity become root canal w/ apical foramen– Damage to nerve and blood supply facilitates infection– Root canal therapy to sterilize before filling

Tooth and Gum Disease

• Caries or cavities when enamel and dentin decay– Dental plaque adheres and acidity increases

• Ca2+ salts removed, organics dissolved by bacteria• Brushing and flossing daily prevent

– Calculus, or tarter causes gingiva separation• Gingivitis, an early indication, causes bleeding and swelling

• Periodontal disease when calculus untreated– Immune system attacks bacteria and periodontal ligaments– Osteoclast activity increased

• Research links w/increase risk of heart attack and stroke

Pharynx

• Oropharynx laryngopharnx– Food and fluids to esophagus– Air to trachea

• Lined w/ stratified squamous and goblet cells• Skeletal muscle layers for swallowing– Inner longitudinal– Outer circular pharyngeal constrictors

Esophagus

• Laryngopharynx esophageal sphincter esophageal hiatus cardiac orifice gastroesophageal sphincter– Heartburn when GE sphincter not tightly closed/leaks

• Histology– Mucosa w/ strat. squ. sim. colum.

• Folds w/o food that flatten w/ food– Submucosa w/ mucus glands to aid passage– Muscularis externa w/ skeletal skeletal & smooth

smooth– Fibrous CT fibrous adventitia instead of serosa

Mouth to Esophagus Digestion• Mouth ingests

– Mastication uses teeth and tongue to start mechanical digestion• Food and structure mov’t influences

– Salivary amylase starts chemical digestion of carbs• Tongue forms bolus for deglutition

– Tongue pushes to oropharynx in buccal phase (voluntary)• Esophageal sphincter constricts

– Pharyngeal-esophageal phase (involuntary)• Uvula and epiglottis protection triggered• Esophageal sphincter relaxes

• Peristalsis moves bolus down esophagus– Gastroesophageal spincter relaxes then constricts– Prevents chyme escape

Stomach• Chemical breakdown of proteins• Regions– Cardia– Fundus– Body– Pyloric region (antrum, canal, and sphincter)

• Mesenteries (greater and lesser omentums) at respective curvatures

• Rugae seen when empty, allow distension

Stomach: Microscopic Anatomy

• Muscularis externa w/ 3rd oblique layer to break food and push to SI

• Mucosa– Simple columnar epithelia w/ goblet cells

• Insoluble alkaline mucus layer w/ bicarbonate fluid below• Protects stomach from acid and enzyme digestion

– Invaginates into gastric pits leading to gastric glands that produce gastric juices, mucus, and gastrin• Pits are primarily mucus cells• Glands vary w/ region

Gastric Glands• Mucus neck cells produce acidic mucus

– Function not understood• Parietal cells secrete H+ and Cl- ions HCl

– Acidity (pH ~ 2) denatures proteins, kill bacteria, and digest cellulose

– Intrinsic factor production: B12 absorption for RBC production

• Chief cells produce pepsinogen pepsin• HCl initiates, but pepsin maintains (positive feedback)

• Enteroendocrine cells release chemical messengers– Histamine, gastrin, serotonin, endorphins, cholecystokinin (CCK),

and somatostatin

Protecting the Stomach

• Insoluble mucus layer on surface• Bicarbonate ions (HCO3

-)• Tight junctions b/w epithelial cells• H+ and Cl- ions secreted separately• Epithelial replaced ~ 3 – 6 days• Gastric ulcers form when mechanisms fail– Chronic can rupture stomach– Helicobacter pylori usually responsible

Controlling Gastric Secretion• Cephalic (reflex) phase before food entry

– Smell, taste, thought, or sight excites– Depression or lack of appetite suppress

• Gastric phase once food enters stomach– Stretch receptors signal ACh to increase gastric juices– Peptides, caffeine, and high pH stimulate gastrin (HCl) release – Low pH, emotions, and SNS inhibit gastrin

• Intestinal phase once food enters duodenum (SI)– W/ low pH and food stimulates intestinal gastrin release – Distension and prolonged low pH inhibit CN X, activate SNS to

tighten pyloric sphincter, and release inhibiting hormones• Fig 23.17

Controlling Mobility and Emptying

• Peristalsis pushes to pyloric for mixing before retropulsion pushes back– Chyme squirts through pyloric valve w/ each cycle

• Duodenum monitoring tightly coupled– Inhibiting hormones from intestinal phase (previous)– Carb rich chyme moves quickly, but fat rich chyme

slower for digestion• Vomiting or emesis• Fig 23.30

Small Intestine (SI)

• Digestion finishes and nutrient absorption occurs• 3 subdivision w/ no clear external distinctions– Duodenum (retroperitoneal)• Receives chyme from pyloric sphincter• Hepatopancreatic ampulla where bile and pancreatic duct

join (sphincterof Oddi controls)– Jejunum (mesentery support)– Ileum (mesentery support) • Ileocecal valve at LI junction

Small Intestine: Microscopic Anatomy

• Mucosa and submucosa form plicae circulares– Simple columnar mucosa folds into villi

• Lacteal, artery, and vein w/i• Intestinal crypts b/w

– Intestinal juice, Paneth cells, and new cells

– PM of mucosa cells form microvilli (brush border)• Enzymes finalize carb and protein digestion

– Increase SA for absorption• Submucosa variations throughout SI region

– Intestinal crypts decrease– Goblet cell number increases– Peyer’s patches (MALT) increase– Brunner’s glands (HCO3

- mucus) in duodenum only

• Muscularis externa– Duodenum w/ adventitia (retroperitoneal)

Liver

• Right, left, caudate, and quadrate lobes– Falciform ligament is the mesentery

• Separates R & L and suspends liver from diaphragm– Ligament teres is remnant of umbilical vein

• Lesser omentum connects curvature to liver– Hepatic artery, vein, and portal vein

• Hepatic ducts hepatic duct common bile duct• Hepatitis and cirrhosis

Liver: Microscopic Anatomy

• Hexagonal liver lobules are functional units– Hepatocytes arranged around a central vein

• Portal triads at each corner• Bile duct, hepatic artery, and hepatic portal vein

• Liver sinusoids (capillaries) b/w– Kupffer cells

Gallbladder

• Stores and concentrates liver produced bile– Hepatic duct cystic duct storage– Cystic duct common bile duct sphincter of Oddi release

• Bile– Alkaline solution to emulsify lipids– Facilitate fat and cholesterol absorption– Bile salts recycled in illeum back to liver (hepatic portal vein)– Bilirubin, chief pigment, absorbed in liver and released in bile

• Gallstones from cholesterol crystallization• Jaundice when bile pigments accumulate in skin from

blockage

Pancreas

• Retroperitoneal organ• Acinar cells produce digestive enzymes that

breakdown all foodstuffs– Pancreatic duct joins common bile to drain to

duodenum (where specifically?)• Islets of Lagerhans produce insulin and

glucagon

Pancreatic Juice

• Water based solution of enzymes and HCO3-

– Neutralize chyme (pH ~ 2 vs pH ~ 8)– Optimal pH for enzymes

• Enzymes hydrolyze monomers– Inactive proteases– Active amylases, lipases, and nucleases

Accessory Secretion Regulation

• Neural (CN X) and hormonal (CCK & secretin) control

• CCK released w/ fatty chyme presence• Gallbladder contract and sphincter of Oddi relax

– Vagus nerve weakly• Bile and pancreatic juice (enzyme rich) secreted

• Secretin released w/ HCl presence– HCO3

- rich pancreatic juice release

• Bile salt presence stimulates more bile release– Positive feedback

Large Intestine• Absorb water & vitamins and eliminate feces• Tone of teniae coli create pouch-like haustra• Divisions

– Cecum– Appendix (MALT)

• Appendicitis when blocked and bacterial growth– Ascending and descending colon are retroperitoneal– Transverse and sigmoid colon w/ mesocolons – Rectum w/ rectal valves to separate feces and gas– Anal canal w/ internal and external sphincters

• Imbalances– Diarrhea vs constipation

Large Intestine: Microscopic Anatomy

• Mucosa– Simple columnar except anal canal (strat. squa.)– No folds, villi, or digestive enzyme cells– Thicker w/ deeper crypts and more goblet cells

• Bacterial flora– Enter via anus or survive SI– Functions

• Breakdown indigestible carbs (cellulose)• Release gases from breakdown• Synthesize vitamins B and K

– Generally peaceful existence• Irritable bowl syndrome (disease) when not

Nutrient Digestion• Catabolic process of hydrolysis• Carbohydrates

– Amylases (salivary and pancreatic) – Brush border enzymes (dextrinase, lactase, maltase, sucrase)– From villi to liver in hepatic portal vein

• Proteins– Pepsin in stomach (pepsinogen + HCl)– Brush border peptidases (amino-, carboxy-, di)– Pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase)

• Lipids– Bile sales emulsifies fats to expose triglycerides– Pancreatic lipase forms glycerol and FA’s

• Nucleic acids– Brush border enzymes (nuclesidases and phosphotases)– Pancreatic nucleases (deoxy- and ribonucleases)

• Fig 23.32

Nutrient Absorption• Carbohydrates

– Glu and gal: active transport (w/ Na+) to epithelium then facilitated diffusion to capillaries

– Fru: only facilitated diffusion• Proteins

– See glucose and galactose above– Inefficient in newborns = food allergies as endocytized whole proteins

‘seen’ as antigens; allow IgA from breast milk into blood• Lipids

– Monomers bind w/ bile salts = micelles to reach epithelium for diffusion– Chylomicrons in epi. when reassembled water soluble lacteal absorb

• Nucleic Acids– See glucose and galactose above

Nutrient Absorption (cont.)• Vitamins – Fat soluble see lipids previous; need to eat w/– Water soluble by diffusion, active, or passive transport

• Electrolytes– Anions move w/ Na+ gradient (glucose & glactose)– Iron and Ca2+ regulated by body needs– K+ related to water volume/absorption

• Water– 95% in SI, rest in LI– Bidirectional, but net osmosis w/ [gradient] from active solute

absorption