Chapter 23 Flashcards
Peristalis
Adjacent segments of GI tract organs alternately contract and relax moving food distally.
Segmentation
Nonadjacent segments of alimentary tract organs alternately contract and relax, moving food forward then backward.
Food mixing and slow food propulsion occur.
Alimentary Canal
Gastrointestinal tract
What do GI mechanoreceptors and chemoreceptors respond to?
Respond to stretch, changes in osmolarity, pH, and presence of substrate and end products in digestion.
What do GI mechanoreceptors and chemoreceptors do in digestion?
Initiate reflexes, activate and inhibit digestive glands, stimulate smooth muscle to mix and move contents in lumen
What is peritonitis
Inflammation of the peritoneum via piercing wound, perforating ulcer, ruptured appendix, peritoneal coverings sticking together.
3 layers of Mucosa
Epithelium - simple columnar epithelium and mucous secreting cells. Secretes mucus: Protects organs from enzyme and eases food passage. May secrete enzymes and hormones (e.g., in stomach and small intestine)
Lamina Propria - Loose areolar connective tissue, Capillaries for nourishment, absorption Lymphoid follicles (part of MALT), Defend against microorganisms
Muscularis Mucosae - smooth muscle – local movements of mucosa
Submucosa
Areolar connective tissue surrounds mucosa
Blood and lymphatic vessels, lymphoid follicles, and submucosal nerve plexus
Muscularis Externa
- Segmentation and peristalsis
- Inner circular, outer longitudinal layers
- Circular layer thickens sphincters
- Myenteric nerve plexus between layers
Serosa
Visceral peritoneum
• Areolar connective tissue covered with mesothelium in most organs
• Replaced by adventitia in esophagus
Hard palate
palatine bones and maxillae
Corrugated to create friction with tongue
Soft palate
Mostly of skeletal muscle
Closes nasopharynx during swallowing
Uvula projects downward from free edge
Tongue
Skeletal muscle
Repositioning,mixing food during chewing
Formation of bolus
Initiation of swallowing, speech, and taste
Lingual lipase is secreted by serous cells
Fat-digesting enzyme functional in stomach
Functions of Saliva
Cleanses mouth
Dissolves food
Moistens food
Enzyme breakdown starch
Two types of secretory cells in the salivary gland
Serous cells- Watery, enzymes, ions, bit of mucin
Mucous cells- Mucus
Composition of Saliva
97–99.5% water, slightly acidic
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
Dental Caries
Cavities - - demineralization of enamel and dentin from bacterial action
Teeth
Tear and grind food for digestion
- 20 deciduous teeth erupt (6–24months)
Roots resorbed, teeth fall out (6-12years)
Permanent dentitions formed by age21
- Third molars at 17–25, or may not erupt
Gingivitis
Plaque calcifies to form calculus (tartar) which disrupts seal between gingivae and teeth
Anaerobic bacteria infect gums
Periodontitis
(from neglected gingivitis) Immune cells attack intruders, tissues Destroy periodontal ligament Activate osteoclasts dissolve bone Possible tooth loss; may promote atherosclerosis and clot formation in coronary and cerebral arteries
Esophagus
Flat muscular tube from laryngopharynx to stomach
Pierces diaphragm at esophageal hiatus
Joins stomach at cardial orifice
Gastroesophageal (cardiac) sphincter
Heartburn
Stomach acid regurgitates into esophagus
Excess food/drink, extreme obesity, pregnancy, hiatal hernia: Part of stomach above diaphragm
Cellular Layers of Esophagus
Mucosa
Submucosa
Muscularis Externa
Adventitia (fibrous connective tissue)
Deglutition
Swallowing
Step 1: Buccal Phase of Eating
During the buccal phase, the upper esophageal sphincter is contracted. The tongue presses against the hard palate, forcing the food bolus into the oropharynx.
Step 2: pharyngeal-esophageal phase of eating
The pharyngeal-esophageal phase
begins as the uvula and larynx rise to prevent
food from entering respiratory passageways.
The tongue blocks off the mouth. The upper
esophageal sphincter relaxes, allowing food
to enter the esophagus.
Step 3: of Eating
The constrictor muscles of the pharynx contract, forcing food into the esophagus inferiorly. The upper esophageal sphincter contracts (closes) after food enters.
Step 4: of eating
Peristalsis moves food through the esophagus to the stomach.
Step 5: of eating
The gastroesophageal sphincter surrounding the cardial oriface opens, and food enters the stomach.
Mesentaries
Tether stomach
Parietal Cells
Secrete: Hydrochloric acid (HCl) pH 1.5–3.5 denatures protein, activates pepsin, breaks down plant cells, kills bacteria
Intrinsic factor: Required for absorption of vitamin B12 in small intestine
Chief cell
secretions:
Pepsinogen - inactive enzyme
Activated to pepsin by HCl and by pepsin itself (positive feedback)
Lipases - Digest ~15% of lipids
Enteroendocrine cell
secretions:
Chemical messengers into lamina propria that act as paracrines
Serotonin and histamine
Hormones: Somatostatin and gastrin
Mechanisms to Protect Stomach from itself
Harsh digestive conditions in stomach Has mucosal barrier to protect Thick layer of bicarbonate-rich mucus Tight junctions between epithelial cells Damaged epithelial cells quickly replaced by division of stem cells
Digestive Processes in the Stomach
Physical digestion
Denaturation of proteins by HCl
Enzymatic digestion of proteins by pepsin (milk protein by rennin, infants)
Lingual lipase digests some triglycerides Delivers chyme to small intestine
Lipid-soluble alcohol and aspirin absorbed Only stomach function essential to life:
- Secretes intrinsic factor for vit. B12 absorption mature red blood cells
- Lack of intrinsic factor: pernicious anemia, treated with B12 injections
3 phases of Gastric Secretion
Cephalic (reflex) phase – conditioned reflex triggered by aroma, taste, sight.
Gastric phase – lasts 3–4 hours
Stimulated by distension, peptides, low acidity, gastrin (major stimulus)
Enteroendocrine G cells stimulated by caffeine, peptides, rising pH gastrin
Intestinal Phase:
Stimulatory component: Partially digested food enters small intestine brief intestinal gastrin release
Inhibitory effects: Chyme with H+, fats, peptides, irritating substances
Enterogastrones released
Secretin, cholecystokinin (CCK), vasoactive intestinal peptide (VIP)
Stimuli of Gastric Phase
Gastrin enzyme and HCl release
Low pH inhibits gastrin secretion (as between meals)
Buffering action of ingested proteins – rising pH – gastrin secretion
Three chemicals - ACh, histamine, and gastrin - stimulate parietal cells
Structural Modifications of Small Intestine
Circular folds: Force chyme to slowly spiral through lumen more nutrient absorption
Villi: Extensions of mucosa with capillary bed and lacteal for absorption
Microvilli: (brush border) – contain enzymes for carb. and protein digestion
Cells of Intestinal Crypts
Intestinal crypt epithelium renewed every 2-4 days, containn secretory cells:
Enteroendocrine cells: enterogastrones
Intraepithelial lymphocytes: Release cytokines that kill infected cells
Paneth cells: antimicrobial agents (defensins and lysozyme)
Peyer’s patches
protect especially distal part of small intestines against bacteria
hepatocytes
Liver Cells increased rough & smooth ER, Golgi, peroxisomes, mitochondria Process bloodborne nutrients Store fat-soluble vitamins Perform detoxification Produce ~900 ml bile per day
Hepatitis
Usually viral infection, drug toxicity, wild mushroom poisoning
Hep C - IV Drugs, Sex, etc.
Hep A - Pathogen, Ingested, replicates in liver CDC
Cirrhosis
Progressive inflammation from chronic hepatitis or alcoholism
Liver fatty, fibrous – portal hypertension
Bile
Secreted by the liver and stored in the Gallbladder. Yellow-green, alkaline solution
Bile salts - cholesterol derivatives that function in fat emulsification and absorption
Bilirubin - pigment formed from heme
Cholesterol, neutral fats, phospholipids, and electrolytes
Gallstones
High cholesterol; too few bile salts
Obstruct flow of bile from gallbladder
May cause obstructive jaundice
Treated with drugs, ultrasound (lithotripsy), laser vaporization, surgery
Pancreas
Endocrine function: Pancreatic islets secrete insulin and glucagon
Exocrine function: Acini (clusters of secretory cells) secrete pancreatic juicev
Pancreatic Juice
1200 – 1500 ml/day
Alkaline solution (pH 8) neutralizes chyme
Electrolytes (primarily HCO3–)
Enzymes: Amylase, lipases, nucleases Proteases secreted in inactive form
Appendix
Part of MALT of immune system
Bacterial storehouse recolonizes gut when necessary
Appenticitis: Blocked or twisted enteric bacteria accumulate and multiply
Large Intestine
Thicker mucosa of simple columnar epith.
- No circular folds, villi, digestive secretions
- Abundant deep crypts with goblet cells
Bacterial Flora: Colonize colon
Synthesize B complex vitamins, vit. K
Metabolize host-derived molecules (mucin, heparin, hyaluronic acid)
Ferment indigestible carbohydrates
Release acids and gases (~500 ml/day)
Processes of Large Intestine
Residue remains in large intest.12–24 hrs.
No food breakdown except by bacteria
Vitamins (made by bacterial flora), water, and electrolytes (Na+ and Cl–) reclaimed
Propulsion of feces to anus; defecation
Haustral contractions
Most contractions Large Intestines
Slow segmenting movements
Haustra sequentially contract in response to distension
Gastrocolic reflex
Initiated by presence of food in stomach
Activates three to four slow powerful peristaltic waves per day in colon (mass movements)
Defecation
Assisted by Valsalva’s maneuver
Closing of glottis, contraction of diaphragm and abdominal wall muscles increased intra-abdominal pressure
Levator ani muscle contracts anal canal lifted superiorly feces leave
Diverticulitis
Inflamed diverticula; may rupture and leak into peritoneal cavity.
Irritable bowel syndrome
Recurring abdominal pain, stool changes, bloating, flatulence, nausea
Stress common precipitating factor
Digestion of Carbohydrates
Monosaccharides absorbed as ingested
Glucose, fructose, galactose
Digestive enzymes
Salivary amylase, pancreatic amylase, dextrinase, glucoamylase, lactase, maltase, and sucrase
Break down disaccharides sucrose, lactose, maltose; polysaccharides glycogen and starch
Absorption of Carbohydrates
Glucose and galactose
Secondary active transport (cotransport) with Na+ epithelial cells
Move out of epithelial cells by facilitated diffusion capillary beds in villi
Fructose
Facilitated diffusion to enter and exit cells
Digestion and absorption of Proteins
Digestion begins in the stomach with HCl denaturing the proteins and pepsin breaking down the proteins. Pancreatic proteases (trypsin, chymotrypsin, and carboxypeptidase), and by brush border
enzymes (carboxypeptidase,
aminopeptidase, and dipeptidase)
of mucosal cells also break down proteins even further. The amino acids are then
absorbed by active transport into the apical membrane of microvilli, and move to the basal side. The amino acids leave the villi via facilitated diffusion and enter the capillary via intercellular clefts.
Emulsification, digestion, and absorption of fats.
Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets). Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa. Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other fatty substances and proteins to form chylomicrons. Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals and are carried away from the intestine in
lymph.
Absorption of Vitamins
In small intestine
Fat-soluble vitamins (A, D, E, and K)
Water-soluble vitamins (C and B vit.)
Vitamin B12, binds with intrinsic factor
In large intestine
Vitamin K and B vitamins from bacterial metabolism are absorbed
Absorption of Electrolytes
Most ions along length of small intestine
Iron and calcium in duodenum
Na+ coupled with active absorption of glucose and amino acids
Cl– transported actively
K+ diffuses in response to osmotic gradients; lost if poor water absorption
Absorption of Water
9L water, most from GI tract secretions, enter small intestine
95% absorbed in the small intestine Most of rest absorbed in large intestine
Water uptake coupled with solute uptake
Celiac Disease
Immune reaction to gluten
Gluten causes immune cell damage to intestinal villi and brush border
Treated by eliminating gluten from diet
