Digestive System Flashcards
1
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Digestive Tract
A
Digestive tract (Gastrointestinal GI tract, Alimentary tract)
- oral cavity, esophagus, stomach, small & large intestines, anus
- associated glands: salivary glands, liver, pancreas
- small intestines lining: absorbs complex CHO, nuclei acids, fats
- large intestines: absorbs most H2O & electrolytes
- Functions:
- obtain molecules necessary for maintenance, growth, energy needs of the body from ingested food.
- inner layer forms an important protective barrier between the contact of the tract’s lumen & the intestinal milieu of the body’s CT & vasculature
- Structures within allow the following:
- Ingestion
- Mastication (digesting food into digestible pieces)
- Motility (muscular movements of materials through the tract)
- Secretion of lubricating & protective mucus, digestive enzymes, acidic & alkaline fluids, & bile
- Hormone release for local control of motility & secretion
- Chemical digestion (enzymatic degradation of large macromolecules to smaller molecules & their subunits
- Absorption of small molecules & H2O into the blood & lymph
- Elimination of indigestible, unabsorbed components of food
2
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4 Main layers of the GI wall
A
4 main layers of the GI wall:
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Mucosa (mucous membrane)
- epithelial lining
- lamina propia of Loose CT, rich in blood vessels, lymphatics, small glands
- macrophages & lymphocytes for production of IgA antibodies
- MALT/GALT can be found here
- Muscularis mucosae- thin layer of smooth muscle separating mucosa from submucosa & allowing local movements of the mucosa
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Submucosa
- dense irregular CT with large blood & lymph vessels
- submucosal (Meissner) plexus of autonomic nerves
- may contain gland & significant lymphoid tissue (MALT/GALT)
- Muscularis (Muscularis Externa)
- smooth muscle cells
- smooth muscle layers in muscularis externa:
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internal sublayer (close to the lumen)- circular fiber orientation
- closes
-
in between layers- contains blood & lymph vessels & myenteric (Auerbach) nerve plexus of autonomic neurons in small ganglia & interconnected pre-and postganglionic nerve fibers
- generates and coordinates contractions of the muscularis which mix and propel
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external sublayer- longitudinal
- pushes forward
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internal sublayer (close to the lumen)- circular fiber orientation
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Serosa or Adventitia (depending on the location)
- thin layer of Loose CT, rich in blood vessels, lymphatics, & adipose tissue with mesothelium (simple squamous epithelium)
- in abdominal cavity:
- serosa is continuous with mesenteries (thin membrane covered by mesothelium that supports intestines)
- mesenteries are continuous with the peritoneum
- Parietal peritoneum – an outer layer which adheres to the anterior and posterior abdominal walls
- Visceral peritoneum – an inner layer which lines the abdominal organs
3
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Enteric nervous system
A
Enteric nervous system
- composed of Submucosal (Meissner) plexus and Myenteric (Auerbach) nerve plexus
- the rich autonomic innervation of the enteric nervous provides an anatomic explanation of the well-known actions of emotional stress on the stomach & other regions of the GI tract
4
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5
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6
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MALT/GALT & IgA Antibodies
A
MALT/GALT
- Mucosa-associated lymphoid tissue
- Gut-associated lymphoid tissue
- found in the mucosa or submucosa
- essential backup to the thin physical barrier of epithelial lining
IgA antibodies
- produced by macrophages & lymphocytes in the lamina propia
- undergo transcytosis into the intestinal lumen bound to the secretory proteins produced by the epithelial cells
- resists proteolysis by digestive enzymes & provides important protection againsts specific viral & bacterial pathogens
7
Q
Hirschsprung disease & Chagas disease
A
Hirschsprung disease
- Congenital aganglionic megacolon
- plexuses in the digestive tract’s enteric nervous system are absent which disturbs digestive tract motility and produces dilations in some areas
Chagas disease
- Trypanosomiasis
- infection with the protozoan Trypanosoma cruzi
- plexuses are severely injured which disturbs digestive tract motility and produces dilations in some areas
8
Q
Esophagus
A
Esophagus
- muscular tube
- transports swallowed material from the pharynx to the stomach
- 4 layers of the GI tract:
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Mucosa- nonkeratinized stratified squamous epithelium
- esophageal cardiac glands- mucosa near the stomach that secret additional mucus
-
Submucosa:
- esophageal glands- small mucus-secreting glands that lubricate and protect mucosa
-
Muscularis externa- important swallowing mechanism begins with voluntary muscle action but finishes with involuntary peristalsis
- upper 1/3 - muscularis is exclusively skeletal muscle (surrounded by adventitia)
- middle portion - combination of the outer skeletal & inner smooth muscle fibers (surrounded by adventitia)
- lower 1/3 - muscularis is exclusively smooth muscle (surrounded by serosa)
- Adventitia or Serosa (see above)
-
Mucosa- nonkeratinized stratified squamous epithelium
9
Q
Gastroesophageal Reflux Disease
A
Gastroesophageal Reflux Disease (GERD)
- erosion of the esophageal mucosa d/t chronic heartburn from an incompetent esophageal sphincter
- can produce metaplastic changes in the nonkeratinized stratified squamous epithelium if untreated (Barett’s esophagus)
10
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Identify
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11
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Stomach
A
Stomach
- a greatly dilated segment of the digestive tract
- Main functions:
- continues digestion of carbohydrates initiated by the amylase of saliva
- adds acidic fluid to the ingested food and mizing its contents into a viscous mass (chyme) by the churning activity of the muscularis
- begins digestion of triglycerides by a secreted lipase
- promotes initial digestion of proteins with pepsin enzyme
- stores food
- 4 major regions of the stomach:
- cardia- narrow transitional zone that is histologically similar with pylorus which is also involved in mucus production
- fundus- identical with histological structures with the body which also site of gastric glands releasing acidic gastric juice
- body- identical with histological structures with the fundus which also site of gastric glands releasing acidic gastric juice
- pylorus- funnel-shaped region that opens into the small intestines that is histologically similar with cardia which is also involved in mucus production
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Rugae:
- large, long, longitudinally directed folds in the mucosa & submucosa of empty stomach
- increase surface area
- flatten when stomach fills with food
- 4 layers of the GI tract:
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Mucosa:
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Gastroesophageal junction
- abrupt transition of epithelium from nonkeratinized stratified squamous epithelium to simple columnar epithelium
- epithelium invaginates deeply into the lamina propia which form gastric pits (opening to the stomach lumen)
- surface mucus cells (line lumen) & gastric pits secrete viscous mucus layer rich in bicarbonate (alkaline) ions & protects the mucosa from abrasive effects of intraluminal food & corrosive effects of acid
- stem cells for the epithelium that lines the glands, pits, & stomach lumen is located in the narrow segment (isthmus) between each gastric pit & gastric glands (some replace surface mucus cells with turnover of 4-7 days)
- lamina propia surrounding and supporting the gastric pits (opening) & glands
- vascularized
- contains smooth muscle fibers, lymphoid cells, capillaries, & lymphatics
- muscularis mucosa- smooth muscle that separated the mucosa from the underlying submucosa
-
Gastroesophageal junction
- Submucosa- CT with large blood & lymph vessels & many lymphoid cells, macrophages, & mast cells
-
Muscularis externa:
- Outer longitudinal layer
- Middle circular layer (thickened at the pylorus)
- Innermost oblique layer (lumen side)
- Covered by a thin Serosa
-
Mucosa:
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18
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Pernicious anemia
A
Pernicious anemia
- reduction of proliferation of erythroblasts d/t low levels of vitamin B12 which is a cofactor required for DNA synthesis
- damaged parietal cells (caused by autoimmunity or other reasons) can cause malabsorption of vitamin B12
19
Q
Gastric and Duodenal ulcers
A
Gastric and Duodenal ulcers
- painful erosive lesions of the mucosa that may extend to deep layers
- can occur between lower esophagus and jejunum
- causes:
- Helicobacter pylori (baterial infection)
- nonsteroidal anti-inflammatory drugs
- overproduction of HCL or pepsin
- lowered production of mucus or bicarbonate
20
Q
Secretory Epithelial cells in fundus & body of the Stomach
A
Secretory Epithelial cells in fundus & body of the Stomach:
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Mucus neck cells
- present in clusters often distorted by neighboring cells
- round nuclei & apical secretory granules
- secrete acidic fluid containing mucin
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Parietal (oxyntic) cells
- produce HCL & intrinsic factor (glycoprotein or cofactor required for uptake of vitamin B12 in small intestines)
- secretory activity is stimulated both by parasympathetic innvervation and paracrine release of histamine and gastrin from enteroendocrine cells
- present among the mucus neck cells
- large, rounded, pyramidal, 1-2 central round nucleus
- eosinophilic cytoplasm d/t high density of mitochondria
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intracellular canaliculi
- formed in an active parietal cell
- fusion of tubular vesicles with cell membrane providing in the generous increase in the surface area of the cell for diffusion and ion pumps (with microvilli)
- prolonged activity may produce more mitochondria
- Synthesis of H2O by parietal cells:
- In parietal cells: H2O-> H+ + OH-
- H+ through the H+K+pump-> lumen of gastric gland
- OH- bonds with CO2 to form bicarbonate ion (HCO3-) by carbonic anhydrase
- HCO3- enters into the blood while Cl- gets in the parietal cell then to the lumen of gastric gland
- H+ + Cl- combine in the lumen of gastric gland to form HCl
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Chief (zymogenic) cells
- predominates in the lower regions of the gastric glands
- active protein-secreting cells
- abundant RER (basophilic) & numerous apical secretory granules
- granules contain inactive enzymes pepsinogen (precursor of pepsin) which converts to pepsin in the acid environment of the stomach to
- Pepsin: endoproteinases, optimum pH=1.8 to 3.5, initiates the hydrolysis of ingested proteins in the stomach
- produce gastric lipase ( digests lipids)
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Enteroendocrine cells
- scattered epithelial cells in gastric mucosa with endocrine or paracrine functions
- in fundus- secrete serotonin at the basal lamina of gastric glands (EC cells)
- in pylorus- produce gastrin to the glandular lumen by stimulating parietal cells to produce HCl (G cells)
- D cells release somatostatin which functions opposite to G cells
- enterochromaffin (EC) cells- stained with chromium for TEM
- argentaffin cells- stained with silver salts for TEM
- APUD cells- amine precursor uptake and decarboxylation activity which is also part of diffuse neuroendocrine system (DNES)
21
Q
Cardiac glands and Pyloric glands
A
Cardiac glands and Pyloric glands
- Simple branched tubular exocrine glands in the mucosa
- lack parietal and chief cells
- primarily secreting mucus producing lots of mucins
- has villi and gastric pits
22
Q
Carcinoids
A
Carcinoids
- tumors that arise from enteroendocrine (EC) cells
- responsible for clinical symptoms caused by overproduction of serotonin
- serotonin increases gut motility
- high levels of serotonin can produce mucosal vasoconstriction & tissue damage
23
Q
Small Intestine
A
Small Intestine
- site where digestive processes are completed
- nutrients are absorbed by cells of epithelial lining
- relatively long (5 m)
- consists of 3 segments (DJI :)
- duodenum
- jejunum (proximal)
- ileum (distal)
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Plicae circulares
- series of permanent circular or semilunar folds consisting of mucosa and submucosa
- core is submucosa
- best developed in the jejunum
- increase mucosal surface area 3-fold which is important for nutrient absorption
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Villi
- short mucosal outgrowth densely covering the entire mucosa of the small intestines
- finger or leaf-like projections
- covered by simple columnar epithelium of absorptive cells (enterocytes) with many interspersed goblet cells
- covering plicae circulares-> core is lamina propia
- has core of loose CT that extends from the lamina propia & contains fibroblasts, smooth muscle fibers, lymphocytes & plasma cells, fenestrated capillaries, & central lymphatic (lacteal)
- crypts of Lieberkuhn- intestinal glands or crypts that are openings of short tubular glands between the villi
- increase mucosal surface area 10-fold which is important for nutrient absorption
- cell types of the small intestines:
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Enterocytes
- absorptive cells
- tall columnar cells with basal oval nucleus
- apical striated or brush border
- densely packed microvilli (actin filaments) covered by glycocalyx
- microvilli increase mucosal surface area 20-fold which is important for nutrient absorption
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Goblet cells
- secrete mucin-> mucus
- protect & lubricate lining of intestines
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Paneth cells
- basal portion of the intestinal crypts below the stem cells
- exocrine cells with large eosinophilic secretory granules in their apical cytoplasm
- release lysozymes, phospholipase A2, hydrophobic peptide (defensins) which break down membrane of microorganisms & baterial cell walls
- role in innate immunity & in regulating the microenvironment of the intestinal crypts
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Enteroendocrine cells (see stomach cells)
- secrete various peptide hormones
- open type- constricted apical end of the cell contracts the intestinal lumen and has chemoreceptors similar to those of taste buds
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M (Microfold cells)
- specialized epithelial cells in the mucosa of the ileum overlying the lymphoid follicles of Peyer patches
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Peyer patches:
- basal membrane invaginations containing many intraepithelial lymphocytes & antigen-presenting cells
- selectively endocytose antigens and transport then to the underlying lymphocytes-> lymph nodes
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Peyer patches:
- specialized epithelial cells in the mucosa of the ileum overlying the lymphoid follicles of Peyer patches
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Enterocytes
- 4 layers of the GI tract:
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Mucosal lamina propia:
- loose CT containing extensive blood & lymph microvasculature, nerve fibers, smooth muscle cells, diffuse lymphoid tissue
- penetrate the core of each microvillus to bring the microvasculature, etc
- smooth muscle fibers from muscularis mucosa:
- produce rhythmic movements of the villi that increase absorption efficacy
- fibers produce movements of plicae circulares that help propel lymph from the lacteal into submucosal & mesenteric lymphatics
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Submucosa:
- has larger blood & lymph vessels & the diffuse interconnected neurons of the submucosal (Meissner) nerve plexus (autonomic)
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proximal part of duodenul (submucosal & mucosal):
- duodenal (or Brunner) glands
- compound tubular exocrine gland
- mucus pH= 8.1 to 9.3 to neutralize chyme entering the duodenum
-
ileum (submucosal & lamina propia):
- contains MALT (Mucosa-associated lymphoid tissue)
- consisting of the large lymphoid nodules (Peyer patches) underlying the epithelial M cells (found where lymphoid tissues are- on top of Peyer patches
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Muscularis externa:
- well developed
- composed of:
- internal circular layer
- neurons of Myenteric (Auerbach) nerve plexus (autonomic)
- external longitudinal layer
- Covered with thin Serosa with mesothelium continuous with the mesenteries
-
Mucosal lamina propia:
25
Q
Mechanism of nutrient absorption
A
Mechanism of nutrient absorption:
Disaccharides & Peptidases-> hydrolyze disaccharides & dipeptides-> into monosaccharides & amino acids to be sent to capillaries through active transport
Gastric & pancreatic Lipases-> digest fats-> into lipase subunits-> then emulsified by bile salts into small micelles-> lipids enter enterocytes by passive diffusion-> chylomicrons-> uptake by lacteal (lymphatic capillaries)
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39
Q
Pancreas
A
Pancreas
- mixed exocrine-endocrine gland that produces both digestive enzymes and hormones
- exocrine- by cells of serous acini
- no myeloepithelial cells
- Compound acinar exocrine gland
- resembles parotid gland histologically but lacks striated ducts
- Acinus->
-
Intercalated duct->
- simple squamous or low cuboidal epithelium
- initial cells extend into lumen of acinus (penetrate the secretory portion) are palestaining centroacinar cells (unique to the pancreas)
- cells secrete a large volume of fluid rich in bicarbonate ion (alkalinize & transports hydrolytic enzymes produced in acini)
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increasing # of columnar epithelial:
- Intralobular ducts->
- Interlobular ducts
- endocrine function involves pancreatic islets (Islets of Langerhans) which are pale staining areas (alpha secrete glucagon and beta secrete insulin)
- exocrine- by cells of serous acini
- elongated retroperitoneal (behind the peritoneum) organ with a large head near the duodenum & more narrow body & tail regions that extend to the left
- thin capsule of CT from which septa extend to cover the larger vessels and ducts to separate parenchyma into lobules
- secretory acini are surrounded by basal lamina that is supported only by a delicate sheath of reticular fibers with a rich capillary network
- secretes approx. 1.51L of alkaline pancreatic juice/day delivered directly to the duodenum where bicarbonate ions neutralize the acidic chyme entering the stomach
- digestive enzymes
- proteases (inactive zymogens):
- chymotrypsinogen
- trypsinogen (cleaved and activated by enteropeptidase in the duodenum generating trypsin)
- a-amylases
- lipases
- nucleases
- DNAase
- RNAase
- proteases (inactive zymogens):
45
Q
Liver
A
Liver
- largest internal organ (1.5kg or 2% of body weight)
- located in right upper quadrant of abdomen just below the diaphragm
- has major left & right lobes with 2 small inferior lobes covered by a thin capsule (dense irregular CT) & mesothelium of visceral peritoneum
- capsule thickens at the hilum (porta hepatis) on inferior side
- dual blood supply from hepatic portal vein & hepatic artery enter the organ
- hepatic vein, lymphatics, & common hepatic (bile) duct exit
- capsule thickens at the hilum (porta hepatis) on inferior side
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Hepatocytes
- key cells of the liver
- large cuboidal epithelial cells with large round central nuclei & eosinophilic cytoplasm rich in mitochondria
- frequently binucleated (50% polyploid-> 2-8X the normal chromosome)
- hepatic lobules- hepatocytes form hundreds of irregular plates (supported by reticular fibers) arranged radially around a small central vein
- Portal triad- 3 interlobular structures (portal venule (from portal vein- rich in nutrients), hepatic arteriole (from hepatic artery- rich in 02), bile ductule of cuboidal epithelium
- functions:
- storage of iron with ferritin
- removal of effete RBCs by Kupffer cells
- storage of vitamin A (in hepatic stellate cells) & other fat soluble vitamins
- storage of glucose in glycogen granules & triglycerides in small lipid droplets
- amino acid deamination producing urea removed from blood in kidney
- detoxification
- conversion of amino acids into glucose (gluconeogenesis)
- synthesis & endocrine secretion into the blood of the major plasma proteins (albumin, fibrinogen, etc)
46
Q
Hepatic Portal system
A
Hepatic Portal System:
- Peripheral branch of Portal vein & Hepatic artery (that form the portal parts of the portal triad) ->
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Vascular sinusoids->
- presence of venous & arterial blood mixture
- thin discontinuous linings of fenestrated endothelial cells surrounded by sparse basal lamina & reticular fibers
- discontinuous linings allow plasma to fill a narrow perisinusoidal space (space od disse)
- perisinusoidal space bathes the many irregular microvilli projecting from hepatocytes
- facilitates most key hepatocyte functions that involve uptake & release of nutrients, proteins, & potential toxins
- discontinuous linings allow plasma to fill a narrow perisinusoidal space (space od disse)
- 2 functionally important cells:
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Kupffer cells (stellate macrophages)
- Immune cells or Antigen-presenting cells
- found within the sinusoidal lining
- recognized & phagocytose aged RBCs, freeing heme and iron for reuse of storage in ferritin complexes
- antigen-presenting cells & remove any bacteria or debris present in the portal blood
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Ito cells (hepatic stellate cells)
- found in perisinusoidal space
- with fat droplets that store vitamin A,D,E,K (fat-soluble vitamins)
- mesenchymal cells that produce ECM (becoming myofibroblasts after liver injury) & cytokines that help regulate Kupffer cell activity
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Kupffer cells (stellate macrophages)
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Central vein->
- endothelium is supported by thin layer of fibrous CT
-
Large hepatic vein
- empty into the inferior vena cava
54
Q
Structure- Function relationship in Liver
A
Structure- Function relationship in Liver:
- Classic Hepatic Lobule
- drains blood from portal vein & hepatic artery to hepatic or central vein
- emphasizes endocrine function of the structure producing factors for uptake by plasma
- from periphery to center
- hexagonal
- Portal Lobules
- drain bile from hepatocytes to the bile duct
- exocrine function of cells (bile secretion)
- bile ductule at the center-> bile moving in the opposite direction as blood
- area drained by each bile duct is triangular
- Portal Acinus
- supplies oxygenated blood to hepatocytes
- liver acinus-> irregular oval or diamond-shaped area extending from 2 portal triads to the 2 closest central vein
- Periportal hepatocytes
- nearest to the hepatic arterioles
- comprising zone I in acinus
- get the most oxygen & nutrients & can most readily carry out functions requiring oxidative metabolism such as protein synthesis
- Hepatocytes in zone III
- near the central vein
- get the least oxygen & nutrients
- preferential sites of glycolysis, lipid formation & detoxification (drug biotransformation)
- first hepatocyte to undergo fatty accumulation & ischemic necrosis
- Intervening zone II
- hepatocytes have intermediate range of metabolic functions between zones I & III
58
Q
Gallbladder
A
Gallbladder
- hollow pear-shaped organ
- attached to the lower surface of the liver
- capable of storing 30-50ml of concentrated bile
- Layers ( do NOT have submucosa & muscularis mucosa)
- mucosa of Simple columnar epithelium (mucosal folds are evident when gallbladder is empty)
- lamina propia
- thin muscularis with bundles of muscle fiber oriented in several directions
- external adventitia or serosa
- Lining epithelial cells have prominent mitochondria, microvilli, and large intercellular space-> indicates heavy transport of water-> Na+ pumps in basolateral membrane-> passive movement of water from bile
- stored bile is moved to duodenum by contraction of gallbladder muscularis induced by CKK (stimulated by presence of ingested fats) or cholecystokinin released from enteroendocrine cells of small intestines
59
Q
Cholelithiasis
A
Cholelithiasis
- gallstones in the lumen of the gallbladder or biliary ducts d/t reabsorption of water from bile in the gallbladder
- supersaturation of cholesterol in bile-> formation of cholesterol stones
- brown or black pigment stones form when bile contains excessive amount of unconjugated bilirubin (chronic hemolysis associated with sickle cell anemia)
