Digestive System Overview Flashcards by Caitlin Heim

General characteristics of GI system

oral cavity ==> anus
- extracellular space/tube running through the body
responsible for the breakdown of consumed foods via enzymes in order to gain “building blocks” for cellular structures

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Lining of the GI system (general)

tube (except esophagus and anal region) is lined by mucosae
mucosae = protective & creates a microenvironment
- resistant to proteolytic digestion

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Regulation/coordination of GI system (general)

intrinsic autonomic nerves = “enteric neurons”
- control local gut motility
CNS input from parasympathetic/sympathetic
- coordinates peristtalsis
- affects blood vessels and glands
hormonal control via endocrine cells @ mucosal epithelium

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Basic structures of layers in GI tract

mucosa = epithelium + lamina propria
submucosa = dense connective tissue
1. large blood vessels, nerve plexes, glands, lymph nodes
muscularis externa = smooth muscle + nerves
1. inner muscular layer = circular
2. outer muscular layer = longitudinal
3. fxn = peristalsis & churning
serosa/adventitia = outer squamous epithelial layer

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Characteristics of mucosa layer

mucosa = epithelium + lamina propria
lamina propria = underying loose tissue + vasculature
- contains scattered lymphs, plasma cells, macrophages
muscularis mucosae = thin smooth muscle layer underneath mucosa

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Characteristics of the Esophagus

structure:
- muscular tube
  - upper 1/3 = skeletal m.
  - middle 1/3 = skeletal + smooth
  - lower 1/3 = smooth m.
- lining = non-cornified squamous epithelium
- some mucous glands, but lacks thick mucosal covering
- small sphincter @ esophogeal-gastric jxn
function
- conveys ingested material from pharynx to stomach
- no digestive fxns

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Characteristics of the Stomach structure (general)

structure
- 3 zones
  - cardia = mucus glands near esophogeal entry
  - fundus = main body of stomach
  - pyloris = near intestinal end
- ruguae (“plicae mucosae”) = longitudinal folds that disappear w/distention
- muscularis externa = 3 layers w/oblique layer deep to circular layer

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Characteristics of stomach fxn (general)

chemical and mechanical digestion
hormone release
fundus
- secretes acid, peptic digestive products, mucus
pyloris
- secretes mucus
- contains endocrine cells that secrete gastrin

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Characteristics of the gastric epithelium

epithelium contains many folds w/mucous-secreting cells on the surface
gastric pits = between epithelial folds
- epithelium @ deep portion of pits becomes tubular gastric glands = important in digestion @ acidic pH
types of epithelial cells
- stem cells
- surface mucous cells
- chief cells
- parietal cells
- enteroendocrine cells

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Stomach protections from autodigestion

stem cells
- surface cells replaced every 3-5 days
- deep cells every 6-12 months
surface mucous cells
- release mucins/bicarb
- provides viscous protective layer from acid and abrasian
specialized digestive enzyme secretion
- secrete inactive enzyme forms (e.g. pepsinogen) which are converted to active forms (e.g. pepsin) @ low pH

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Characteristics of parietal cells

fxn = acid-producing cells

pump protons into lumen of gastric glads against a steep gradiant
- microvilli ==> large surface area
- mitochondria ==> energy for active transport
secrete intrinsic factor = required for uptake of vitamin B₁₂

stimulated by gastrin (local hormone) and histamine

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Characteristics of enteroendocrine cells

cells of the GI tract that produce hormones involved in regulation of digestion
many types present throughout digestic tract

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A cells: location, hormone, hormonal action

location: stomach & small intestine
hormone: glucagon (entero-glucagon)
action: stimulates glycogenolysis by hepatocytes ==> elevated blood glucose

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D cells: location, hormone, hormonal action

location: stomach, small & large intestine
hormone: somatostatin
action: inhibits release of hormones by other enteroendocrine cells nearby

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EC cells: location, hormone, hormonal action

location: stomach, small & large intestine
hormone:
- stomach/small = serotonin
- large = substance P
action: increases peristalsis

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I cells: location, hormone, hormonal action

location: small intestine
hormone:cholecystokinin
action: stimulates release of pancreatic hormone and contration of gall bladder

K cells: location, hormone, hormonal action

location: small intestine
hormone: gastric inhibitory peptide
action: inhibts HCl secretion

ECL cells: location, hormone, hormonal action

location: stomach
hormone: histamine
action: stimulates HCl secretion

N cells: location, hormone, hormonal action

location: small intestine
hormone: neurotensin
action:
- increases blood flow to ileum
- decreases peristaltic action of small/large intestines

S cells: location, hormone, hormonal action

location: small intestine
hormone: secretin
action: stimulates bicarb-rich fluid from pancreas

Segments of the small intestine

duodenum
- pyloric sphincter = between stomach and duodenum
jejunum
ileum

Characteristics of structure of small intestine

fxn = major center of digestine and absorption
large surface area neccessary created by topographic features:
- plicae circulares = transverse folds covered w/villi
- enterocytes = absorptive/digestive eptithelial cells covered w/microvilli
goblet cells = mucous cells between enterocytes
other structures include:
- intestinal glands
- enteroendocrine cells
- lymphoid tissue

Characteristics of intestinal glands

intenstinal glands = tubular glands = “crypts of Lieberkuhn”
“penetrate from the base of the villi deeper into the mucosa” (https://www.youtube.com/watch?v=Q8p1PPjht-I)
Paneth cells contain large eosinophilic granules w/defensin (antibacterial peptides)
Brunner’s glands = only @ duodenum
- secrete bicarb/mucins to neutralize stomach acid

Characteristics of intestinal villi

villi = loose lamina propria core + small blood vessels + some lymphatic structures
- lymphatics =
  - lympocutes
  - small lymphatic spaces join @ lacteal (large lymph vessel)
- lacteal = passes fluid + transports chylomicrons (lipoprotein dops)
capillaries ==> hepatic portal system
lacteal ==> lymphatic system ==> bloodestream

Characteristics of lymphoid tissue @ GI tract

* lymph tissue @ GI = scattered + nodules * Peyer's patches = groups of lymphatic nodules * Plasma cells @ nodules release IgA ==\> lumen to act as antibacterial agents * M-cells = antigen-uptake and presentation

Regional differences of small intestine: Brunner's glands

* Duodenum = present * Jejunum = absent * Ileum = absent

Regional differences of small intestine: Goblet cells

* Duodenum = + * Jejunum = ++ * Ileum = +++

Regional differences of small intestine: lymphatic tissue

* Duodenum = + * Jejunum = ++ * Ileum = ++++

Regional differences of small intestine: plicae circulares

* Duodenum = + * Jejunum = best developed * Ileum = +

Regional differences of small intestine: numer of villi

* Duodenum = most numerous * Jejunum = decrease distally * Ileum = less abundant

Structure of exocrine pancreas

* acini = clusters of pancreatic acinar cells arranged at the end of a common duct * @ basal portion = rough ER for protein synthesis * @ apical = zymogen granules = secretory granules * centroacinar cells = begining of duct system: secrete pancreatic juice (water + bicarb) * secreted enzymes ("zymogens") pass through progressively larger ducts ==\> main pancreatic duct ==\> common bile duct near entry to duodenum

Enzymes released by pancreas

* zymogens = inactive enzymes released by pancrease; require partial degredation before becoming active * e.g. trypsin, chymotrypsin, elastase, carboxypeptidase, tracylglycreol lipase * trypsinogen (pro-trypsin) is released and activated by entorokinase @ duodenal apical membrane * trypsin activates other zymogens * certain enzumes released in active form * amylase ==[starch & glucose] * ribonuclease ==[RNA]

Locations/mechanisms of digestion (general)

* Oral cavity * mechanical * enzymatic (e.g. amylase) * Stomach * mechanical * chemical (pH) * enzymatic (pepsin) * Small intestine * enzymatic (pancreatic enzymes) * @ glycocalyx (=glycoprotein layer at the surface of enterocytes) * e.g. starch, lactose

Characteristics of paristalsis (general)

* muscularis externa = inner circular and outer longitudinal layers * provides for movement of luminal contents by peristalsis * segmented movement = alternate contraction and relaxation of segments cause ==\> agitates the lumenal contents * Efferent fibers of the myenteric nerve plexus (of Auerbach) innervate the externa.

Characteristics of large intestine

* major regions = cecum and appendix, transverse and descending colons, and the rectum * structure * smooth, lacks plicae and villi * contains tubular glands/cyrpts * cells = mucous producing & absorptive * contains many lymphocytes * very muscular wall * fxn * recovery of water and salt * concentration of fecal material

Characteristics of oral cavity

* major fxn = initiate digestion via chewing + saliva * salivary glands * submandibular = mixed secretor * sublingual = mucous secretor * mucous = lubricative adn protective * parotid glands = serous secretor * serous secretions = watery w/enzymes and IgA