Digestive System Overview

Question 1

General characteristics of GI system

Answer 1

• 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

Question 2

Lining of the GI system (general)

Answer 2

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

Question 3

Regulation/coordination of GI system (general)

Answer 3

• 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

Question 4

Basic structures of layers in GI tract

Answer 4

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

Question 5

Characteristics of mucosa layer

Answer 5

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

Question 6

Characteristics of the Esophagus

Answer 6

• 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

Question 7

Characteristics of the Stomach structure (general)

Answer 7

• 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

Question 8

Characteristics of stomach fxn (general)

Answer 8

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

Question 9

Characteristics of the gastric epithelium

Answer 9

• 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

Question 10

Stomach protections from autodigestion

Answer 10

• 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

Question 11

Characteristics of parietal cells

Answer 11

• fxn = acid-producing cells

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

• stimulated by gastrin (local hormone) and histamine

Question 12

Characteristics of enteroendocrine cells

Answer 12

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

Question 13

A cells: location, hormone, hormonal action

Answer 13

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

Question 14

D cells: location, hormone, hormonal action

Answer 14

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

Question 15

EC cells: location, hormone, hormonal action

Answer 15

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

Question 16

I cells: location, hormone, hormonal action

Answer 16

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

Question 17

K cells: location, hormone, hormonal action

Answer 17

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

Question 18

ECL cells: location, hormone, hormonal action

Answer 18

• location: stomach
• hormone: histamine
• action: stimulates HCl secretion

Question 19

N cells: location, hormone, hormonal action

Answer 19

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

Question 20

S cells: location, hormone, hormonal action

Answer 20

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

Question 21

Segments of the small intestine

Answer 21

• duodenum
  
  • pyloric sphincter = between stomach and duodenum
• jejunum
• ileum

Question 22

Characteristics of structure of small intestine

Answer 22

• 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

Question 23

Characteristics of intestinal glands

Answer 23

• 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

Question 24

Characteristics of intestinal villi

Answer 24

• 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

Question 25

Characteristics of lymphoid tissue @ GI tract

Answer 25

• 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

Question 26

Regional differences of small intestine: Brunner’s glands

Answer 26

• Duodenum = present
• Jejunum = absent
• Ileum = absent

Question 27

Regional differences of small intestine: Goblet cells

Answer 27

• Duodenum = +
• Jejunum = ++
• Ileum = +++

Question 28

Regional differences of small intestine: lymphatic tissue

Answer 28

• Duodenum = +
• Jejunum = ++
• Ileum = ++++

Question 29

Regional differences of small intestine: plicae circulares

Answer 29

• Duodenum = +
• Jejunum = best developed
• Ileum = +

Question 30

Regional differences of small intestine: numer of villi

Answer 30

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

Question 31

Structure of exocrine pancreas

Answer 31

• 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

Question 32

Enzymes released by pancreas

Answer 32

• 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]

Question 33

Locations/mechanisms of digestion (general)

Answer 33

• 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

Question 34

Characteristics of paristalsis (general)

Answer 34

• 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.

Question 35

Characteristics of large intestine

Answer 35

• 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

Question 36

Characteristics of oral cavity

Answer 36

• 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