Small and Large Intestine Anatomy / Function

Question 1

3 Levels of Intestinal Folding

Answer 1

• 1- Plicae Circulares - folds of entire mucosa w/ core of submucosal tissue (similar to stomach rugae)
• 2- Villi and Crypts - pattern of epithelial folding (villi project up from surface into lumen w/ lamina propria core while crypts project down into lamina propria)
• 3 - Microvilli on ea individual enterocyte w/ core of actin cytoskeleton

Question 2

What makes up the core of the villa? What 2 cell types line the villa?

Answer 2

-Lamina propria contains fenestrated capillaries, lacteals (lymphatic capillaries), unmyelinated nerves, isolated smooth muscle cells, arterioles from submucosal artery, a lot of lymphocytes and plasma cells (in normal person - not chronic infection)

1- Enterocytes (columnar, absorptive cells)
-Tight junctions w/ claudins for water between

2- Goblet cells (secrete mucin coating to lubricate)

Question 3

What 3 cell types make up the crypts?

Answer 3

• Immature enterocytes (secrete bicarb and Cl- which drags Na+ out of cells too for co-transport in absorption)
• Paneth cells (SI only) - large basal RER and apical storage granules (make lysozymes, beta-defensins and TNF-alpha)
• Enteroendocrine Cells - secrete from basal end

Question 4

2 Major Components of Microvilli

Answer 4

• Glycocalyx - glycoprotein coat on outside of microvilli
  
  • Contain hydrolytic enzymes (enterokinase, dipeptidases, disaccharidases)
• Nutrient Transporters- on lateral side of microvilli
  
  • Di-Tri-Peptide
  • AA - co-transported w/ Na+
  • Sugar - co-transported w Na+
  • Lipids diffuse (free Fas, cholesterol, 2-monoglycerides, etc)
  • And aquaporins for water

Question 5

What does enterokinase do?

Answer 5

• converts pancreatic trypsinogen –> trypsin which then activates all other pancreatic digestive enzymes (pancreatic enzymes in pro-enzyme form so do not digest self - must be activated)

Question 6

What are Peyer’s Patches?

Answer 6

• large nodule of lymph surrounding lumen (esp in ileum)

- contain M cells that reach lumen - pinocytosis

Question 7

Duodenum

Answer 7

• SHORTEST 25 cm
• Ampula of Vater for pancreatic enzymes
• Brunner’s glands in sub-mucosa (secrete alkaline mucous to neutralize stomach chyme), villi and crypts, few goblet cells, leaf-like villi
• Secretes CCK and secretin
• Absorption of: folate, iron, calcium (Vit D-dep), Vit D, Vit Bs (except B12)

Question 8

Jejunum

Answer 8

• 2.5 meters
• Medium # goblet cells, numerous/longest finger-like villi, most defined plicae circulares
• Can distinguish normal lymphocytes/immune cells v state of inflammation - many dark lymphocytes penetrating epithelium + plasma cells (clock chromatin) and eosinophils (bi-lobe)
• Final digestion and absorption

Question 9

Ileum

Answer 9

• LONGEST (3.5 meters)
• Peyer’s patches, short/club-like villi, only shallow crypts, low plicae circulares, large # goblet cells, easy to see Paneth cells (at base of crypts)
• Terminal ileum absorbs: Vit B12 and intrinsic factor, recycle bile acids, recycle secretory IgA

Question 10

Appendix

Answer 10

• Only crypts w/ numerous goblet cells
• Very small lumen diameter (key distinguishing feature); often also have debris in lumen
• Lymphatic nodules surrounding lumen

Question 11

Colon

Answer 11

• Crypts only / no villi, simple columnar enterocytes (w/ glycocalyx but no hydrolytic enzymes), many goblet cells, few enteroendocrine cells
• NO paneth cells - want microbiome
• Outer muscle layer organized into 3 clumps (taeniae coli)

Question 12

Rectus

Answer 12

• Lined by plicae transversal recti (semilunar mucosal folds) -“speed bumps”
• Below peritoneal cavity so retroperitoneal (ADVENTITIA)
• Last chance to reabsorb - deeper crypts w/ large veins for drainage (can become hemorrhagic)
• Muscularis externa reforms continuous sheet of longitudinal smooth muscle (not taeniae coli)
• Rectum-Anus Junction (simple columnar crypts –> stratified squamous)

Question 13

Anus

Answer 13

• Pacinian corpuscle - sense deep pressure changes
• Form anal mucosa folds –> anal column or anal valves
• Internal anal sphincter - thickening of muscularis externa
• External anal sphincter - skeletal muscle innervated by pudental nerve

Question 14

SI Carb Digestion

Answer 14

• Pancreatic amylase cleaves alpha 1,4 bonds of carb THEN brush border disaccharidases break down specific dissacharides
  - Sucrose (glucose + fructose) = sucrase
  - Lactose (glucose + galactose) = lactase
  - Maltose (glucose + glucose alpha 1,4) = maltase
  - Isomaltose (glucose + glucose alpha 1,6) = isomaltase

*Challenges arise when lack of specific transporter or dec # transported (ex- lactose intolerance or complex carbs - tri or tetrasaccharides in beans, whole grains, b sprouts, etc)

Question 15

Protein Digestion and Absorption in SI

Answer 15

• Additional peptidases in SI lumen to continue breaking protein –> AA
• Mult AA Transporters - coupled w/ Na+ influx or proton influx and specific transporter dep on charge of AA

Question 16

Fat Digestion and Absorption in SI

Answer 16

• Bile acids are amphiphatic and they emulsify otherwise insoluble triglycerides to form a micelle (lipid core w/ polar heads on outside) so they can be targeted by lipases –> free FAs and glycerol
• FAs diffuse into enterocyte
• Then re-esterified –> apoliproteins and released as chylomicron or VLDL –> lymphatics

Question 17

SI Carb Absorption

Answer 17

• Sugar Transporters
  
  • SGLT1 - apical membrane transporter that moves glucose/galactose in w/ 2 Na+
    
    • Must maintain Na+ gradient w/ basolateral Na-K pump
  • GLUT-5 - apical transporter for fructose but works via passive diffusion based on fructose gradient (EASILY OVERWHELMED - extra fructose to colon - water drag- diarrhea)
  • GLUT2 - glucose exits cells down conc gradient thru this basolateral transporter

Question 18

SI Motility in Fast v Fed

Answer 18

• Fasting…MMC (function is to move luminal contents - flushing; helps maintain low bacteria count that would otherwise compete to absorb nutrients)
  
  • 1- Quiescence
  • 2- Irregular waves
  • 3- Max wave activity
• Fed… disrupts MMC for prolonged irregular activity for mixing and propagation

Question 19

What is used for oral rehydration?

Answer 19

**Glucose uptake drags water into cell w/ it so water paired w/ glucose and sodium used for oral rehydration (gatorade)

Question 20

Low FODMAP Diet

Answer 20

• fermentable, oligo, di- and monosaccharides and polyols
• If eat less of these –> less hard to digest foods –> less excess in colon –> less water drag and flatulence (in IBS)

Question 21

LI Bloody Supply

Answer 21

• Midgut - thru transverse colon (splenic flexure) - Superior Mesenteric Artery
• Hindgut - descending, sigmoid colon - Inferior Mesenteric Artery
  
  • **Watershed area at splenic flexure is most common site of ischemia
• Cloaca - lower rectum - Hemorrhoidal Vessels

Question 22

3 Types of LI Contractions

Answer 22

• Short duration - short spike bursts (mix in place)
• Long duration - long spike bursts (move things slightly forward/backward to mix)
• Giant migration contractions - actually move food forward

Question 23

6 General LI Functions

Answer 23

• Mixing/ dehydration
• Storage - Compliance (esp rectum)
• Electrolyte and Water Absorption (about 20% of overall GI absorption)
• Fermentation
• Mass Movements - High Amp Peristaltic Contractions
• Defecation (usually 24-72 hrs between eating and defecation - more so in L colon)

Question 24

Main Components of Defecation

Answer 24

• Sphincters /Muscles
  
  • IAS - (autonomics) slow twitch muscle
  • EAS - (skeletal) stimulated by rectal distention, posture change, cough, perianal pinprick
  • Puborectalis Muscle - maintains kink in anorectal canal (relaxes to defecate)
• Sensations
  
  • Sense inc wall tension
  • Sense type of material (solid, liquid, gas)