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GI Week 2 > Lecture 11: GI Physiology, Small Bowel and Colon > Flashcards

Lecture 11: GI Physiology, Small Bowel and Colon Flashcards

1
Q

How long is the small bowel?

A

21 feet

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2
Q

What are the key characteristics of the small bowel?

A
  1. gradient of peristaltic contraction rates
  2. pacemaker cells throughout (intestinal cells of cajal) that have rate of 9/12 contracts per minute
    End result = mixing of luminal contents, enanced contact time with epithelium, 3-5 hours transit time
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3
Q

What regulates the small bowel?

A
  1. Distention for mechanoreceptors (activates intrinsic reflex arc)
    • particularly in duodenum
  2. Extrinsic nerves
    • parasympathetic stimulates and sympathetic depresses
  3. Gastrin
    • activates segmentation in ileum with meal
    • gastroileal reflex
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4
Q

What is the migrating motor complex?

A
  1. occurs during the fasted state
  2. initiated by rise in motilin from ileum
  3. sequential, organized short peristaltic waves
  4. abrupt termination with meal (gastrin)
  5. starts in stomach and progresses caudally
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5
Q

Where is the vomiting center?

A

In the medulla (lateral reticular formation near respiratory center)
Input = irritant receptors in gut
Output = vagal, phrenic, spinal, sympa and parasympathetic nerves

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6
Q

What is MoA of vomiting?

A

Saliva  intestine contract  pylorus contract  abdomen and diaphragm contract  LES and esophagus dilate  glottis closes

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7
Q

What are the duodenal luminal contents?

A
  1. Ingestion
  2. saliva
  3. gastric secretion
  4. Bile
  5. Pancreatic secretion
    7-8 L/day of outside stuff
  6. Small bowel secretion = 1.5 L/day
    So total small bowel volume = 9-10 L/day
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8
Q

What does small bowel secrete?

A

Water and mucus to lubricate and protect cells
H2O for enzymatic hydrolysis of nutrients
Driven by Cl- in epithelial crypt cells via the CFTR
Regulated by cAMP

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9
Q

Why does cystic fibrosis affect GI?

A

Because water secretion in small intestine depends on CFTR

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10
Q

What amplifies the absorptive surface of the small intestine?

A
  1. folds
  2. villi
  3. microvilli (on top of villi)
    600x
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11
Q

What are the key characteristics of absorption in small bowel?

A

Not regulated
Small intestine is designed to absorb ALL nutrients presented to it
Does not concentrate shit

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12
Q

What are the distribution of apical channels in small bowel?

A

Sodium channel on the villus (where the absorption takes place)
CFTR in the crypts where the secretion takes place

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13
Q

What can be found in the crypts (of Lieberkuhn)?

A

Cl- channels

Stem cells

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14
Q

How are carbs digested?

A
  1. pancreatic amylase secretion
  2. majority absorbed in jejunum
  3. degrade complex carbs to disaccharides like maltose, sucrose and lactose
  4. final step = brush border dissacharidases that breakdown disaccharides and COTRANSPORTS the monosaccharides along with Na
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15
Q

Where is glucose absorbed?

A

All in the small intestine

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16
Q

Where is lactose absorped?

A

Differences in lactase ability among individuals so meal lactose absorption is variable between individuals

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17
Q

How are proteins digested?

A
  1. initiated in stomach (HCl-pepsin)
  2. Pancreatic proenzymes activated in the lumen
    • enterokinases catalyzes trypsinogen  trypsin
      - trypsin activates other proteases
    • proteins degraded to AAs
  3. Brush border aminopeptidases
  4. Cotransport of AAs with Na
  5. Contransport of di/tripeptides with H
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18
Q

Why is the presence of sodium contransport channels on the villi significant?

A

ecause carbs and protein are cotransported with Na so this is where absorption takes palce

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19
Q

What is the significance of enterokinase? Trypsin?

A

Enterokinase makes trypsin (by converting trypsinogen to trypsin)
Trypsin activates proteases, and is a protease itself

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20
Q

How are fats digested?

A

Insoluble in water so has to be in 3 phases

i. water in lumen
ii. lipid in cell membrane
iii. water in lymphatics and blood stream 1. fat  emulsion 2. apical cell membrane transfers products via micelle 3. cell reconstitutes into bigger lipid molecules 4. basolateral cell membrane exports this to blood stream
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21
Q

What are the key characteristics of the emulsification step of fat digestion?

A
  1. gastric lipase converts Triglyceride to FA and diacylglycerol (DG)
    • DG and Phospholipids stabilize emulsion
  2. bile salts stabilize emulsion
  3. pancreatic lipase converts TG to FA and MG
  4. stable emlsuion product includes an inner core of cholesterol, fat vitamins and TG while outer core = phospholipid, FA, bile saltes
22
Q

What is colipase?

A

Enzyme secreted by pancrease to bring lipase with TG (to break it down in small bowel)

23
Q

What is phospholipase A2?

A

Secreted by pancreas
Digests PL on surface of emulsion
Helps anchor lipase to emulsion
Activated by bile salts

24
Q

Is there digestion of fat by brush border enterocytes?

A

NO

25
Q

What is the key step of transferring emulsified fat into cells?

A

The formation of micelles by bile salts

Amphipathic allows for transport across water-lipid interface

26
Q

Where do bile salts get reabsorbed?

A

Terminal ileum

27
Q

Once inside the small bowel cells, what does the fat get transformed and packaged into?

A
  1. MG + FA = TG
  2. TG, cholesterol and PL packaged as CHYLOMCRONS for export in lymphatics of lacteals
    Coated with ApoA and B
28
Q

What is the significance of chylomicrons?

A

Transport of TG or fat in body from small intestine to liver

Transport through lacteals or lymphatics

29
Q

What are the key characteristics of vitamin digestion?

A 
Absorbed whole and are NOT digested
Water soluble = B and C
Fat soluble = A, D, E, K
B12 needs intrinsic factor (from parietal cells) in terminal ileum
Lack of B12 = pernicious anemia
30
Q

How is B12 digested?

A

B12 binds to R protein (from stomach, saliva and bile)
Pancreatic enzymes free B12 from R protein and then binds intrinsic factor
IF-B12 complex is taken up by terminal ileum

31
Q

How does the rate of absorption compare?

A

Ileum > jejunum > duodenum

32
Q

What drives water absorption?

A

NaCl absorption (no concentrating, all isosmotic)

33
Q

What is the active process of digestion?

A

Na-K-ATPase on basolateral membrane
Pumps out 3 Na for every 2 K brough in
Blood is electropositive compared to inside of cell

34
Q

Why is blood electropositive when compared to enterocyte?

A

Because 3Na+ is pumped out for every 1 K+ taken in

35
Q

What is the function of the colon?

A
  1. salt and water absorption, but also does NOT concentrate (so contents iso-osmotic)
  2. Waste storage
  3. No digestive function
    Colon is a big garbage can so it is more of a luxury item
36
Q

Is there digestive function in colon?

A

NO

37
Q

What is the significance of the ileocecal valve?

A

Regulates entry of material into colon
“Flutter” valve because it is one way movement
-opens with distention of ileum
-closes on contraction of cecum
Ileal brake = sensors for fat inhibit gastric motility and mediated by Peptide YY

38
Q

What does Peptide YY do?

A

It inhibits the ileocecal valve from contracting

39
Q

What are the taenia coli?

A

Outer longitudinal smooth muscle layer does not completely surround colon but exists as discrete longitudinal bands

40
Q

What are haustra?

A

Colon is shaped as group of pouches which move with circular smooth muscle activity
The segments of colon

41
Q

What are the characteristics of absorption/secretion in colon?

A
  1. large permeable to ions than small bowel
    More like distal renal tubule because it reabsorbs Na, Cl, water
    Absorbs = na, Cl, water
    Secretion = K, HCO3-
    -important exit pathway for K
42
Q

What are the two types of cells in colon?

A
  1. surface cells (absorb Na and Cl and secretes K and HCO3)

2. Crypt cells (secrete Cl, dependent upon cAMP)

43
Q

How much gets reabsorbed in the bowel?

A

99%

44
Q

Which is better at absorbing water?

A

Small bowel is better than colon

45
Q

What are the characteristics of colonic motility?

A
  1. segmentation, 2 contractions/hr
  2. regulated by autonomic nervous system
  3. propulsive via peristaltic waves, intrinsic nerves
46
Q

What is the gastrocolic reflex?

A

Meals that stimulate strong contractions in proximal colon (mass movement)
Feces moves 1/3 length of colon in few seconds after eating
Responds to GASTRIN

47
Q

What are the characteristics of colonic bacteria?

A

A shitload of bacteria in colon
Slow flow = great for bacteria growth
Anaerobes
Produce gas and vitamin K
Stimulate immune development, IgA secretion
Normal flora competes against pathogenic flora

48
Q

What are the characteristics of defecation?

A
  1. voluntary muscles and involuntary muscles
  2. under hypothalamic control
  3. reflexs = feces in rectum  rectal distention  relaxation of internal anal sphincter
49
Q

What are the smooth muscles of defecation?

A
  1. colon/rectum

2. internal anal sphincter

50
Q

What is the defecation sequence?

A
  1. stool in rectum
  2. hypothalamus says it’s a good time
  3. contraction of abdomen muscles, diaphragm, colon and rectum
  4. relaxation of internal anal sphincter
  5. relax puborectalis, contraction of levator ani
  6. Straigtening of rectoanal angle (which happens when you sit)
  7. Relaxation of external anal sphincter

GI Week 2 (15 decks)

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