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Phys 210 > GI tract > Flashcards

GI tract Flashcards

1
Q

Functions of the GI tract

A
  1. Digestion
    - chemical alteration of food into absorbable molecules
  2. Absorption
    - movement of digested food from the intestines into blood
  3. Excretion
  4. Host defense
    - highly developed immune system
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2
Q

Components

A

mouth, pharnyx, esophagus, stomach, small intestine (duodenum, jejunum, illeum), large intestine, accessory organs (pancreas, liver, gallbladder)

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3
Q

Layers of GIT

A
  1. Mucosa
  2. submucosa
  3. muscularis externa
  4. serosa
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4
Q

Muscosa layers - epithelial

A

a. epithelial layer
- layer of cells that lines all body cavities and surfaces
- apical surface = faces lumen
- basolateral = faces blood stream
- tight junctions confine transport proteins to specific regions
-provides selective uptake
- surface area is amplified by villi and crypts

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5
Q

paracellular pathway

A

limited by tight junction seal
water and small ions can diffuse through tight junctions

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6
Q

transcellular pathway

A

two step process which requires a transport protein on the apical and basolateral surfaces of cell

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7
Q

Mucosa layers - lamina propria

A

b. lamina propria
- connective tissue
- small blood vessels
- nerve fibres
- lymphatic vessels

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8
Q

mucosa layers - muscularis mucosa

A

c. muscularis mucosa
- thin layer of smooth muscle (not involved in contractiom of GIT)

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9
Q
  1. Submucosa
A

plexus of nerve cell bodies
- relays info to and away from mucosa
blood and lymphatic vessesls

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10
Q
  1. muscualris externa
A

thin layer of circular muscle
- narrowing of lumen
myenteric nerve plexus
- network of nerve cells regulate muscle function
thinner outer layer of longitudinal muscle
- shortens the tube

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11
Q
  1. serosa
A

thin layer of connective tissue
- forms connections so intestines do not freely float

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12
Q

Blood supply to the liver

A

portal circulation (series circulation)
- blood is taken from digestive tract and empties directly into liver
- allows liver to remove harmful substances (like a filter)
-liver also receives systemic oxygenated blood (10%) (parallel blood flow)

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13
Q

Regulation of GI processes

A

reflexes initiated by:
distension of wall by volume of luminal contents
osmolarity of contents
pH of contents
conc. of specific digestion contents

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14
Q

intrinsic neural regulation of GI processes
(short reflexes)

A

enteric nervous system ( contained within GI tract)
- controls activity of secretomotor neurons
- contained within walls of GIT
- can function independantly of CNS
1. myenteric plexus
2. submucosal plexus

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15
Q

myenteric plexus

A

influences smooth muscle

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16
Q

submucosal plexus

A

influences secretion (closer to the lumen)

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17
Q

Intrinsic vs extrinsic

A

Intrinsic → contained wholly within the organ
Occurs through nerve plexi located in the GIT wall itself Nerve plexi = branching networks of intersecting nerves
Extrinsic - regulation is through systems outside of the organ/ GI tract
Through the parasympathetic and sympathetic nervous system

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18
Q

Extrinsic regulation of GI processes
(long reflexes)

A

done by autonomic nervous system
-parasympathetic
stimulates flow of saliva, stimulates peristalsis and secretion, stimulates release of bile

-sympathetic
stimulates flow of saliva, inhibits peristalsis and secretion

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19
Q

Neurocine

A

a nerve cell produces a electrical signal resuting in a neurotransmitter to be released which travels across to synapse onto another nerve cell or effector cell

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20
Q

Endocrine

A

a chemical messenger passes from cell which produced it into blood and is carried by blood to its distant target

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21
Q

Paracrine

A

chemical messenger diffuses through interstitial fluid to nearby cells

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22
Q

Autocrine

A

chemical messenger acts on cells which produced it

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23
Q

CCK

A

a GI hormone
- fatty acids and amino acids in the small intestine trigger CCK secretion from cells in small intestine into blood

  • CCK stimulates:
    pancreas to increase digestive enzyme secretion
    gall bladder contraction - release of bile acids for fat breakdown!!
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24
Q

Secretin

A

GI hormone
released from S cells in small intestine
acid stimuli’s release
lowers HCl and stomach motility
increases bicarbonate/ H2O from pancreas and bile

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25
Q

Gastrin

A

GI hormone
released from G cells in stomach antrum
- peptides/amino acids in stomach cause release

increases HCl and stomach/ileum motility

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26
Q

Peristalsis

A

Circular and longitudinal muscle contraction of two outer smooth layers of the GIT
- propels contents of lumen towards the anus

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27
Q

Segmentation

A

mixing
- contraction and relaxation of the intestinal segments with little net movement to allow mixing of contents and enzymes, and slows down food movement for absorption

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28
Q

Basic electrical rhythm

A

GIT has pacemaker cells throughout the smooth muscle that are constantly undergoing spontaneous depolarization-repolarization cycles
These slow waves are the basic electrical rhythm - do not cause muscle contractions on their own!!

excitatory hormones/NTs further depolarize to bring to threshold and contract

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29
Q

Phases of gastrointestinal control

A
  1. cephalic
  2. gastric
  3. intestinal
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30
Q

control of food intake

A

hypothalamus
- feeding centre in lateral region
- satiety centre in ventromedial region

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31
Q

activation of feeding centre in lateral region increases what?

A

hunger

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32
Q

acitvation of satiety centre in ventromedial region does what?

A

makes you feel full

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33
Q

Leptin

A

inhibits neuropeptide Y
- neurotransmitter that stimulates hunger

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34
Q

regulation of water intake
stimualtion

A
  1. Increased plasma osmolarity (osmoreceptors)
  2. Decreased plasma volume (baroreceptors)
  3. Dry mouth and throat
  4. Prevention of over-hydration by GI tract
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35
Q

three main salivary glands

A
  1. parotid - watery (serous) secretion
  2. submandibular - serous/mucous secretion
  3. sublingual - mucous secretion
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36
Q

components of saliva

A

mostly water - hypotnic + alkaline

contains digestive enzymes
- amylase + lipase

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37
Q

functions of saliva

A

Moistens and lubricates food
Initiates digestion
Dissolves a small amount of food which effects appetite and food intake
Prevents microbial colonization

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38
Q

cells of salivary glands

A
  1. Acinar
  2. ductal
  3. myoepithelial
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39
Q

Acinar

A

proteins (amylase mucin)
- released by exocytosis
water, and electrolyte (Cl-,Na+) secretion

= primary secretion (isotonic)

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40
Q

ductal

A

lets HCO3 (bicarbonate) in and Na+ and Cl- out
= secondary modification so its alkaline and hypotonic

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41
Q

myoepithelial cells

A

contract and expel formed saliva from acinus into ducts

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42
Q

Saliva regulation

A

NO HORMONE REGULATION

primarily parasympathetic + sympathetic

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43
Q

Role of saliva in digestion

A

starch digestion is initiated in the mouth by amylase (cleaves inernal a-1,4 linkages)
- creates maltose, maltotriose and a-limit dextrin

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44
Q

how is swallowing initiated?

A

complex reflex initiated by pressure receptors in the wall of the pharnyx
- stimulated by food/ liquid entering pharnyx

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45
Q

steps of swallowing

A

a. tongue pushes food bolus to the back of the pharnyx
b. soft palate elevates to prevent food entering the nasal passages
- swallowing centre inhibits respiration, raise the larynx, and close the glottis
c. epiglottis covers glottis to prevent food or liquid entering the trachea
d. food descends into the esophagus

46
Q

esophageal sphincter

A

upper esophageal sphincter
- ring of skeletal muscle just below pharnyx

lower esophgeal sphincter
- ring of smooth muscle at stomach

47
Q

functions of the stomach

A

+ storage of food
+ mechanical breakdown of food
+ chemical breakdown of food
- secretes pepsinogen (digests proteins)
- secretes HCl (dissolves food, partially digests macromolecules)

+ controls the rate food enters the small intestine
+secretes intrinsic factor
- critical for absorption of B12

48
Q

What are the names of the different regions of the stomach?

A
  1. Fundus
  2. Body
    - both are thin layer of smooth muscle
    - secretes mucus, pepsinogen, Hcl
  3. antrum
    - thicker smooth muscle layer
    - secretes mucus, pepsinogen, gastrin
49
Q

Mucous cell

A

secretes mucous to protect stomach lining from acid

50
Q

parietal cell

A

secretes intrinsic factor and HCl
- not found in antrum

51
Q

Chief cell

A

secretes pepsinogen
(precursor to pepsin- which digests proteins)

52
Q

Enteroendocrine cells/ G cells

A

secrete gastrin (hormone that stimulates HCL production by parietal cell and stimulates GI motility)
- mostly in antrum

53
Q

zymogen

A

means that it is a precursor for a protein/enzyme

54
Q

Enterochromaffin-like (ECL) cells

A
  • gastric glands
  • secretes histamine (stimulates HCl release)
55
Q

D-cells

A

-secretes somatostatin (negative regulator of HCL secretion!!)

56
Q

acidification of stomach lumen

A
  1. H+/K+ ATPase
    - pumps H+ into lumen in exchange for K+ into cells
    - ACTIVE transport (uses atp)
  2. Carbonic anydrase (CA)
    -catalyses formation of H2CO3 from H2O and CO2
    - H2CO3 dissociates H+ (for lumen secretion) and HCO3-
  3. Cl-/HCO3- exchanger
    - excess HO- is taken out of cell INTO BLOOD as HCO3- in exchnage for Cl-
  4. K+ channels
    K+ diffuses back into stomach lumen
    - loss of positive charge (compensated when Cl- leaves cell too)
  5. Cl- channels
    - Cl- leaks back into stomach lumen through diffusion
57
Q

4 chemical messengers that regulate acid secretion

A
  • gastrin (gastric hormone)
  • acetlycholine (NT)
    -histamine (paracrine)
  • somatostatine (paracrine)
    from D-cells
58
Q

what activates pepsinogen?

A

-released from chief cells
-activated by acidic pH in stomach lumen

59
Q

factors that increase acid secretion?

A

Acetylcholine, gastrin and histamine all directly increase acid secretion

60
Q

factors that inhibit acid secretion?

A

Somatostatin directly inhibits acid secretion by parietal cells

61
Q

Different GI phases

A

1.cephalic - stimulation in the brain - sight , smell, taste etc causes exciatiry stimuli via vagus nuclei

  1. Gastric phase - occurs when food reaches the stomach
    Major phase, regulates gastric production
  2. Intestinal phase - occurs when food that has been partially broken down by the stomach enters the SI (duodenum)
    Mainly inhibitory, wants to slow down food for the intestine to absorb/digest
    - Mediated by secretion and CCK
62
Q

Gastric motility

A

Waves of contraction is set by the basic electrical rhythm, but strength of contraction is determined by stimulatory input.

63
Q

What causes vomiting?

A

Motion sickness/inner ear infection
Alcohol/toxins
Pressure on CNS
GIT disturbances (i.e infection)
sight /smells etc

64
Q

The pancreas, exocrine gland

A

Exocrine gland = releases substances into a duct that drain onto epithelial surfaces
Source for majority of enzymes required for digestion
Carbohydrate, protein, aft, nucleic acid
!!!!!secretes HCO3- which is critical for enzyme function

65
Q

The pancreas, endocrine gland

A

= secretes products directly into blood stream
Produces hormones that regulate the entire body i.e insulin

66
Q

Pancreatic ducts

A

Duct cells secrete H2O and HCO3-

Acinar cells produce and secrete digestive enzymes through exocytosis

67
Q

Pancreatic juices

A

isotonic, alkaline
contains electrolytes
- high HCO3-, low Cl-
contains digestive enzymes
( secreted by acinar cells)

68
Q

production of HCO3- by pancreatic duct cells

A
  1. CFTR opens
    - allows diffusion of Cl- into duct lumen
  2. Cl- in lumen is exchanged for HCO3- in cell (Cl- goes back into the cell)
  3. H2O and Na+ follow paracellulary in response to electrochemical gradient across epithelium
  4. Neutral pH pf cytosol is maintained by exchange of H+ (exported) and Na+ (imported)
    • this is dependant on the chemical gradient generated by Na+/K+-ATPase
69
Q

Alkaline tide

A

after a big meal, when the parietal cells are producing lots of acid in stomach - lots of HCO3- is pumped into the blood

tide = moving into blood

70
Q

Acid tide

A

after a big meal, duct cells in pancreas are producing and secreting HCO3-, so large amounts of H+ are pumped across the basolateral surface into the blood stream

71
Q

acinar cells in pancreas

A

synthesize and package pro-enzymes into zymogen granules

72
Q

proteases

A

digest proteins into peptides and amino acids

73
Q

amylolytic enzymes

A

digest starch into sugars

74
Q

lipases

A

digest triglycerides into free fatty acids and monoglycerides

75
Q

nucleases

A

digest nucleic acids into free nucleotides

76
Q

how to activate enzymes

A

enterokinase turns trypsinogen into trypsin which then activates enzymes

77
Q

Major proteases secreted by the pancreas

A

trypisinogen (activated by enterokinase to form trypsin)

chymotrypsinogen (activated by trypsin to form chymotrypsin)

Pro-elastase (activated by trypsin to form elastase)

all of them hydrolyze interior peptide bonds of proteins and polypeptides

78
Q

Regulation of Pancreatic juice secretion

A

Acid in duodenum stimulates Scells - secrete secretin - leads to more HCO3- production (duct cells)

Digested fat/protein in small intestine stimulates I cells - secrete CCK - stimulates production of Enzymes (from acinus cells)

79
Q

Secretin and CCK both

A

inhibit gastrin secretion
- reduced stomach motility
- reduced acid secretion

80
Q

The hepatic and lobule structure

A

Hexagonal structure with central vein running through the centre
Portal triads at each corner
Has hepatic artery, portal vein and bile duct

81
Q

What does the liver do?

A
  • filter the blood
  • forms and secretes bile into a duct (exocrine gland)
82
Q

blood flow to the liver

A

receives blood from systemic (25%) and portal circulation (75%)

83
Q

what is bile made out of

A
  1. bile acids
  2. cholesterol
  3. salts (Na+, K+, HCO3-)
  4. phospholipids
  5. bile pigments
  6. trace metals
84
Q

How does bile help in fat digestion

A

pancreatic lipase can only work at the surface of lipid molecules
- larger droplets need to be emulsified into smaller ones for lipase

  • emulsifying agents prevent aggregation of droplets
  • ampipathic bile acids and phospholipids
85
Q

biile acids also form mixed micelles with ?

A

phospholipids and products od lipase ( free fatty acids + monoglycerides) = micelle

86
Q

what do micelles do?

A

keep monoglycerides and fatty acids in small soluble aggregates
like a “holding station” for small insoluble lipids until they free form and diffuse across the small intestine epithelium

87
Q

what produces bile?

A

hepatocytes
produce and secrete bile

88
Q

how much bile is recovered?

enterohepatic circulation

A

95%
- occurs through enterohepatic circulation
- reabsorbed across the small intestine into portal circulation
then they are transported back into hepatocytes

89
Q

Regulation of bile

A

CCK Increases contraction of gallbladder and relaxes the sphincter of oddi

Secretin Increases HCO3- secretion by the bile duct cells (+ pancreas)

90
Q

sections of small intestines

A
  1. duodenum
  2. jejunum
  3. illeum
91
Q

illeum

A

digestion and absorption
- bile acids
- vitamin B12

92
Q

jejnumun

A

digestion and absorption

93
Q

duodenum

A

mixing of pancreatic digestive enzymes and bile with food
absorption of nutrients,iron, and calcium

94
Q

how is surface area increased

A

intestinal villi, crypt, and micro villi

95
Q

Cell types of the small intestine

A
  1. absorptive cell
    - brush border enzymes
  2. goblet cell
    - secretion of mucus
  3. enteroendocrine cell
    - release of hormones
    - I or S cells
  4. paneth cell
    - secrete antibacterial proteins
96
Q

what are brush border enzymes

A

enzymes anchored to the brush border with catalytic activity in the lumen
- break down carbohydrates and peptides into sugars and amino acids

97
Q

digestion of carbohydrates

A

starch is broken down into maltose, maltriose and a limit dextrins by salivary and pancreatic amylases

then they are broken down into monosacchardies glucose by brush border enzymes

sucrose = glucose + fructose
lactose = glucose + galactose

98
Q

how to absorb glucose/galactose

A

by SGLT transporter

99
Q

how to absorb fructose

A

by GLUT5

100
Q

protein digestion

A

proteins are broekn down by pepsin in the stomach and in the small intestine by pancreatic proteases

then they are broken down further into free amino acids by pancreatic proteases and brush border enzymes

101
Q

fat digestion

A

lipid droplets are emulsified by bile acids and phospholipids and allow pancreatic lipase to release free fatty acids and monoglycerides

102
Q

Absorption of Iron

A

stored by ferritin (a protein) in enterocyte

iron that is not stored is released on blood side of enterocyte and transported through the blood attached to the plasma protein transferrin

103
Q

where is the majority of water/fluid reabsorbed

A

by the small intestine

104
Q

where does absorption and secretion of water

A

water absorption at the villi
secretion of the crypts

105
Q

absorption of water in small intestine

A

predominantly depends on Na+ gradients generated during secondary active nutrient uptake

water follows Na+ into the cell

106
Q

secretion of water in the small intestine

A

predominately depends on Cl- gradients generated by secondary active Na+/K+/2Cl- transporter

H2O follows the Cl- out of the cell

107
Q

Regions of the large intestine

A
  • cecum/appendix
    No apparent function in humans

Ascending/transverse/descending/sigmoidal colon
- re absorption of water
- Reservoir for storage of wastes
and indigestible materials prior
to elimination by defecation
Rectum
- Reservoir for feces
Anus
- Two sphincters that control
defecation

108
Q

does large intestine contain villi

A

NO! only crypts (smaller surface area)

109
Q

cells of large intestine

A
  1. absoprtive cells/ enterocytes
    Similar to small intestine but no brush border enzyme
  2. Golblet cells
    Secrete mucin to create protective mucus layer
110
Q
A

Phys 210 (2 decks)

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