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1-5 GIT > Physiology > Flashcards

Physiology Flashcards

1
Q

GIT function

A

Motility, digestion, absorption

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2
Q

Enzymes involved in chemical digestion of carbohydrates

A 
Pancreatic amylase (main)
Alpha-dextrinase, sucrase, lactase, maltase in brush border 
---> all end in monosaccharides
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3
Q

Enzymes in the chemical digestion of proteins

A
  • Trypsin, chymotrypsin, carbopeptidase and elastase from pancreas
  • aminopeptidase and dipeptidases in brush border
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4
Q

Enzymes in lipid chemical digestion

A
  • most lipase from pancreas
  • pancreatic lipase most important in TGL digestion
  • emulsification by bile salt to increase SA
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5
Q

Nucleic acid enzymes in chemical digestion

A

Nucleases

  • ribo/deoxyribonuclease in pancreatic juice
  • nucleosidases and phosphates in brush border
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6
Q

What is located on the apical membrane of brush border?

A

Brush border enzymes

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7
Q

What is muscle contraction stimulated by?

A

Calcium ions, via calmodulin –> MLCK (not troponin C)

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8
Q

What are the two types of waves in the phasic pacemaker of enteric smooth muscle?

A
  1. slow waves - basal rhythm, not AP/not contraction

2. spikes - when slow waves pass threshold and AP generated

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9
Q

Effect of ACh and Adrenaline on digestion

A

Ach increases

Adr decreases

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10
Q

What is peristalsis and what induces it?

A 
Feeding induces it
Gut wall (any part) reflex  initiates retrograde circular contraction and anterograde circular relaxation
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11
Q

What are involuntary sphincters controlled by?

A
  1. myogenic tone (ANS/ENS/reflexes/hormones)

2. relative pressures

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12
Q

What are the direct and indirect effects of ACh on enteric muscle?

A

Direct - constriction, prevents flow

Indirect - relaxation, excites interneurons and facilitates flow

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13
Q

Nerve plexuses of enteric nervous system (intrinsic)

A 
Submucosal plexus (Meissner’s): between mucosa and circular muscle layer 
Myenteric plexus (Auerbach’s): between circular and longitudinal muscle layer
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14
Q

What are the components of the extrinsic autonomic nervous system in digestion

A

Parasympathetic stimulation increases secretion and activity by stimulating ENS

Sympathetic stimulation decreases secretions and activity by inhibiting ENS

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15
Q

GIT > prevertebral SNS ganglia > GIT reflexes

A

Gastrocolic reflex: stomach colon evacuation

Enterogastric reflex: colon small intestine to inhibit stomach motility/secretion

Colonoileal reflex: colon ileum to inhibit ileal emptying

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16
Q

GIT > CNS > GIT reflexes

A

Control gastric motility/secretion: stomach/duod brainstem GIT

Pain reflexes general inhibition of entire GIT

Defecation reflexes: colon/rectum spinal cord powerful colon/rectum/abdominal contractions

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17
Q

4 phases of digestion and main control

A
  1. Interdigestive phase
  2. Cephalic phase (neural)
  3. Gastric phase (neural + hormonal)
  4. Intestinal phase (hormonal)
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18
Q

Coordination of the cephalic phase in digestion

A

smell/taste > reflex from taste buds > stimulate vagal centre in CNS > cranial nerves stimulate secretions (saliva, pancreas, stomach) > vagus also increases motility of stomach and intestine

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19
Q

Coordination of the gastric phase in digestion

A

Food stimulates vago-vagal reflexes and hormone release > stimulates gastric and pancreatic secretion > hormones further stimulate secretion > vagal activity stimulates gastric motility

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20
Q

Coordination of the intestinal phase

A

food > hormones released from gut mucosa > stimulate secretion of exocrine pancreas > H. stimulate gallbladder contraction > inhibitory hormones act on stomach to inhibit acid secretion (overload protection)

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21
Q

Interdigestive phase coordination in digestion

A

Stomach - small volume
Jejunum - empty
Pancreas - v little secretion
Liver and gallbladder - sphincter of oddi closed, bile diverted to gall bladder to be stored and concentrated

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22
Q

What are the 4 layers of the stomach and what cells do they contain?

A
  1. mucosa
  2. 3 types of exocrine glands: mucous neck, parietal and chief cells
  3. G cell
  4. D cell
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23
Q

What do mucous neck cells secrete and what is their function?

A

Mucus - physical barrier between lumen and epithelium

Bicarbonate - buffers gastric acid

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24
Q

What do parietal cells secrete and what is the function?

A

Gastric acid - activates pepsin and kills bacteria

Intrinsic factor - B12 absorption

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25
Q

What do ECL cells secrete and what is their function?

A

Histamine - stimulates gastric acid secretion from parietal cells

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26
Q

What do chief cells secrete and what is their function?

A

Pepsin - digest protein

Gastric lipase - digest fats

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27
Q

What do D and G cells secrete?

A

D - somatostatin: inhibit gastric acid secretion

G - gastrin: stimulates gastric acid secretion

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28
Q

How are solutes moved from gut lumen into the blood stream ?

A

Paracellular – Gut lumen into paracellular junction = passive, then dependent on structures of the tight junctions

Transcellular - movimento of solute via active transport processes

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29
Q

How is water absorbed?

A

Mostly in the small intestine (jejunum): all transported bia aquaporins but a concentration gradient must be setup before (via electrolytes)

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30
Q

Where is sodium absorption highest? and vitamin B12?

A

sodium - colon (to allow water transport)

B12 - lower small intestine

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31
Q

How is Cl- absorbed? and secreted?

A
  • passive diffusion
  • CL- HCO3 exchange
  • parallel Na+, H+ and HCO3 exchange

secreted via Cl- channels (CFTR cystic fibrosis)

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32
Q

How is K+ absorbed and secreted?

A

Absorption - mostly passive diffusion, also active K+H+ exchange
Secretion - passive, also apical K+ channel powered by basal Na+ K+ pump

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33
Q

Which group of minerals can enter enterocytes?

A

Soluble

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34
Q

What does mineral bioavailability depend on?

A

pH
Redox state of metal
Dietary complexes which enhance solubility
dietary complexes which diminish absorption

35
Q

Two routes of calcium absorption

A
  1. Active transcellular route = duodenum and vitD dependent

2. Paracellular diffusion = through SI

36
Q

How are the two forms of dietary iron absorbed?

A

Haem iron: more readily absorbed and taken up as Fe-porphyrin complex

Non-haem-iron: absorbed in duodenum, either ferrous (2+) or ferric (3+)

37
Q

Importance of ferritin in iron overload?

A

It complexes with iron inside cells so that it is not toxic and will be shed from the intestinal lumen within a few days

38
Q

Which are the fat soluble vitamins?

39
Q

Which are the water soluble vitamins?

A

B1, 2, 3, 5, 6, 7, C

40
Q

Which enzymes break down starches?

A

Alpha-amylases (saliva and pancreatic juice)

41
Q

Which enzyme breakdown di/tri-saccharides?

A

Maltase/glucoamylase, lactase and sucrase-isomaltase on intestinal brush border

42
Q

Which enzymes digest proteins and by what proportion?

A

Pepsins in the stomach digest 10-15%

Pancreatic proteases in the SI, convert 70% luminal protein to oligopeptides + 30% AA

43
Q

Which protease enzymes are located in the SI lumen?

A
  • trypsin
  • chymotrypsin
  • elastase
  • carboxypeptidase
44
Q

Main functions of the liver

A

Synthesis, storage, secretion, excretion and detoxification

45
Q

What increases and decreases blood flow to the liver?

A

Increases - local modulators (an increase in metabolic demand)
Decreases - sympathetic NS

46
Q

Effect of NA on hepatic artery and portal vein

A

Vasconstriction

Hepatic artery = results in diverting blood away from liver

47
Q

What controls vasodilation in the hepatic circulation?

A

No known vasodilator fibres

  • local control
  • adenosine (reduced flow rate = adenosine released to dilate and increase flow rate)
48
Q

How is bile formed from blood?

A

Blood is passed through the portal venue and hepatic arteriole and forced into hepatocytes and bile is secreted on the other side into the hepatic duct

49
Q

What is:
Glycogenesis
Glycogenolysis
Gluconeogenesis

A

Glycogenesis: glucose > glycogen
Glycogenolysis: glycogen > glucose
Gluconeogenesis: glucose produced from fats/amino acids etc

50
Q

Fat metabolism in the liver

  • energy supply
  • synthesis
  • conversion
A

Energy supply: primary site of beta oxidation (fat breakdown)
Synthesis: fats from CHO/AA, cholesterol from acetyl CoA
Conversion: XS acetyl CoA to ketone bodies for release to plasma (low glucose)

51
Q

Summarise metabolism of protein in the liver

A

Amino acids are either

  1. synthesised into plasma proteins
  2. interconverted
  3. deamination into ammonia and then synthesised into urea for excretion
52
Q

Summary of iron storage in the liver

A
  • iron bound to ferritin
  • controls storage and release
  • liver sense elevated plasma iron and releases HEPCIDIN which switches of FERROPORTIN channels on the intestine/spleen and reduces [Fe]
53
Q

Primary and secondary bile acids

A

Primary bile acids synthesised by hepatocytes and secreted into bile canaliculi
Converted to secondary bile acids by bacteria in intestinal lumen

54
Q

Which forms of bilirubin are the small intestine and colon permeable and impermeable to?

A

Permeable - urobillinogen, free bilirubin

Impermeable - conjugated BG/BG2

55
Q

Causes of jaundice

A
  1. haemolytic: XS RBC destruction

2. Hepatic: impaired uptake, disturbed processing - disturbed secretion, obstruction

56
Q

Functions of pancreatic secretions

A
  • Digestion of lipids, proteins & carbohydrates
  • Neutralizing pH of acid chyme entering the duodenum
  • Create suitable pH for pancreatic digestive enzymes.
57
Q

Why is the pancreas not digested?

A
  • Protease enzymes are synthesised in inactive form (trypsinogen –> trypsin)
  • trypsin then activates other things
  • trypsin inhibitor also produced in acini
58
Q

How are ACh, secretin and cholecystokinin involved in pancreatic secretion control

A

ACh - from vagus, stimulates enzyme release from acing cells into lumen
Secretin: secreted in response to duodenal pH <4.5 and stimulates production of HCO3-
CCK: secreted in response to fat/protein content in chyme in duodenum, stimulates enzyme secretion and relaxes/opens sphincter of Oddi

59
Q

Main action of CCK

A
  • stimulates pancreatic acing cell enzyme production
    • Relaxation of the sphincter of Oddi (+ GB contraction)
  • Also functions as a neuropeptide (CNS: regulates food intake, anxiety, analgesia)
  • Inhibits gastric emptying, stimulates enterokinase synth, enhances intestinal motility, glucagon release.
60
Q

The origin of CCK

A

Synthesised by chemosensitive “I” cells in the gut mucosa

61
Q

Stimulus for and action of secretin

A

Stimulus - H+ ion contact with mucosa (pH<4)
Action - stimulates HCO3- secretion by pancreas duct cells, decreases gastric H+ secretion, causes pyloric sphincter contraction

62
Q

Dust cell bicarbonate production

A
  1. CO2 diffuses bloodductal cells, CO2 + H2OH2CO3 (carbonic anhydrase), which dissociates into HCO3- + H+. HCO3- is actively transported (with Na+) into the lumen.
  2. H+ (from H2CO3) is exchanged for Na+ ions (active transport) through blood, to neutralise the –ve charges of HCO3-
  3. The movement of HCO3- and Na+ ions to the lumen sets up an osmotic gradient, & water moves from blood duct cells lumen, creating an isosmotic HCO3- solution.
63
Q

Phases of pancreatic secretion

A
  1. Cephalic phase:
    - Vagus releases Ach, 20% of total secretion
  2. Gastric phase:
    - Gastrin phase adds another 5-10%
    - Gastrin also binds CCK receptors
  3. Intestinal phase:
    - Most important (80%)
    - CCK, secretin and Ach
64
Q

Describe hepatic handling of bilirubin

A

Albumin bound bilirubin enters hepatocytes via OATP. Dissociates and conjugates with glucouronic acid. Forms bilirubin di glucouronide BG2 which is water soluble. This is actively pumped into the canaliculi with bile

65
Q

What do duct cells in the pancreas secrete?

A

A solution of HCO3-, Na+, K+ and water

66
Q

What do acinar cells in the pancreas secrete?

A

Trypsin, chymotrypsin, carboxypeptidases, elastase, nucleases, pancreatic amylase, pancreatic lipase, cholesterol esterase, phospholipase

67
Q

Which 3 substances control pancreatic secretion?

A
  1. ACh - stimulates enzyme release from acinar cells into lumen
  2. Secretin - secreted in response to pH <4.5 and stimulates production of HCO3-
  3. CCK - secreted in response to fat and protein content and stimulates enzyme production and relaxation of sphincter of Oddi
68
Q

What causes fusion of the tubulovesicles, containing H+/K+ ATPase pumps, to the apical membrane of parietal cells?

A

Histamine, gastrin and ACh

69
Q

Where does passive sodium absorption occur?

A

Distal colon

70
Q

Where does glucose/sodium co transport occur?

A

Jejunum and ileum

71
Q

Where does counter/antiport transport of sodium with H+ occur?

A

Duodenum and jejunum

72
Q

Where does parallel Na+/H+ and Cl- Hco3- exchange occur?

A

Ileum and proximal colon

73
Q

Site of active and passive potassium absorption

A

Active - distal colon

Passive - jejunum and ileum

74
Q

What does glucose require to be absorbed?

A

Sodium - SGLT1

75
Q

Amino acid uptake and release

A

AA transporters present on apical surface of enterocytes. Distinct transporters for distinct AAs
Basal membrane - specific AA transporters that are Na+ dependent

76
Q

What does pancreatic lipase digest?

A

Emulsified fat into fatty acids and 2-monoglycerides

77
Q

Which substances are actively pumped into bile and which passively diffuse?

A

Active - bile acids, phosphatidylcholine, BG/BG2, cool .xenobiotics

78
Q

Fat metabolism in the liver

A
  1. energy supply: beta oxidation
  2. synthesis: fat from protein/glucose and cholesterol from acetyl coa
  3. Conversion: acetyl coa to ketone bodies
  4. storage
  5. cholesterol converted to bile salts
  6. lipoprotein formation
79
Q

Carb metabolism in the liver

A
  1. blood glucose barrier
  2. gluconeogenesis - from fat or protein
  3. glycogenolysis - via 6 glucose phosphatase
  4. glycogenesis - storage
  5. formation and interconversion of other CHO intermediates
80
Q

Protein metabolism in the liver

A
  1. deamination: AA to kero-acid to NH3
  2. transamination: interconversion of non essential aa
  3. synthesis of plasma proteins: albumin, ACP, hormone binding proteins, coagulation factors
  4. synthesis of urea to remove ammonia from body
81
Q

Detoxification in the liver

A
  1. bugs by kupferr cells
  2. drug metabolism
  3. metabolises hormones
  4. abundant CYP450 enzymes
82
Q

Vitamin storage in the liver

A

A, D, B12, K Fe

83
Q

Liver iron storage

A

Ferroxidase converts FE2+ to 3+
Ferroreductase converts Fe3+ to 2+

Liver also releases hepcidin which switches of ferroportin channels to reduce plasma Fe

1-5 GIT (7 decks)

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