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X Physiology 1B > Gastrointestinal System > Flashcards

Gastrointestinal System Flashcards

1
Q

What are the 3 main functions of the GI tract?

A

digestion
absorption
protection

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2
Q

define digestion

A

the process by which foodstuffs are degraded to produce smaller molecules that can be absorbed

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3
Q

define absorption

A

the process by which nutrient molecules are absorbed by cells that line the GI tract and enter the blood stream

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4
Q

define protection in terms of the GI tract

A

the process by which the lining of the GI tract is protected from damage during digestion of foodstuffs

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5
Q

what does the GI tract require protection from?

A

harsh environment created to break down food, e.g. acids and enzymes

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6
Q

name all the main parts of the GI tract

A

mouth, pharynx, oesophagus, stomach, duodenum, jejunum, ileum, colon, rectum, anus

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7
Q

is the GI tract structured the same from mouth to anus?

A

has a common design but there is difference between regions depending on function

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8
Q

what are the accessory organs of the GI tract?

A

liver, gall bladder, pancreas, salivary glands

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9
Q

what are the layers of the GI tract wall from lumen to basement layer?

A

mucosa, submucosa, muscularis externa, serosa

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10
Q

how does the mucosa structure in the stomach aid its function?

A

gastric pits with glands which secrete acid

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11
Q

how does the mucosa structure in the small intestine aid its function?

A

villi over apical surface to increase surface area for absorption

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12
Q

how does the mucosa structure in the large intestine aid its function?

A

has crypts full of mucous secreting cells to protect epithelium

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13
Q

what are the 3 layers of the mucosa?

A

epithelium, lamina propina, muscularis mucosae

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14
Q

within the lumen of the small and large intestine what are villi separated by?

A

crypts

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15
Q

what does the epithelium of the mucosa form?

A

the interface between external environment and the body

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16
Q

what is the epithelium made up of?

A

single layer of specialised cells

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17
Q

what is the lamina propina made from?

A

loose connective tissue

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18
Q

which of the 3 layers of mucosa is the largest?

A

lamina propina

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19
Q

what is the name of the most outer layer of mucosa?

A

muscularis mucosae

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20
Q

what is the muscularis mucosae formed of?

A

thin layer of muscle

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21
Q

what is the function of the muscularis mucosae?

A

local gut movement e.g. glands

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22
Q

what is the major site of gut motility that makes the GI tract a muscular tube?

A

muscularis externa

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23
Q

what is the next layer superficially to the mucosa?

A

sub mucosa

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24
Q

what is the submucosa formed from?

A

loose connective tissue with blood vessels and nerves

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25
Q

what the next gut layer located superficially to the sub mucosa?

A

muscularis externa

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26
Q

what 2 muscle layers is the muscularis externa formed of?

A 
circular muscle (wraps around)
longitudinal muscle (travels along length of tube)
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27
Q

which muscle layer of the muscularis externa is more deep to the lumen of the GI tract?

A

circular muscle with longitudinal superficial to it

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28
Q

what is the most outer layer of the GI tract?

A

serosa

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29
Q

what is the serosa made of?

A

connective tissue

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30
Q

what is the main function of the serosa?

A

holds tube together

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31
Q

where are lymph nodes found in the GI tract?

A

within mucosa

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32
Q

why are lymph nodes needed in the GI tract?

A

ensure immune response isn’t mounted incorrectly and also ensures quick immune response to pathogens when needed

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33
Q

where in the GI tract are glands found in the submucosa?

A

oesophagus and small intestine

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34
Q

where are myenteric plexi found in the GI tract?

A

between the 2 layers of muscularis externa all the way along GI tract from top of oesophagus to rectum

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35
Q

where are submucosal plexi found?

A

between muscularis externa and submucosa only in small and large intestine

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36
Q

what are the 3 levels of amplification of the surface area of the GI tract?

A

circular or spiral folds
villi
microvilli

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37
Q

where are microvilli located?

A

on apical membrane of epithelial cells (facing into the lumen)

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38
Q

what are glycocalyx?

A

network of proteins on the outside of microvilli which create an undisturbed area for final products of nutrient breakdown to sit in and then be absorbed

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39
Q

what are the 2 membranes found on absorptive cells?

A

apical and basolateral

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40
Q

what are the 2 pathways of movement through epithelium in the GI tract?

A

transcellular

paracellular

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41
Q

where do substances travel through the paracellular pathway?

A

gap between two epithelial cells

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42
Q

where do substances travel through the transcellular pathway?

A

through the epithelial cell from apical to basolateral membrane

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43
Q

what is the function of tight junctions?

A

act to separate proteins through the apical and basolateral membranes

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44
Q

give an example of a transcellular pathway

A

glucose and sodium pathway

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45
Q

what are parietal cells for?

A

vital for GI tract function to secrete acids

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46
Q

what does the presence of 2 membranes mean for the cell?

A

it is polar

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47
Q

what are the 5 main sites of secretion in the GI tract?

A

salivary glands, gastric glands, exocrine pancreas, liver-billiary system and small intestine

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48
Q

what is total saliva secretion throughout the day?

A

6-7 litres a day

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49
Q

what are the 3 types of gland?

A

in epithelium
in sub mucosa
gland outside GI tract

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50
Q

what does saliva contain?

A

enzymes, ions, water and mucus

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51
Q

what is the function of saliva?

A

breakdown of large compounds, regulate pH, dilute and protect

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52
Q

what is the name of the system that supplies the GI tract with blood?

A

splanchnic circulation

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53
Q

where does all blood from the GI tract drain before meeting the IVC?

A

into the hepatic portal vein and into the liver before exiting the liver into hepatic vein and the IVC

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54
Q

how is a villus supplied with blood?

A

central arteriole branching into capillaries and then exiting in a venule

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55
Q

what is absorbed into the blood stream through the villi?

A

simple sugars and amino acids

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56
Q

how do fats/fatty acids enter the blood stream?

A

drain into lacteals where they join the intestinal lymphatic system before entering circulation at the thoracic duct

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57
Q

where are fats reassembled?

A

in the epithelium of the villi

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58
Q

what are 3 types of regulation of blood flow to GI tract?

A

nerves
hormones
vasodilatory metabolites released by epithelial cells as they work to absorb nutrients

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59
Q

how does blood supply increase in the GI tract when moving from fasted to fed state?

A

sequentially as the different areas require blood

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60
Q

what are the 3 regulatory mechanisms that control GI function

A

endocrine
paracrine
neuronal

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61
Q

how does endocrine regulation of GI function work?

A

release of a transmitter (e.g. peptide) into the blood for delivery to distant target cells

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62
Q

how does paracrine regulation of GI function work?

A

release of a transmitter from a sensor cell that will have a local effect on adjacent cells with no travel in the blood stream

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63
Q

how does neuronal regulation of GI function work?

A

electrical signalling via neurons

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64
Q

where are endocrine cells located in the GI tract?

A

mucosa of the stomach, intestine and pancreas

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65
Q

what cells produce gastrin?

A

G cells

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66
Q

where are gastrin producing G cells located?

A

stomach

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67
Q

what is the role of gastrin?

A

stimulates acid secretion

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68
Q

what cells produce cholecystokinin (CKK)?

A

I cells

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69
Q

where are cholecystokinin (CKK) producing I cells located?

A

wall of small intestine

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70
Q

what cells is secretin produced by?

A

S cells

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71
Q

where are secretin producing S cells located?

A

small intestine

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72
Q

what cells is glucose dependent insulinotropic peptide (GIP) produced by?

A

K cells

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73
Q

where are the glucose dependent insulinotropic peptide (GIP) producing K cells located?

A

small intestine

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74
Q

what is the main role of glucose dependent insulinotropic peptide (GIP)?

A

regulates insulin production

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75
Q

what cells is somatostatin produced by?

A

D cells

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76
Q

where in the body are somatostatin producing D cells located?

A

stomach, pancreatic islets, small intestine

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77
Q

what is a key role of somatostatin?

A

inhibits acid secretion

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78
Q

how can the location of D cells affect somatostatin?

A

some are paracrine factors and others are hormones depending on location

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79
Q

what 2 hormones regulate secretion and motility in the GI tract?

A

gastrin and CCK

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80
Q

What is a key role of CCK in secretion and motility?

A

contraction of gall bladder to release bile into duodenum for fat digestion
pancreatic secretion of enzymes

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81
Q

what hormone regulates blood flow in the GI tract?

A

CCK - stimulates blood flow

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82
Q

what hormone regulates secretions of endocrine cells in the GI tract?

A

GIP - insulin release

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83
Q

what is the nervous system of the GI tract?

A

enteric nervous system

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84
Q

describe the nervous pathway in the enteric nervous system for local reflexes

A

sensory neurons stimulated by chemo/mechanoreceptors
signal transmitted to submucosal/myentric plexus (inter neurons)
response transferred via motor neurons to effectors (e.g. smooth muscle, secretory cell, blood vessel)

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85
Q

is there any intervention from the brain within the enteric nervous system?

A

no - local reflex control in a self-contained network

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86
Q

what is the name of the branch of the nervous system that relays information from the enteric nervous system to the brain?

A

extrinsic nervous system

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87
Q

what branches of the nervous system have effect on the GI tract?

A

sympathetic and parasympathetic nerves

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88
Q

what is the effect of sympathetic nerves on the GI tract?

A

direct blood away from GI tract

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89
Q

what is the vagovagal reflex?

A

direction of sensory information from the gut to the CNS

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90
Q

what are the 3 phases of GI control?

A

cephalic phase
gastric phase
intestinal phase

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91
Q

what happens during the cephalic phase of GI control?

A

before entry of food into stomach involves sight, smell and taste

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92
Q

what happens during the gastric phase of GI control?

A

distention and change in acidity of stomach as food enters

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93
Q

what happens during the intestinal phase of GI control?

A

distention, osmolarity and acidity of intestines

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94
Q

what are epithelial cells lining the GI tract responsible for?

A

nutrient absorption

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95
Q

What is the function of the GI tract tightly regulated by?

A

nerves, hormones and paracrine factors

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96
Q

what do epithelial cells lining the GI tract control?

A

quality and composition of secretions in the GI tract

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97
Q

what are the 3 main functions of motor activity of the GI tract?

A

non-propulsive movements (segmentation)
peristaltic movements
reservoir function

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98
Q

what is involved in non-propulsive movements in the GI tract?

A

churning and mixing of digestive products to ensure adequate exposure to acids and enzymes and aid absorption through the gut wall

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99
Q

what are peristaltic movements?

A

relaxation ahead of a bolus and contraction behind in order to move contents of gut through GI tract

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100
Q

what happens during reservoir function of the GI tract?

A

contraction of specialised circular muscles (sphincters) which causes isolation of a specific region (e.g. stomach)

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101
Q

what is a sphincter formed of?

A

specialised muscularis externa

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102
Q

what are the two types of muscle contraction seen in the GI tract?

A

phasic

tonic

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103
Q

how long do phasic contractions last in the GI tract?

A

seconds

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104
Q

what functions of the GI tract utilise phasic contractions?

A

non-propulsive movements (segmentation)

peristaltic movements

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105
Q

how long do tonic contractions last in the GI tract?

A

minutes to hours

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106
Q

what functions of the GI tract utilise tonic contractions?

A

reservoir functions for long term contraction of sphincters

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107
Q

is GI tract muscle single or multi unit?

A

single

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108
Q

how do innervated smooth muscle cells communicate with those that are not innervated?

A

through gap junctions

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109
Q

how is the membrane potential of GI smooth muscle cells described?

A

slow waves - oscillations of membrane voltage/potential

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110
Q

how does the resting potential of smooth muscle cells compare to nerves/skeletal muscle?

A

RMP of smooth muscle cells is more depolarised

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111
Q

what is smooth muscle activity and the slow waves of RMP regulated by?

A

hormones and nerves

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112
Q

what is the frequency of contractions in GI smooth muscle dictated by?

A

frequency of action potential firing which is determined by the frequency of slow waves

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113
Q

how do sphincters mediate reservoir function?

A

through long lasting tonic contractions

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114
Q

what are sphincters?

A

specialised circular muscles that separate two adjacent compartments of the GI tract

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115
Q

what does separation of different areas of the GI tract encourage?

A

regional specificity

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116
Q

what are the 6 sphincters found in the GI tract?

A 
upper oesophageal sphincter
lower oesophageal sphincter
pyloric sphincter
sphincter of Oddi
ileocecal sphincter
internal and external anal sphincters
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117
Q

which sphincter has the highest resting pressure of all sphincters in the tract?

A

upper oesophageal sphincter

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118
Q

what is the key role of the lower oesophageal sphincter?

A

prevents acid reflux into oesophagus from the stomach

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119
Q

which sphincters isolate the stomach?

A

lower oesophageal and pyloric

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120
Q

what sphincters isolate the small intestine?

A

pyloric and ileocecal

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121
Q

what sphincters isolate the large intestine?

A

ileocecal and internal and external anal sphincters

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122
Q

what muscle is the external anal sphincter formed of?

A

skeletal

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123
Q

what is the role of the sphincter of Oddi?

A

smooth muscle around bile duct and pancreatic duct which regulates enzyme and bile delivery to the small intestine

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124
Q

histologically how does the pyloric sphincter appear?

A

large bulge of circular muscle (from muscularis externa) into lumen

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125
Q

what is the difference between physiological and anatomical sphincters?

A

physiological sphincters do not have evident bulges of muscle into the lumen forming the sphincter like anatomical ones do

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126
Q

histologically how can the junction between oesophagus and stomach be identified?

A

change from stratified squamous epithelium and sub mucosal glands to gastric pits

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127
Q

what is the role of stratified squamous epithelium in the oesophagus?

A

protects from damage by abrasion/acid

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128
Q

what sort of resting pressure is mostly found in sphincters due to tonic contractions?

A

positive resting pressure

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129
Q

what types of movement do sphincters regulate?

A

forward/antegrade (mostly) and backwards/retrograde

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130
Q

what is sphincter regulation coordinated with?

A

smooth muscle contractions of adjacent compartments

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131
Q

what is the key role of sphincters?

A

mediate reservoir function through tonic contractions

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132
Q

is swallowing initiated voluntarily or involuntarily?

A

voluntarily

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133
Q

once swallowing is initiated what control is it under?

A

reflex

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134
Q

describe the process of the swallowing reflex?

A

food bolus reaches back of the mouth as directed by voluntary control of the tongue
touch receptors activated
stimulate the medulla and lower pons
vagal nerves stimulate the oesophagus and cranial nerves stimulate the pharynx and upper oesophagus

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135
Q

where are most of the touch receptors located that have a key role in swallowing?

A

those near the opening of the pharynx

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136
Q

what are the 3 phases of swallowing?

A

oral/voluntary phase
pharyngeal phase
oesophageal phase

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137
Q

what happens during the oral/voluntary phase of swallowing?

A

the tongue presses the food against the hard palette and the bolus is forced into the pharynx stimulating touch receptors

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138
Q

what happens during the pharyngeal phase of swallowing?

A

soft palette elevates, epiglottis close the trachea and upper oesophageal sphincter relaxes

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139
Q

what is the main aim of the pharyngeal phase of swallowing?

A

protection of the respiratory system

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140
Q

what does the elevation of the soft palette and closure of the trachea by the epiglottis prevent?

A

food entering the nasal cavity and trachea respectively

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141
Q

what happens during the oesophageal phase of swallowing?

A

upper oesophageal sphincter is closed and peristalsis starts

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142
Q

define peristalsis

A

sequential contraction of rings of muscle behind a bolus

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143
Q

what are the two regions of the stomach distinguished by motility called?

A

orad and caudad

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144
Q

what is the function of the orad area of the stomach?

A

accommodation of food

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145
Q

what is the function of the orad and caudad areas of the stomach?

A

gastric emptying once small intestine is ready

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146
Q

what happens to the orad region during swallowing?

A

relaxes in coordination with peristalsis and opening of lower oesophageal sphincter

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147
Q

what is the relaxation of the orad following swallowing known as?

A

receptive relaxation

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148
Q

describe the process involved in vomiting

A

reverse peristalsis to remove toxic material from SI
pyloric sphincter and the stomach relax to receive contents
forced inspiration occurs against the closed epiglottis
sharply elevated intra-abdominal pressure causes forceful contractions of abdominal muscles
reflex relaxation of the upper oesophageal sphincter which drives vomit out

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149
Q

what are the 3 results of forced inspiration against the closed epiglottis?

A

lowered diaphragm
decrease in intrathoracic pressure
elevated intra-abdominal pressure

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150
Q

where do peristalsis contractions begin in the stomach?

A

corpus

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151
Q

where do contractions travel towards when they have been initiated in the corpus?

A

travel towards pylorus

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152
Q

what is the travel towards pylorus by peristalsis from the corpus known as?

A

propulsion

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153
Q

what happens to peristalsis contractions as they travel towards gastroduodenal junction (pyloric sphincter)?

A

increase in force and velocity

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154
Q

where does most grinding/mixing occur in the stomach?

A

mainly in the antrum

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155
Q

what is retropulsion?

A

movement of gastric contents back towards corpus from the antrum

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156
Q

what is retropulsion very effective at?

A

mixing and breaking down gastric contents

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157
Q

what does the tonic contraction of the pyloric sphincter control?

A

the speed of stomach emptying

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158
Q

in the fed state what size must gastric contents be to pass through pyloric sphincter?

A

<2mm in diameter

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159
Q

what is the most frequent type of movement in the small intestine?

A

non-propulsive movements

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160
Q

what are non-propulsive movements in the small intestine caused by?

A

rhythmic contraction and relaxation of the muscularis externa

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161
Q

what is the purpose of non-propulsive movements in the small intestine?

A

effectively mixes chyme and brings digested nutrients into contact with the mucosal surface (unstirred area to be absorbed into villi)

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162
Q

what is low frequency peristalsis in the small intestine caused by?

A

contraction of successive sections of the muscularis externa

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163
Q

what is the purpose of peristalsis in the small intestine?

A

propels chyme for a short distance, allowing time for digestion and absorption

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164
Q

how does gut motility appear in the fed state?

A

continuous contractile activity (segmentation and peristalsis)

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165
Q

how does gut motility appear in the fasting state?

A

presence of the migrating motor complex in the stomach and small intestine

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166
Q

what is the function of the migrating motor complex?

A

cleans stomach and small intestine of undigestable items

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167
Q

what happens to the pyloric sphincter during the migrating motor complex?

A

opens wider than 2mm to allow non-digested items to pass through

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168
Q

what hormone controls mobility in the fasting state?

A

motalin

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169
Q

what are the 4 phases of migrating motor complex?

A 
no activity (silent)
build up of mechanical activity
short lived maximal activity 
drop back down to no activity
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170
Q

how does gut muscle contraction vary between fed state and fasting state?

A

during fasting state the MMC contractions are larger than normal contractions seen in fed state. However fed state contractions are ongoing whereas MMC is sporadic with silent phases

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171
Q

what are the 3 main functions of motility in the large intestine?

A

mixing of chyme to improve absorption of water and salts from the colon (mainly ascending and transverse)
kneading of semi solid contents
movement of contents towards anus

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172
Q

how quickly do non propulsive movements move products towards anus?

A

5-10cm per hour

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173
Q

what is mass movement/mass peristalsis?

A

specialised movement that happens 1-3 times a day and moves colonic contents towards the anus.
up to 20cm in one go

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174
Q

what system is motility in the large intestine controlled by?

A

enteric nervous system

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175
Q

what happens during the gastrocolic reflex?

A

distention of the wall of the stomach initiates mass peristalsis

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176
Q

what is the rectosphincteric reflex?

A

distention of the rectum causes signalling from pelvic nerves to the sacral spinal cord
internal anal sphincter relaxes in response to stimulus from spinal motor nerves

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177
Q

describe the route to defecation

A

distention of rectum
rectosphincteric reflex
relaxation of internal anal sphincter
relaxation of external anal sphincter
contraction of abdominal muscles and relaxation of pelvic wall muscles
flexure of hips and descent of pelvic floor
defaecation

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178
Q

what happens if it is not acceptable to defecate?

A

distention of rectum
rectosphincteric reflex
relaxation of internal anal sphincter
contraction of external anal sphincter

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179
Q

how much saliva is produced per day?

A

1.5l

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180
Q

is saliva hyposmotic, hyperosmotic of isosmotic compared to plasma?

A

hyposmotic

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181
Q

what is the pH of saliva?

A

~7

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182
Q

what is found within saliva?

A

mucin glycoproteins, lysozyme and alpha amylase

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183
Q

what is the only hyposmotic solution in the body?

A

saliva

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184
Q

what are the 3 main functions of saliva?

A

lubricate food to aid swallowing
clean and protect the cavity of the mouth
reduce starch to oligosaccarides and triglycerides to fatty acids

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185
Q

what elements of saliva are involved in lubrication of food to aid swallowing?

A

mucin glycoproteins and water

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186
Q

what elements of saliva are involved in cleaning and protecting the cavity of the mouth?

A

lysozyme

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187
Q

what elements of saliva are involved in digestion of starch and triglycerides?

A

alpha amylase and lipase respectively

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188
Q

what sort of saliva is released by the parotid gland?

A

serous (watery) secretion rich in alpha amylase

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189
Q

what sort of saliva is released by the submandibular and sublingual glands?

A

seromucous (water and mucous) secretion

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190
Q

what are minor salivary glands?

A

small glands found throughout the mouth

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191
Q

what sort of secretion is released by minor salivary glands?

A

mucous secretion rich in mucin glycoproteins

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192
Q

what is the main duct of a salivary gland called?

A

excretory duct

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193
Q

what are excretory ducts of salivary glands fed by?

A

intercalated duct

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194
Q

what are intercalated ducts of salivary glands fed by?

A

acinus

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195
Q

what are acinus formed of?

A

150-200 acinar cells

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196
Q

what is the functional unit of the salivary gland?

A

acinus

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197
Q

what intracellular structure is found in large quantities in acinar cells?

A

endoplasmic reticulum due to high levels of protein production

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198
Q

what are zymogen granules?

A

pockets of enzymes on the apical membrane of acinar cells which perform exocytosis to release enzymes into the intercalated ducts

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199
Q

what are acini lined by?

A

epithelial cells

200
Q

what are acinar cells responsible for?

A

producing elements of saliva that are washed through the ducts

201
Q

what type of epithelia do acinar cells have?

A

‘leaky’

202
Q

what sort of transport do ‘leaky’ epithelial cells allow?

A

transcellular movement and paracellular movement

203
Q

what sort of transport across the epithelium occurs in the duct lining cells of salivary glands?

A

transcellular movement only as the epithelia are tightly bonded

204
Q

what is in the primary secretion?

A

isotonic sodium chloride and water

205
Q

what is the role of Na/K ATPase in NaCl secretion by the salivary gland?

A

uses energy from the hydrolysis of ATP to transport sodium (out) and potassium (in) across the membrane creating a steep concentration gradient of sodium ions that is used by other transport channels

206
Q

what is the role of the sodium/potassium/2chloride cotransporter in NaCl secretion by the salivary gland?

A

through secondary active transport using the gradient created by Na/K ATPase sodium ions move into the cell down electrochemical gradient and bring potassium and chloride ions with them. This sets up a chloride concentration within the cell

207
Q

how much potassium and chloride is brought into the cell alongside 1 Na+?

A

1 K+ and 2Cl-

208
Q

what is the sodium/potassium/2chloride cotransporter also known as?

A

NKCC

209
Q

what is the role of basolateral K+ channels in the secretion of NaCl by the salivary glands?

A

recycling of potassium ions across the membrane creating a source of extracellular potassium that can be used by NKCC and Na+/K+ ATPase
hyperpolarises the basolateral membrane which increases drive for Cl- to exit the cell across the apical membrane

210
Q

what is the basolateral K+ channel gate opened by?

A

increase in intracellular Ca2+ concentration

211
Q

is exit of Cl- across the apical membrane an active or passive process?

A

passive

212
Q

what is the apical Cl- channel gated by?

A

Ca2+ concentration within the cell

213
Q

how is salt brought into the lumen of the duct?

A

cl- channel opens and Cl- moves out down electrochemical gradient. The negative charge created by Cl- movement into duct draws Na+ across the apical membrane paracellularly completing movement of NaCl into duct

214
Q

what is the effect of having NaCl in the lumen of the duct?

A

exerts osmotic effect that pulls water into the lumen of the duct via paracellular pathway

215
Q

what does transcellular Cl- movement lead to?

A

paracellular sodium and water movement

216
Q

how are salivary glands regulated?

A

parasympathetic nervous system

217
Q

what does signalling from the parasympathetic nervous system cause in the epithelial cell of the salivary gland?

A

increase in intracellular Ca2+ concentration which opens the basolateral K+ channel and apical chloride channel

218
Q

summerise NaCl secretion by salivary gland acinar cells

A

Na+/K+ ATPase creates inwardly directed Na+ gradient across basolateral membrane
The NKCC accumulates Cl- ions intracellularly driven by the Na+ gradient
Ca2+ activated K+ channels recycle K+ ions across the basolateral membrane and maintain the driving force of Cl-exit across the apical membrane
Ca2+ activated Cl- channels in the apical membrane provide a pathway for Cl- to exit the cell into the duct
the movement of Cl- ions into the duct lumen draws Na+ and water through paracellular pathways to complete the process of isotonic NaCl secretion

219
Q

how is the primary secretion of the acinar cells modified in the duct?

A

duct lining epithelial cells reabsorb Na+ and Cl- by transcellular pathway
potassium bicarbonate is excreted across the same cells on the same pathway

220
Q

what causes the final ionic composition of hyposmotic saliva?

A

reabsorption of Na+ and Cl- by transcellular pathway and potassium bicarbonate excretion

221
Q

what is composition of saliva determined by?

A

ion transport mechanisms of acinar and ductal epithelial cells

222
Q

what happens to the ionic composition of saliva as flow rate increases?

A

K+ remains at constant rate
Na+ rises exponentially
HCO3- increases quickly and then remains at a steady level
Cl- increases linearly

223
Q

does saliva flow rate affect its osmolality?

A

no - remains hyposmotic throughout, whatever happens to flow rate

224
Q

how is saliva secretion controlled?

A

nerves

225
Q

what branches of the ANS stimulate saliva secretion/

A

sympathetic and parasympathetic

226
Q

what is the effect of acetylcholine on acinar cells?

A

stimulates production of 0.9% NaCl (fluid) and enzyme release by exocytosis

227
Q

what is the effect of acetylcholine on duct cells?

A

indirect - due to increased saliva production less NaCl is absorbed in the duct

228
Q

what is the effect of noradrenaline on acinar cells?

A

stimulates release of enzymes by exocytosis

229
Q

what is the effect of noradrenaline on duct cells?

A

stimulates secretion of bicarbonate by raising th eICF concentration of cyclic AMP

230
Q

What are the 3 main roles of the stomach in digestion?

A

reservoir to aid gastric motility
digests proteins through the action of pepsins
essential for the absorption of vitamin B12 through production of intrinsic factors

231
Q

what sort of environment do pepsins require in order to perform their function properly?

A

highly acidic

232
Q

what is the role of the intrinsic factor in absorption of vitamin B12?

A

prevents the destruction of vitamin B12 in the small intestine by proteolytic enzymes from the pancreas

233
Q

what is the name of the majority of the stomach?

A

corpus

234
Q

where is the cardia of the stomach located?

A

just below lower oesophageal sphincter

235
Q

what is the mucosa of the cardia of the stomach specialised for?

A

mucous secretion in order to protect the oesophagus from acid

236
Q

how much gastric secretion is produced in the stomach per day?

A

2 litres

237
Q

what is the pH of gastric secretion in the stomach?

A

0.9-1.5

238
Q

is gastric secretion in the stomach isosmotic, hyposmotic or hyperosmotic?

A

isosmotic

239
Q

what is found within gastric secretions?

A

HCl, pepsins, intrinsic factors, mucous and HCO3

240
Q

what is the role of mucous and HCO3?

A

protection of GI mucosa from highly acidic gastric secretions

241
Q

what is the gastric mucosa of the stomach made up of?

A

gastric pits

242
Q

what is the name of the cell that produces HCl and intrinsic factors?

A

parietal cell

243
Q

what is the most dominant cell in gastric glands?

A

parietal cells

244
Q

what do parietal cells secrete?

A

HCl and intrinsic factors

245
Q

where are gastric glands located?

A

beneath gastric pits

246
Q

what is produced by chief cells?

A

pepsinogen (inactivated pepsin)

247
Q

what are the 2 types of endocrine cell in the gastric mucosa?

A

ECL - (entro chromafin like)

D cells

248
Q

what is produced by ECL endocrine cells in the gastric mucosa?

A

histamine

249
Q

what is produced by D endocrine cells in the gastric mucosa?

A

somatostatin

250
Q

what type of factor is histamine in the stomach?

A

paracrine

251
Q

what type of factor is somatostatin in the stomach?

A

depending on location is either paracrine or peptide hormone

252
Q

what is the role of histamine in the stomach?

A

stimulates acid secretion

253
Q

what is the role of somatostatin in the stomach?

A

inhibits acid secretion

254
Q

which cells produce HCO3 in the gastric mucosa?

A

surface epithelial cell

255
Q

which cells produce mucous in the gastric mucosa?

A

mucous neck cells

256
Q

where are mucous neck cells located?

A

in the top of glands

257
Q

what are the dominant epithelial cells in the gastric mucosa?

A

surface epithelial cells

258
Q

what do G cells produce?

A

gastrin in the antrum

259
Q

what cells are found close to the G cells in the antrum?

A

D cells producing somatostatin

260
Q

what cell mechanism causes the release of pepsinogens from chief cells?

A

increase in intracellular Ca2+ concentration

261
Q

how are pepsinogens released from the chief cell into the lumen of the gastric gland across the apical membrane?

A

exocytosis

262
Q

what is the role of pepsins?

A

digestion of proteins

263
Q

what is the optimal pH for pepsins?

A

less than 3

264
Q

what is the spontaneous conversion of pepsinogens to pepsins stimulated by?

A

HCl

265
Q

what happens to the speed of pepsinogen conversion to pepsin at pH above 5

A

very slow

266
Q

what does increased pepsin presence stimulate?

A

more pepsinogen to convert to pepsin

267
Q

what 2 things is low pH int he stomach required for?

A

pepsinogen activation and pepsin activity

268
Q

what process does HCl speed up once pepsin is produced?

A

breakdown of proteins to peptides

269
Q

what are the 3 functions of HCl?

A

promotes activation and activity of pepsins
kills or inhibits microorganisms
stimulates secretions in the small intestine and so process of digestion is completed

270
Q

what are tubulovesicle?

A

vesicle contained within the apical membrane of resting parietal cells that holds ion transporters essential for secretion of acid

271
Q

what happens to tubulovesicles when the parietal cell is activated to secrete HCl?

A

form canaliculi on the apical membrane

272
Q

what are canaliculi?

A

finger like projections on the apical membrane of parietal cells that are formed of tubular vesicles inserting into the apical membrane

273
Q

how doe canaliculi affect the surface area of the apical membrane of a parietal cell?

A

50-100x increase in surface area

274
Q

how does an active parietal cell appear?

A

larger secretory membrane with more H+ (proton) pumps, K+ and Cl- channels

275
Q

what is the role of the H+/K+ ATPase pump (proton pump) on the apical membrane in secretion of HCl?

A

uses energy from the hydrolysis of ATP to move hydrogen out of the parietal cell and potassium into the parietal cell

276
Q

what is required for the H+/K+ ATPase pump to work?

A

extracellular K+ source

277
Q

how is the extracellular K+ source maintained to ensure the H+/K+ ATPase pump works?

A

apical K+ channel allows K+ to exit the cell so that it can be used by the proton pump to move back into the cell

278
Q

what is required within the parietal cell to maintain the movement of protons across the apical membrane?

A

source of protons

279
Q

what does the parietal cell do with water and CO2?

A

converts them to H+ (protons) and HCO3 (bicarbonate)

280
Q

how does CO2 enter the parietal cell for binding with water to create H+ and HCO3?

A

diffuses across basolateral membrane

281
Q

how does water enter the parietal cell for binding with CO2 to create H+ and HCO3?

A

requires specific aquaporin channels to cross the cell membrane

282
Q

what are water and CO2 converted to initially before forming H+ and HCO3?

A

carbonic acid

283
Q

what enzyme is used to speed up the process of conversion of water and CO2 to H+ and HCO3?

A

carbonic anhydrase

284
Q

what must happen to the bicarbonate produced during proton production?

A

cell must release it into the blood vessels of the mucosa

285
Q

what transporter is used to transfer bicarbonate into the blood vessels of the mucosa from the cell?

A

basolateral membrane exchanger (antiporter)

286
Q

what is exchanged at the basolateral membrane of the parietal cell so that HCO3 may enter the blood vessels of the mucosa?

A

Cl- enters the parietal cell

287
Q

what is an alkaline tide?

A

when parietal cells secrete acid so blood cells are alkalised by influx of HCO3

288
Q

under what conditions will an alkaline tide only occur?

A

only occurs in the fed state when parietal cells secrete acid

289
Q

how does Cl- exit the apical membrane of the parietal cell?

A

through gated channel into the lumen of gastric gland

290
Q

how is HCl produced in the lumen of the gastric gland?

A

Cl- combines with H+

291
Q

what is the role of Na+/K+ ATPase in the parietal cell?

A

creates a steep concentration gradient for sodium across the basolateral membrane which is used by other channels (e.g. regulation of cell pH where Na+ is moved out and H+ in)

292
Q

what is the role of basolateral K+ channels in the parietal cell?

A

provides a drive for Cl- and K+ to exit the cell via apical membrane channels

293
Q

summerise the cellular mechanism of HCl secretion

A

H+/K+ ATPase uses energy from ATP hydrolysis to pump H+ ions out of the cell across the apical membrane into the lumen of the gastric gland and K+ions into the cell. Apical membrane K+ channels recycle K+ ions across the apical membrane
The secretion of H+ ions across the apical membrane causes intracellular pH to rise. This rise in pH triggers passive uptake of CO2and H2O by the cell across the basolateral membrane. The production of H+ and HCO3-from H2O and CO2 via H2CO3 is catalysed by carbonic anhydrase
HCO3- ions produced by carbonic anhydrase are removed from the cell across the basolateral membrane by the anion (Cl-/HCO3-) exchanger. In exchange, Cl-ions are taken up by the cell
HCO3- ions that exit the cell across the basolateral membrane cause the alkalinisation of local blood vessels termed “alkaline tide”
The Cl-ions that enter the cell across the basolateral membrane via the Cl-/HCO3-exchanger exit passively across the apical membrane via a Cl-channel to complete the process of acid (HCl) secretion
The Na+-K+-ATPase creates the inwardly directed Na+ gradient across the basolateral membrane used by other transporters
Basolateral membrane K+ channels maintain the driving force for Cl- exit across the apical membrane

294
Q

what 2 factors regulate mucus and HCO3- secretion?

A

acetylcholine

prostaglandins

295
Q

what is the role of acetylcholine in the regulation of mucus and HCO3- secretion?

A

nervous stimulation leading to secretion of mucus and HCO3-

296
Q

how is HCO3- drawn into surface epithelial cells?

A

secondary active transport alongside Na+ using the Na+ gradient set up by Na+/K+ ATPase

297
Q

what is the main role of prostaglandins?

A

protect the integrity of gastric mucosa

298
Q

what 3 ways do prostaglandins protect the integrity of gastric mucosa?

A

inhibit acid secretion by parietal cells
stimulate mucous secretion
stimulate bicarbonate secretion

299
Q

how is the gastric mucosa protected?

A

gastric mucosal barrier

300
Q

what is the gastric mucosal barrier formed of?

A

mucus impregnated with bicarbonate

301
Q

how do acid and bicarbonate leave the gland without contact with surface epithelial cells?

A

viscous fingering - muscularis mucosae squeezes gastric glands and shoots the acid out of the glands

302
Q

what is the bicarbonate and proton concentration like close to the epithelial cells of the mucosa?

A

bicarbonate concentration is high
proton concentration is low
bicarbonate neutralises any protons

303
Q

what is the bicarbonate and proton concentration like close to the lumen of the stomach?

A

bicarbonate concentration is low

proton concentration is high

304
Q

what is the physiological protection of the gastric mucosa?

A

mucous and HCO3-

305
Q

what forms the anatomical protection of the gastric mucosa?

A

apical surface of the surface epithelial cells is impermeable to protons
tight junctions do not allow protons to pass through the paracellular pathway

306
Q

how many physiological agonists stimulate HCl secretion directly?

A

3

307
Q

what are the 3 physiological agonists that stimulate HCl secretion directly?

A

histamine
gastrin
acetylcholine

308
Q

how do ECL cells stimulate HCl secretion directly?

A

secrete histamine as a paracrine factor

309
Q

how do G cells stimulate HCl secretion directly?

A

secrete the hormone gastrin

310
Q

how do enteric neurons stimulate HCl secretion directly?

A

release ACh

311
Q

how is HCl secretion stimulated indirectly?

A

nerves stimulate ECL cells to release histamine

gastrin targets ECL cells causing them to release histamine

312
Q

what nerve is gastric secretion in the cephalic phase entirely dependent on?

A

vagus

313
Q

how much does secretion in the cephalic phase contribute to the total volume of secretion?

A

30%

314
Q

when does gastric secretion in the cephalic phase occur?

A

before food enters the stomach

315
Q

what does the vagus nerve stimulate in the cephalic phase of gastric secretion?

A

ACh to stimulate parietal cell
ECL cell to release histamine to stimulate parietal cell
G cell to release gastrin to stimulate parietal cell and ECL cell

316
Q

what cells does the the vagus nerve inhibit in the cephalic phase of gastric secretion?

A

D cells and so stops inhibition of acid secretion

317
Q

what is the gastric phase of gastric secretion controlled by?

A

vagovagal reflexes, hormones and paracrine factors

318
Q

how much does secretion in the gastric phase contribute to the total volume of secretion?

A

50%

319
Q

what mechanical and chemical factors stimulate secretion of acid during the gastric phase?

A

distention of the stomach

digested proteins and amino acids

320
Q

what receptors is distention of the stomach detected by?

A

mechanoreceptors

321
Q

what receptors detect presence of digested proteins and amino acids?

A

chemoreceptors

322
Q

what does distention of the stomach cause in the gastric phase of gastric secretion?

A

mechanoreceptors via vagovagal reflexes trigger parietal cells and G cells to release/trigger release of HCl

323
Q

what does digested proteins and amino acids cause in the gastric phase of gastric secretion?

A

g cell stimulates secretory cell

324
Q

how is secretion of HCl inhibited if levels become to high during gastric phase of gastric secretion?

A

HCl triggers chemoreceptors which trigger D cells that inhibit G cells by paracrine factors and parietal cells by endocrine hormones

325
Q

what happens to gastric secretion during early gastric emptying?

A

gastric chyme pH is >3 so stimulation dominates

326
Q

what happens to gastric secretion during late gastric emptying?

A

gastric chyme is <3 pH so inhibition dominates

327
Q

what factors stimulate secretion of acid during the intestinal (gastric emptying) phase?

A

distention of duodenum (mechanoreceptors)

digested proteins and amino acids (chemoreceptors)

328
Q

what does distention of the duodenum cause in the intestinal phase of gastric secretion?

A

parietal cell and G cell stimulated due to signalling from mechanoreceptors

329
Q

what does digested proteins and amino acids cause in the intestinal phase of gastric secretion?

A

g cell stimulates parietal cell due to signalling from chemoreceptors

330
Q

what factors inhibit secretion of acid during the intestinal (gastric emptying) phase?

A

HCl concentration in duodenum

products of digestion in duodenum

331
Q

what does higher HCl concentration cause in the intestinal phase of gastric secretion?

A

D cells produce somatostatin which inhibits g cells, ECL cells and parietal cells
s cells produce secretin which shut down G cells in antrum and parietal cells

332
Q

what does digestion products cause in the intestinal phase of gastric secretion?

A

cholysystekinine (CCK) released by I cells which directly inhibits parietal cells

333
Q

what is gastric emptying?

A

delivery of chyme from stomach to duodenum

334
Q

what are the fastest and slowest products to be digested and so leave the stomach?

A

carbs-proteins-fats-indigestable solids

335
Q

what items from digestion do not exit the stomach during the fed state?

A

indigestible solids

336
Q

due to control mechanisms in the intestinal phase what does gastric emptying not exceed?

A

the rate at which: acid can be neutralised, fat can be emulsified, the small intestine processes chyme

337
Q

where in the GI tract are all foodstuffs digested completely?

A

the small intestine

338
Q

how much pancreatic secretion is produced per day?

A

1.5 litres

339
Q

what is the pH of pancreatic juice?

A

7.8-8.4

340
Q

what is pancreatic juice formed of?

A

salts and enzymes

341
Q

what are the 2 types of pancreatic secretions?

A

endocrine and exocrine

342
Q

what is in endocrine pancreas secretions?

A

insulin and glucagon

343
Q

what is in exocrine pancreatic secretions?

A

salts and water

enzymes

344
Q

what are the main functions of the salts and water in pancreatic juice?

A

create the right environment for enzymes to work

345
Q

what is the salts and water portion of pancreatic juice formed of?

A

HCO3-
NaCl
water

346
Q

what is the main role of HCo3- in pancreatic juice?

A

neutralise acid and ensure correct mucous physical and chemical properties

347
Q

what are the main functions of the enzymes in pancreatic juice?

A

important for the digestion of all the major classes of foodstuffs and therefore essential for life

348
Q

what enzymes are found in pancreatic juice?

A

proteases
lipases
alpha-amylase

349
Q

what do protease enzymes do?

A

digest proteins

350
Q

what do lipase enzymes do?

A

digest fats

351
Q

what do alpha-amylase enzymes do?

A

digest carbohydrates

352
Q

what is the structure of pancreatic glands similar to?

A

salivary glands

353
Q

what are the cells of the pancreatic glands?

A

acinar cells

354
Q

what is the name of the secretory unit of the pancreatic/salivary glands?

A

lobule

355
Q

what cells secrete enzymes in the pancreas?

A

acinar cells

356
Q

which of the enzymes are released into the lumen of the gland of the pancreas already activated?

A

lipases and alpha-amylase

357
Q

why can lipases and alpha-amylase be released from the gland as activated enzymes?

A

they won’t digest the cells of the pancreas on the way out of the duct

358
Q

how do zymogen granules leave the cell?

A

exocytosis

359
Q

what are zymogens?

A

inactivated versions of enzymes that must be transported to site of action before being activated

360
Q

what enzymes are released as zymogens from the pancreatic acinar cells?

A

proteases:
trypsin
chrymotrypsin
carboxypeptidases

361
Q

why must proteases be released from the gland as zymogens?

A

as they would digest pancreatic cells if activated in the pancreas

362
Q

where are zymogens activated after leaving the pancreas?

A

small intestine

363
Q

what is trypsin known as while inactive?

A

trypsinogen

364
Q

what is chrymotrypsin known as while inactive?

A

chrymotrypsinogen

365
Q

what are carboxypeptidases known as while inactive?

A

procarboxypeptidases

366
Q

what is the main hormonal stimulator of secretion of enzymes from the pancreas?

A

CCK

367
Q

what cells is CCK relesed by?

A

I cells

368
Q

as well as CCk, what are the 2 other elements involved in regulation of enzyme secretion?

A

ACh and secretin

369
Q

what is composition of pancreatic juice decided by?

A

acinar and ductal epithelial cells

370
Q

what is the difference between duct lining cells of the salivary and pancreas glands?

A

salivary gland has tight epithelia so all transport of ions is transcellular whereas pancreas has leaky epithelia so paracellular transport is possible

371
Q

what moves by paracellular transport in the leaky epithelia of pancreatic duct cells?

A

water and NaCl

372
Q

what is the only difference in production of isotonic NaCl between salivary and pancreatic acinar cells?

A

regulation - pancreatic acinar cells are regulated by nerves and CCK

373
Q

how is isotonic NaCl solution modified in the lumen of the duct lining epithelial cells?

A

transcellular - bicarbonate is added and chloride exits

paracellular - Na and H2O enter the lumen

374
Q

what is the role of Na+/K+ATPase in the secretion of NaHCO3 from the pancreas?

A

creates a steep Na+ concentration gradient across the basolateral membrane which is used by other channels in secondary active transport.

375
Q

how is bicarbonate produced in the ductal epithelial cell of the pancreas?

A

Co2 diffuses into the cell and water delivered through aquaporins. They are then converted to carbonic acid and then to protons and bicarbonate

376
Q

what is the production of protons and bicarbonate catalysed by?

A

carbonic anhydrase

377
Q

what is a secondary source of bicarbonate?

A

there are sodium bicarbonate channels on the basolateral membrane

378
Q

how are the excess protons removed from the cell once bicarbonate has been produced?

A

Na+/H+ antiporter utilising the Na+ gradient produced by Na+/K+ ATPase

379
Q

how does bicarbonate leave the cell?

A

exits through anion exchanger in exchange for Cl-

380
Q

where does the supply of extracellular Cl- come from to supply the anion exchanger?

A

apical Cl- channel

381
Q

what is the role of basolateral K+ channels in secretion of NaHCO3 from the pancreas?

A

hyperpolarises membrane and so drives Cl- and HCO3- to leave the cell across the apical membrane

382
Q

how do Na+ and H2O move into the duct of the pancreatic gland?

A

via paracellular channels

383
Q

how is secretion of NaHCO3 from the pancreas regulated?

A

secretin increases the intracellular concentration of cAMP which phosphorylates apical Cl- channels and opens them

384
Q

summerise secretion of NaHCO3 by pancreatic duct cells

A

Na+, K+-ATPase creates the inwardly directed Na+gradient across the basolateral membrane
HCO3-accumulates intracellularly by two mechanisms: (i) direct entry across the basolateral membrane by the Na+-HCO3-cotransporter and (ii) intracellular generation from CO2 and H2O via carbonic anhydrase3
The Na+-H+exchanger removes H+ from the cell generated by HCO3-synthesis from CO2 and H2O4. The Cl-/HCO3-exchanger secretes HCO3-into the lumen of the duct in exchange for Cl-
cAMP-stimulated Cl-channels secrete the Cl-required by the Cl-/HCO3-exchanger
Basolateral membrane K+channels maintain the driving force for Cl-exit across the apical membrane
HCO3-secretion into the duct lumen draws Na+and H2O through the paracellular pathway to complete the process of isotonic NaHCO3-secretion

385
Q

what enzyme activates pancreatic enzymes from zymogens?

A

enteropeptidase

386
Q

where is enteropeptidase found?

A

brush boarder of the small intestine mostly although can be dislodged by bile and then found in the lumen

387
Q

what substance can dislodge enteropeptidase from the brush boarder?

A

bile

388
Q

what does enteropeptidase do in order to activates pancreatic enzymes from zymogens?

A

converts trypsinogen to trypsin which then converts chrymotrypsinogen and procarboxypeptidase to chrymotrypsin and carboxypeptidase therefore activating them

389
Q

what is the role of a trypsin inhibitor?

A

prevents premature activation of trypsin in the pancreas

390
Q

what cells release enteropeptidase?

A

epithelial cells of duodenum and jejunum

391
Q

what cells release trypsin inhibitor?

A

acinar cells

392
Q

what are the major stimulants of pancreatic secretion in the cephalic phase of digestion?

A

vagal impulses

393
Q

what proportion of pancreatic secretion occurs in the cephalic phase?

A

25% of total volume

394
Q

what does distention of the stomach in the gastric phase lead to with regards to pancreatic secretion?

A

distention stimulates mechanoreceptors and so sets off vagovagal and gastropancreatic reflexes and stimulates secretion

395
Q

what proportion of pancreatic secretion occurs in the gastric phase?

A

10% of total secretions

396
Q

is the enzyme concentration of the pancreatic secretion during the gastric phase high or low?

A

high

397
Q

what system does the major stimulation of pancreatic secretion come from during the intestinal phase?

A

hormones

398
Q

what is triggered by presence of acid with regard to pancreatic secretion?

A

chemoreceptors detect H+ and lead cells to release secretin which in turn causes ductal cells to release NaHCO3 leading to a large volume of secretion low in enzymes

399
Q

what does the presence of fatty acids, monoglycerides and peptides in the duodenum stimulate with regard to pancreatic secretion?

A

CCK is released by I cells and stimulates acinar cells to release enzyme rich pancreatic juice

400
Q

what does distention and osmolarity increase in the duodenum trigger with regard to pancreatic secretion?

A

mechanoreceptors with a vagovagal enteropancreatic reflex increase pancreatic secretion

401
Q

where are all foodstuff completely digested?

A

the small intestine

402
Q

how much bile is produced per day?

A

0.5l

403
Q

what is the pH of bile?

A

7.4

404
Q

what is bile formed of?

A

bile salts, cholesterol, phospholipids

405
Q

what is bile essential for?

A

digestion of fats

406
Q

what cells is bile secreted by?

A

hepatocytes

407
Q

where is bile stored?

A

in the gall bladder

408
Q

what makes up bile?

A

bile salts (65%)
cholesterol (4%)
phospholipids (20%)
bile pigments (0.3%)

409
Q

what is the role of bile salts in digestion?

A

emulsify and so improve the digestion of lipids

410
Q

why is cholesterol found in bile?

A

mechanism for cholesterol excretion

411
Q

what do bile salts, cholesterol and phospholipids form?

A

mixed micelles

412
Q

what is the role of mixed micelles found in bile?

A

aid delivery of fatty acids and monoglycerides to unstirred layer of intestine which helps absorption

413
Q

what are bile pigments formed of?

A

breakdown products of haemoglobin (bilirubin)

414
Q

why is bilirubin found in bile?

A

so it can be excreted via the GI tract

415
Q

what happens to bile between meals?

A

concentrated and stored in the gallbladder

416
Q

what happens to the gall bladder during the cephalic phase of digestion?

A

neuronal stimulation of secretion causing the contraction of the gallbladder and relaxation of the sphincter of Oddi

417
Q

what happens to the gall bladder during the gastric phase of digestion

A

distention of stomach stimulates mechanoreceptors. Vagovagal reflex via the parasympathetic nervous system causes contraction of the gallbladder and relaxation of the sphincter of Oddi

418
Q

how is the intestinal phase of digestion regulated?

A

hormones

419
Q

what stimulates release of bile from the gall bladder during the intestinal phase?

A

presence of products of digestion (fatty acids, monoglycerides, peptides) in the duodenum stimulate release of CCK which causes contraction of gallbladder and relaxation of the sphincter of Oddi

420
Q

what effect do sympathetic nerves have on the gallbladder?

A

stop contraction of the gallbladder and contract the sphincter of Oddi

421
Q

why are bile acids reabsorbed?

A

bile synthesis is hugely metabolically expensive and so only a fraction of required bile is produced per day

422
Q

where in the GI tract are bile acids reabsorbed?

A

ileum

423
Q

what happens to reabsorbed bile acids once they leave the ileum?

A

returned to the liver and taken up by hepatocytes

424
Q

what type of mechanism is recycling of bile acids?

A

homeostatic

425
Q

what mechanism is used to reabsorb bile acids from the ileum?

A

Na+ exchanger, Na+ dependent mechanism

426
Q

how many rounds of bile recycling can take place per meal?

A

up to 5 depending on richness of meal

427
Q

how much secretion does the small intestine produce per day?

A

1l

428
Q

what is the pH of small intestine secretion?

A

7.6

429
Q

what is small intestine secretion formed of?

A

mucous, enteropeptidase, salt and water

430
Q

what is enteropeptidase important for?

A

activation of trypsinogen

431
Q

in what part of the intestine are villi found?

A

dominant in SI

432
Q

what is the role of crypts in the small intestine?

A

secretion of NaCl and water

433
Q

what is the role of crypts in the large intestine?

A

secretion of mucous to protect the surface of the colon

434
Q

what is the length of the small and large intestine?

A

small - 6m

large - 2.4m

435
Q

what is the surface area of the small and large intestine?

A

small - 300m2

large - 25m2

436
Q

are there villi in the small and large intestine?

A

small - yes

large - no

437
Q

are there microvilli in the small and large intestine?

A

yes

438
Q

are there crypts in the small and large intestine?

A

small - yes (NaCl and water)

large - yes (mucous)

439
Q

is there nutrient absorption in the small and large intestine?

A

small - yes

large - no

440
Q

is there active Na+ absorption in the small and large intestine?

A

yes

441
Q

is there active K+ secretion in the small and large intestine?

A

small - no

large - yes

442
Q

is there secretion of HCO3- in the small and large intestine?

A

for protection
small - yes
large - yes

443
Q

how much water is usually taken in through the mouth per day?

A

2l

444
Q

what fluids are secreted by the GI tract?

A 
salivary glands 
stomach
bile duct
pancreas
small intestine
445
Q

how much fluid does the small intestine absorb throughout the day?

A

6.5l

446
Q

how much fluid does the small intestine absorb throughout the day?

A

1.9l

447
Q

what is fluid balance in the intestine achieved by predominantly?

A

absorption in the small intestine

448
Q

what process occurs on the villi of the small intestine?

A

absorption

449
Q

what happens in the crypts of the small intestine?

A

secretion of salt and water

450
Q

what are the 5 mechanisms of digestion and absorption in the small intestine?

A

none needed (e.g. for glucose)
brush boarder hydrolysis of oligomer to monomer
luminal hydrolysis of polymer to monomer
intracellular hydrolysis
luminal hydrolysis followed by intracellular resynthesis

451
Q

what happens during brush boarder hydrolysis of oligomer to monomer in the small intestine?

A

final breakdown of products (e.g. sucrose to glucose and fructose) occurs in the brush boarder of apical membrane

452
Q

what happens during luminal hydrolysis of polymer to monomer in the small intestine?

A

breakdown occurs in the lumen of small intestine (e.g. proteins to amino acids)

453
Q

what happens during intracellular hydrolysis in the small intestine?

A

partial digestion occurs in the lumen of small intestine (e.g. protein to tri-peptide), this is then absorbed and broken down (e.g. to amino acids) in the cell

454
Q

what happens during luminal hydrolysis followed by intracellular resynthesis in the small intestine?

A

triglyceride is broken down to monoglyceride and fatty acids in the lumen and absorbed across the apical membrane. These are then recombined into triglycerides in the cell and absorbed into lacteals

455
Q

by what enzyme are carbohydrates originally broken down?

A

alpha amylase

456
Q

what are the intermediate products of carbohydrate breakdown?

A

maltose, maltotriose and alpha-limit dextrins

457
Q

what enzymes are used to split maltose, maltotriose and alpha-limit dextrins into glucose?

A

maltase, lactase and sucrase-isomaltase

458
Q

what is maltose broken down into by maltase?

A

2 glucose molecules

459
Q

what enzyme is used to break down maltose into glucose?

A

maltase

460
Q

where are maltase, lactase and sucrase-isomaltase found in the small intestine?

A

brush boarder

461
Q

what is maltotriose split into?

A

1 maltose and 1 glucose molecule

462
Q

what are alpha-limit dextrins split into?

A

1 unit maltose and 1 unit maltotriose

463
Q

what enzyme are alpha-limit dextrins split by?

A

sucrase-isomaltase

464
Q

what is the difference between maltase and lactase and sucrase-isomaltase?

A

maltase and lactase sit in the apical membrane alone, sucrase-isomaltase is a complex of 2 enzymes

465
Q

what is produced from the breakdown of sucrose?

A

fructose

466
Q

what is produced from the breakdown of lactose?

A

galactose

467
Q

what are the transport mechanisms found in the apical membrane of the small intestine?

A

SGLT1

GLUT5

468
Q

what does SGLT1 stand for?

A

Na coupled glucose transporter 1

469
Q

what does GLUT5 stand for?

A

glucose transporter 5

470
Q

what does SGLT1 transport?

A

glucose and galactose

471
Q

how does SGLT1 transport glucose and galactose?

A

secondary active transport using gradient created by Na+/K+ ATPase in the basolateral membrane

472
Q

what does GLUT5 transport?

A

fructose

473
Q

how does GLUT5 transport fructose?

A

facilitated diffusion

474
Q

what is the transport mechanism for products of carbohydrate digestion across basolateral membrane into blood vessels called?

A

GLUT2

475
Q

what does GLUT2 transport?

A

glucose, galactose and fructose

476
Q

how does GLUT2 transport glucose, galactose and fructose across the basolateral membrane?

A

facilitated diffusion

477
Q

where is sugar absorption greatest in the GI tract?

A

duodenum

478
Q

what are the 3 methods of digestion and absorption of protein in the small intestine?

A

Luminal enzymes digest proteins to amino acids before absorption by villus epithelial cells.
Luminal enzymes digest proteins to peptides, enzymes in the apical membrane of villus epithelial cells digest the peptides to amino acids before absorption by villus Luminal enzymes digest proteins to peptides, which are absorbed by villus epithelial cells and then digested within the cell (intracellular enzymes) to amino acids

479
Q

how many mechanisms exist to move amino acids across the apical membrane?

A

7

480
Q

are all mechanisms to move amino acids across apical membrane sodium dependent?

A

no - some not all

481
Q

what is transport of peptides across the membrane coupled to?

A

secondary active transport alongside H+ (cotransport)

482
Q

how many mechanisms exist to move amino acids across the basolateral membrane?

A

3

483
Q

where does most amino acid and peptide absorption occur?

A

duodenum

484
Q

what are emulsification droplets formed of?

A

lipid droplets and bile salts

485
Q

what is the function of an emulsion droplet?

A

increases the surface area on which enzymes can act

486
Q

what enzyme is used to break down lipids into monoglycerides and fatty acids?

A

pancreatic lipase

487
Q

what is the key preliminary step in reducing the size of lipid droplets and increasing oil water interface?

A

emulsion

488
Q

when is pancreatic lipase effective?

A

only at the oil water interface of an emulsion droplet

489
Q

what is formed with monoglycerides and fatty acids?

A

mixed micelles

490
Q

what is found in mixed micelles?

A

phospholipid and cholesterol, monoglycerides and fatty acids

491
Q

what is the role of mixed micelles?

A

diffuse into acid unstirred layer on apical membrane of epithelial cells

492
Q

how do fatty acids and monoglyceride cross the apical membrane?

A

diffusion

493
Q

what happens to fatty acids and monoglyceride in the smooth ER of the cell?

A

reassemble into triglyceride and form chylomicrons

494
Q

what happens to chylomicrons once formed in the smooth ER?

A

exit villus epithelial cells across basolateral membrane via exocytosis and enter lacteals

495
Q

what is key to emulsion droplet creation and movement?

A

the non- propulsive movement of the small intestine

496
Q

what happens to the bile salt micelle once the fatty acids and monoglycerides have crossed the apical membrnae?

A

they are recycled and form more mixed micelles

497
Q

In what part of the small intestine does most lipid absorption occur?

A

jejunum

X Physiology 1B (5 decks)

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