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DRE Summary
DIGESTION
The digestive system is important in putting food into the circulatory system for
cellular processes
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The alimentary tract is the tube that passes from the mouth to anus but cannot
alone digest food completely and accessory organs such as the salivary glands,
pancreas liver and gall bladder are needed (exocrine glands)
...

Endocrine glands secrete the substances directly into the blood
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- Gastrointestinal Tract physical functions
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Ingestion – taking food into mouth
Mastication – chewing and mixing food with saliva
Deglutition – swallowing food
Motility – movement of food through GIT by peristalsis (propels food by waves of
contractions) and segmentation (churns and breaks up food by contractions of the wall at
regular intervals)
Elimination – removal of waste products through the anus

Physiological functions
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Digestion: breaking the polymers of fats, carbs and proteins into subunits that can be
absorbed
Absorption: the movement of subunits into the blood or lymph as well as water and
vitamins, can be diffusion, facilitated diffusion or active transport
Secretion: Gut secretes many enzymes, acids and alkalis to aid digestion, per 2L of food
taken in there are 7 litres of secretions
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The simple cylinder only has a small surface area but there are many folds of it known as the
folds of kerckring which are large folds of the mucosa and submucosa which triple the surface area
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The GI tract contains its own regulators of hormones, nerves and paracrine factors which aid in the
control of motility of the GIT, secretions from secretory cells and blood flow to the GIT
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- Hormones of the Gut
Hormone
Gastrin
Cholecystokinin

Released from
Stomach – G cells
Small intestine – I cells

Secretin

Small intestine – S cells

Main effects
Gastric acid secretion
Gall bladder contraction
Pancreatic secretion
Inhibit Gastric acid secretion
Pancreatic secretion
(bicarbonate solution)

- Neural control of the Gut
There is extrinsic control from the autonomic nervous system which acts to modulate the ENS
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The sympathetic nervous innervation comes from spinal nerves level T5-L2 and act to reduce
peristalsis, motility and decrease secretions
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- Mastication
Mashes and crushes food in preparation for swallowing to create a bolus that gets mixed with saliva
to make food small enough to fit down the oesophagus and to increase the surface area for
enzymatic action, more mastication = more salvia
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Feedback from the mouth is also very important as we can sense the food that we
are eating and how much force we need to eat it
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It is a collection of nerves located in the
brainstem which initiates and maintains the opening and closing of the jaw without the need for
somatic innervation as it is done automatically
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It is also possible for
the afferent nerves of the mouth and teeth to send information to the brainstem about how hard a
food is, there is also coordination from the cortex which uses sight and previous experience to
influence the pattern
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The central pattern generator is thought to be comprised of a rhythm generator and a burst
generator, which generates the basic masticatory rhythm and alters the rhythm according to sensory
inputs that fine tune the masticatory pattern
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Diseases within the
teeth and gums also affect the control of chewing as without teeth or damage to the nerves within
the gums there is a lack of feedback to the central pattern generator so it does not work as
effectively
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There are 3 phases to swallowing:
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Oral – voluntary
Pharyngeal – involuntary
This is the most dangerous phase as it is the one where there
is the largest risk of things going wrong, it is possible for the
food to leave the pharynx and move out of the nose or
alternatively down the trachea which we do not want to occur
and so there are safety mechanisms which involve the palate
lifting up and closing off the nose so food cannot leave the
nose and the epiglottis also shuts off the trachea
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Food is passed by peristaltic waves which moves the bolus by
the muscles of the oesophagus contracting behind the bolus and relaxing in front of it, the
peristalsis is controlled by extrinsic vagal nerves and intrinsic nerves
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At the top
and bottom of the oesophagus are two sphincters the upper oesophageal sphincter and the
lower oesophageal sphincter, the latter of which is important in inhibiting acid reflux from
the stomach

- Saliva
Saliva is a very important secretion as it has many functions:
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It cools, moistens and aids the chewing of food and allowing it to form a bolus
Mucins within saliva aid in lubrication and allow easier swallowing and speech as well as
protecting the oesophagus
The solubilising of food also aids in tasting it
There are also immunological properties such as lysozymes, antibodies and lactoferrin, the
latter of which binds iron and protects it from digestion by bacteria
Salvia also starts the digestion process as α-amylase and lingual lipases start the digestion
process, lingual lipases are especially important in new-borns as their pancreas is not yet
fully developed
Bicarbonate within saliva acts as a buffer and neutralises acid in food and in vomit by
reacting with it and making H2CO3 carbonic acid which can dissociate into water and CO2, it is
important that we can neutralise the acid as it demineralises the enamel
Minerals such as calcium and phosphate aid in mineralisation of teeth and aids in the
protection of enamel from de-mineralisation
Saliva also secretes the pellicle, a protein rich film that covers teeth and protects against acid
but is highly stained by coffee, tea, cigarettes and bacteria like to colonise it

Salivary hypofunction can arise from a variety of causes:
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Head and neck cancer radiotherapy
Autoimmune diseases such as Sjogren’s, Lupus
Drug therapy
Methamphetamine can induce xerostomia

If hypofunction is severe then the patient can have Xerostomia which is no salivary production
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There are other smaller salivary glands in the lips, cheeks, palate and tongue and they are all
composed of clusters of acinar cells that deliver saliva to the mouth through ducts so are exocrine
glands
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There are also
proteins such as alpha amylase, lingual lipase, mucins and immunoglobulins
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1 and 8 depending on the HCO3 concentration and the flow rate also the volume of saliva can vary
from one litre to 2 L per day
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If there are more secretions into the saliva then
the osmolality of the solution increases as there is a greater number of moles of solute that
contribute to the osmotic pressure of a solution
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Due to this the primary secretion is very similar in
composition to plasma and so it becomes isotonic
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Along the duct there are many cells with transport
proteins in that remove sodium and chlorine, and add potassium
and bicarbonate
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It is these channels that move the
ions that can be affected by drugs
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It is important for the saliva to be hypotonic because then
water does not move out of the mucosa of the mouth
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When there is no food in the mouth
there is no need for HCO3 to be in the saliva to neutralise acid and so HCO3 is not secreted into the
saliva, this is important as HCO3 is a very useful molecule
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The autonomic nervous system controls salivation, it gets information from the nose, mouth
and eyes
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The sympathetic nervous system decreases salivation by constricting the vesicles
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At
rest they have many mitochondria to
provide the energy for acid secretion, there
are also many tubulovesicles that, upon
activation (by histamine [paracrine], gastrin
[endocrine] and acetyl choline from the
parasympathetic nervous system of the
vagus nerve) join together and increase the
parietal cells surface area
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1
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Carbon dioxide enters the P cell from the blood and binds
with water to make carbonic acid in a reaction catalysed
by carbonic anhydrase
3
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The resulting proton is pumped into the lumen by the
hydrogen potassium ATPase pump, whilst the bicarbonate
is exchanged for chlorine in the blood
5
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People with acid secretion are often put on proton pump inhibitors to stop the K+/H+ ATPase pump
from working
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There are 3 phases of gastric secretion:
Cephalic phase: thinking about food which causes ACh release from the vagus nerve which binds to
muscarinic receptors on the surface of parietal cells
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Longest
phase of acid secretion and so secretes the most acid
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- Pancreas
The pancreas has both exocrine and endocrine
functions
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It is 1% by
mass endocrine, the endocrine cells are the islets of
Langerhans which secrete insulin and glucagon into
the blood stream
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Pancreatic juice and bile enter the duodenum through the ampulla of Vater which is a muscular ring
which surrounds the two ducts
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Duct cells carry out ion movement, moving HCo3
into the ducts and moving Chlorine out of the duct movement of ions depends on the rate of flow:
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High flow rate when food has entered the duodenum so there is more ductal modification
therefore more HCO3 and less Cl-

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1
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3
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5
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If
there is a mutation in this protein then cystic fibrosis arises which means that Cl- is not transported
out so water does not follow by osmosis which leaves thick and sticky mucous
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This is because the thick mucus prevents enzymes
reaching the small intestine so they can start to digest the pancreas
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The enzymes are produced as inactive zymogens so no
autodigestion occurs, trypsinogen is activated by enterokinase in the small intestine and becomes
trypsin
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Chymotrypsinogen
Procarboxypeptidase
Pro-elastase
Prophospholipase A
Trypsin

Control of pancreatic secretion
1
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Fatty chyme causes secretin to be released from S cells in the
small intestine
2
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3
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Carbonic acid is produced which – catalysed by carbonic anhydrase – dissociates into H+ and
bicarbonate
5
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Bicarbonate moves into lumen with enzymes in exchange for ClSmall intestine
Mucus prevents the autodigestion of the small intestine and the compartmentalisation of the
enzymes also helps
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The intestine secretes many substances:

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Enzymes
Mucus
Water: moves by osmosis out of crypt cells in the small intestine after chlorine is pumped
out of the crypts by the CFTR channel
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Produces bile which is important in the digestion and absorption of fats, bile stored in the gall
bladder and made by hepatocytes
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Liver has 2 blood supplies: hepatic portal vein and hepatic artery, former transports all digested food
from the intestines latter provides oxygen and nutrients to the hepatocytes
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Portal triads consist of the hepatic portal vein, the hepatic artery and bile duct
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Hepatocytes make bile salts which are amphipathic and in high concentrations form micelles which
allow fatty substances to be transported in them
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After this it is sent back to the liver in the
enterohepatic circulation, 5% is lost
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The liver plays an important part in bilirubin formation and excretion
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If for any reason this
fails then jaundice sets in
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Bile salts from the
liver emulsify the fat
droplets in the lumen
of the SI
2
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Bile salts continue
along the SI until
they are absorbed
4
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5
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If fat cannot be absorbed then we have steatorrhea, a lack of the essential fatty acids linoleic acid,
alpha linoleic acid, DHA and EPA
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- Absorption of carbohydrates and proteins
Carbohydrates are digested into disaccharides by alpha amylase, pancreatic amylase and enzymes in
the intestine, they are then digested into monosaccharides by intestinal maltase, sucrase and lactase

1
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Sodium conc is decreased so sodium
diffuses into the cell from the lumen across the
SGLT-1 with glucose or galactose
3
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Fructose diffuses into the cell by the GLUT 5
transporter
Once absorbed the monosaccharides can be sent to tissues where they are needed/converted into
glycogen/stored as fat
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In the small intestine pancreatic enzymes such as trypsin,
chymotryspsin (exopeptidases) digest the peptides from one end into amino acids
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Di and tripeptides
are also able to enter the cell by FD and are digested within the enterocyte
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- Absorption of ions and water
It is important that the osmolality of body compartments does not change greatly, as it can lead to
bloating or dehydration
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We secrete 6-7 litres of water into the gut so we must have a good
method of reabsorption, 95% of this occurs in the small intestine where water molecules can move
by osmosis between enterocytes or through aquaporins
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Ions are very important to be absorbed as their absorption is interdependent with that of the
nutrients
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Sodium is absorbed into the gut by facilitated diffusion along a concentration gradient established by
a sodium potassium atpase pump which moves 2 Na+ out of the enterocyte and into the blood
which allows sodium to diffuse into the cell along the concentration gradient either through protein
channels or through cotransport
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If there is not this ion
gradient then water remains in the lumen leading to diarrhoea
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If the pyloric sphincter is dysfunctional then dumping syndrome occurs where excessive amounts of
chyme moves into the duodenum which stimulates excessive H2O secretion due to osmosis
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Both the membrane carrier
proteins are only synthesised in the presence of vitamin D
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They are comprised of an
outer layer of dense collagen which forms the Capsule
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More deep is the Medulla, which is the site
of the blood filtration, these form medullary pyramids which filter into the
calyx, from here the urine filters into the ureters where they travel into the
bladder
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In males the ureter must pass beneath the ductus
deferens before it enters the bladder and in females the ureter passes inferiorly to the uterine artery
anteriorly to the vagina
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The bladder sits posteriorly to the pubic symphysis when empty and upon filling it rises above the
symphysis
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- Filtration
The kidneys have a large blood flow and remove many soluble substances from the blood to be
excreted in urine:
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Metabolic waste products:
o Ammonia is extremely toxic and is produced from protein catabolism
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High
levels of this within the body signals non-functioning kidneys
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Erythropoetin – produced by vascular endothelial cells of the kidney amongst others in
response to hypoxia
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Prostaglandins – these are autocrine and paracrine hormones produced by the kidneys
which induce vasodilation locally to help kidney function, especially PGE2
Vitamin D – cholesterol is converted into inactive vitamin D by sunlight hitting the skin, this
is then converted to 25(OH)D in the liver and finally into the active form of vitamin D in the
kidney which allows for control of calcium absorption
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4 by the kidneys
blood pressure
red blood cells

The kidneys only come into contact with the blood plasma, however due to osmosis through
aquaporins in the plasma membrane and starling’s forces (hydrostatic pressure) which pushes
plasma out of the capillaries, the plasma gives a good representation of other fluid compartments
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The plasma and the interstitial fluid have similar ion concentrations due to the endothelium being
permeable to ions, however not as much to proteins which results in the plasma having greater
protein content in comparison to the extracellular fluid
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- The Nephron
The nephron exists as a tubule that extends through the renal cortex and medulla that is involved in
the production of urine
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The glomerulus is a small highly twisted bundle of capillaries that sits in
the Bowman’s capsule, blood then leaves via the efferent artery
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Once the proximal convoluted tubule enters the medulla it is known as the loop of Henle that then
loops back on itself to once again leave the medulla forming the distal convoluted tubule where
further modification occurs before if feeds into the collecting duct
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It is important in the filtration of the blood to ensure no large substances get through
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These many layers allow the blood to

be well filtered so that no blood cells or proteins are able to enter the nephron and only the plasma
and ions can get through
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If the filter is damaged we can see blood in the urine
(haematuria) or protein in the urine (proteinuria) which is characterised by frothy urine and possibly
oedema in the body
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The
filtrate is maintained within the Bowman’s capsule due to the glomerular hydrostatic pressure being
able to overcome the glomerular oncotic pressure and the hydrostatic pressure of the Bowman’s
capsule
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Endocrine
The Pancreas
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insulin and carbohydrate metabolism
Carbohydrate – glucose and all sugars
Glucose important as energy source for neurones, no glucose = death within seconds
Glucose kept around 5mmole/L
Source from diet or gluconeogenesis in liver and kidneys
Most cells, liver can metabolise fatty acids sometimes in preference to provide energy
Brain can only use glucose and cannot store it

Normal situation = starved situation, breaking down our stores to release energy
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Insulin is a signal released during feeding to stop breaking down of stores
In presence of insulin glucose is taken up by muscle and liver and fat cells to metabolise and
store it as glycogen or fat
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Lots of
glycogen in muscle for endurance
in presence of glucagon we use our stores to create energy

Ingested Glucose
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5% stored as glycogen
30-40% converted to fat as it does not retain water so is a lighter energy store
50% metabolised immediately

Starved state
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Glycogenolysis
Glucose produced from amino acids of muscle
Fatty acids released from fat, used in krebs cycle

Pancreas
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Exocrine – digestive fluids
Endocrine pancreas, irregular cells in the islets of Langerhans
Alpha cells – glucagon
Beta cells – insulin
Delta cells – somatostatin

Importance of insulin:
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Pre proinsulin to proinsulin then c chain released to release insulin
Released constantly at basal level, very high when eating
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Glut 2 transporter on Beta cells raising extracellular
glucose so glucose diffuses into the beta cell through
the glut 2 transporter
2
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the ATP can be used to close ATP sensitive potassium
ion channels = depolariseation
4
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Insulin – anabolic function
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Increases glycogen and fat and protein synthesis stops glucose production

Glucose uptake is allowed by insulin as it increases the number of glut4 receptors in cell membranes
so more glucose can diffuse into cells
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Liver does not receive
insulin signal so releases glucose as it perceives that it is in a starved state
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Called ketoacidosis, acetone can be exhaled from the lung
which can be smelled as a sweet alcoholic smell, acidosis can cause brain damage, only occurs in
type 1 diabetics
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However as some glucose can get in there is not the ketosis
Symptoms of diabetes:
Polyphagia – can’t detect glucose so think they are hungry
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Polydipsia – losing lots of urine so thirsty
Blurred vision glucose in eye draws water in
Fatigue, lack of glucose for respiration
Dry mouth, thirsty
Hyperglycaemia damages vasculature and nerves so people cannot notice damage to their body
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