Regulation of blood glucose level

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Regulation of blood glucose level

• Transport of the monosaccharide glucose to all
  cells is a key function of the blood circulation.
  
• In humans, the normal level of blood glucose is
  about 90 mg of glucose/100 cm3 of blood, but this
  can vary.

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• For example, during an extended period without
  food, or after prolonged and heavy physical
  activity, the blood glucose level may drop to as
  low as 70 mg.
• After a meal rich in carbohydrate has been
  digested, the blood glucose level may rise to 150
  mg.

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• Respiration is a continuous process in all living
  cells. To maintain their metabolism, cells need
  a regular supply of glucose, which can be quickly
  absorbed across the cell membrane.
• Glucose is the principal fuel used for
  respiration
• Most cells hold additional glucose reserves in
  the form of glycogen, which is quickly converted
  to glucose during prolonged physical activity
  however, glycogen reserves may be used up
  quickly.
• In the brain, glucose is the only fuel the cells
  can use and there are no glycogen reserves held
  there at all.

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• If our blood glucose falls below 60 mg/100cm3 a
  condition called hypoglycaemia develops. If this
  is not quickly reversed, the person may faint.
  If the body and brain continue to be deprived of
  adequate glucose levels, then convulsions and
  hypoglycaemic coma follow, which can be fatal.

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• An abnormally high concentration of blood
  glucose, known as hyperglycaemia, is also a
  problem. Since high concentrations of any
  soluble metabolite lower the water potential of
  the blood plasma, water is drawn out of the cells
  and tissue fluid by osmosis, back into the blood.
  As the volume of blood increases, water is
  excreted by the kidney in an attempt to maintain
  the correct concentration of blood. As a result
  the body tends to become dehydrated, and the
  circulatory system is deprived of fluid.
  Ultimately, the correct blood pressure cannot be
  maintained.

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Regulation of blood glucose

• After the digestion of carbohydrates in the gut,
  glucose is absorbed across the epithelial cells
  of the villi into the hepatic portal vein.
• The blood carrying the glucose therefore reaches
  the liver first.
• If the blood glucose level is too high then
  glucose is removed from the blood and stored as
  glycogen.
• But, even so, blood circulating in the body after
  a meal has a raised level of glucose.

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• At the pancreas the presence of an excess of
  blood glucose is detected by groups of cells
  within the organ, known as the islets of
  Langerhans. These islets are hormone-secreting
  glands (endocrine glands) they have a rich
  capillary network, but no ducts that would carry
  secretions away. Instead, their hormones are
  transported all over the body by the blood. The
  islets of Langerhans contain two types of cell,
  known as alpha cells and beta cells.

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How blood glucose is regulated by insulin and
glucagon in relation to diabetes

• In the presence of a raised blood glucose level,
  the beta cells are stimulated.
• They secrete the hormone insulin into the
  capillary network.
• Insulin stimulated the uptake of glucose into
  cells all over the body, but especially in cells
  in the liver and the skeletal muscle fibres.

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• It also increases the rate at which glucose is
  used in respiration, in preference to alternative
  substances (such as fats).
• Another effect of insulin is to trigger
  conversion of glucose to glycogen for storage in
  cells (glycogenesis), and of glucose to fatty
  acids and fats, and finally the deposition of fat
  around the body.

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• As the blood glucose level reverts to normal this
  is again detected in the islets of Langerhans.
  The beta cells respond by stopping insulin
  secretion.
• Meanwhile, the hormone is excreted by the kidney
  tubules, and the blood insulin level falls.

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• When the blood glucose level falls below the
  normal, the alpha cells of the pancreas are
  stimulated.
• These secrete a hormone called glucagon.
• This hormone activates the enzymes in the liver
  that convert glycogen and amino acids to glucose
  (gluconeogeneis)
• Glucagon also reduces the rate of respiraton.

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• Low blood glucose also triggers the secretion of
  other hormones in the body, particularly
  adrenaline by the adrenal glands.
• One effect of adrenaline is to stimulate the
  conversion of glycogen and amino acids to
  glucose, which raises the level of blood glucose.
• As the blood glucose level reverts to normal,
  glucagon production and adrenaline production
  cease, and these hormones in turn are removed
  from the blood in the kidney tubules.

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Diabetes

• Diabetes is the name for a group of diseases in
  which the body fails to regulate blood glucose
  levels.
• Type I diabetes results from a failure of insulin
  production by the beta cells.
• Type II is a failure of the insulin receptor
  proteins on the cell membranes of target cells.

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• As a consequence, blood glucose levels are more
  erratic and, generally, permanently raised.
• Glucose is also regularly excreted in the urine.
• If this condition is not diagnosed and treated,
  it carries an increased risk of circulatory
  disorders, kidney failure, blindness, strokes and
  heart attacks.

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Questions

• 1. Describe a simple test the nurse could use at
  the clinic to detect glucose in urine.
• 2. What result would you expect to see if the
  patient were excreting glucose?
• 3. The nurse decided to send the patient to
  hospital for some further health checks. In the
  hospital, the patient was asked, by a specialist
  nurse, to take a glucose tolerance test. Write
  down the stages involved in conducting a glucose
  tolerance test.

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• 4. Some people can control diabetes by insulin
  injection. Explain two other ways that diabetes
  could be managed by somebody who suffers from the
  condition.

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Answers

• 1. Indicator sticks/clinistix/multistix/unistix
  dipped in urine.
• 2. Multistix/unistix indicator turns from green
  to brown OR clinistix turns from pink to purple
  OR indicator/stick changes colour.

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• 3. Patient fasts for several hours
• Measured amount of glucose given/ingested
• Blood glucose (not urine) levels
  monitored/measured at regular intervals over next
  few hours
• Blood glucose level taken before and after
  glucose meal to detect change.