Download Regulation of blood glucose level

Regulation of blood glucose level
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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.
Regulation of blood glucose
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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 hormonesecreting 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.
How blood glucose is regulated by insulin and
glucagon in relation to diabetes
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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.
Diabetes
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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.
Questions
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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.
Answers
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1. Indicator sticks/clinistix/multistix/unistix
dipped in urine.
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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.