Blood glucose concentration should remain at a fairly constant value of about 100 mg
glucose per 100 cm3 of blood.
If blood glucose concentration falls well below this level, the person is said to
be hypoglycaemic. Cells do not have enough glucose to carry out respiration, and so
metabolic reactions may not be able to take place and the cells cannot function
normally. This is especially so for cells such as brain cells, which can only use glucose
and not other respiratory substrates. The person may become unconscious and various
tissues can be damaged.
If blood glucose concentration rises well above this level, the person is said to
be hyperglycaemic. The high glucose concentration decreases the water potential of
the blood and tissue fluid, so that water moves out of cells down a water potential
gradient. Again, unconsciousness can result.
When blood glucose concentration rises too high, this is sensed by the β cells. They
respond by secreting greater quantities of insulin into the blood. The insulin has several
effects, including:
causing muscle and adipose tissue cells (fat cells) to absorb more glucose from
the blood;
causing liver cells to convert glucose to glycogen for storage.
These effects cause the blood glucose concentration to fall.
When blood glucose concentration falls too low, this is sensed by the α cells. They
respond by secreting greater quantities of glucagon into the blood. This has several
effects, including:
causing liver cells to break down glycogen to glucose, and releasing it into the
blood;
causing liver cells to produce glucose from other substances such as amino acids
or lipids.
These effects cause blood glucose concentration to rise.
Summary:
After eating a meal, blood glucose rises:
• Food eg. starch are hydrolyzed by digestive enzymes into glucose
• Glucose is absorbed across the gut wall into the blood capillaries
• Rise in blood glucose detected by β-cells of islets of langerhans in the pancreas
Insulin is secreted into the bloodstream
• Promotes the uptake of glucose by the liver and muscle cells
• Glucose then converted to glycogen
During exercise, blood glucose concentration falls:
• Detected by α- and β-cells of islets of langerhans in the pancreas
• Fall inhibits further insulin secretion
• Secretion of glucagons by α-cells into the blood
• Binds to receptors on liver cell surface membrane
• Activation of phosphorylase
• Promotes conversion of glycogen to glucose in the liver (glycogenolysis)
• Promotes gluconeogenesis, the production of glucose
• Glucose is released into the bloodstream
• Fats are broken down and respired.
VIDEO Insulin and Glucagon:
https://www.youtube.com/watch?v=e-3N7w2sWps
Insulin and Regulation of Glucose in the Blood:
https://www.youtube.com/watch?v=OlHez8gwMgw