Insulin and glucagon

Blood glucose is the best known regulation example, and the most useful to understand.

Why it must stay stable

   too LOW (< 0.6 g/L)   ->  the brain runs short of fuel
                             faintness, seizures, coma
   too HIGH (> 1.8 g/L)  ->  glucose attaches to proteins
                             damage to vessels, kidneys, nerves

The brain consumes about 120 g of glucose a day and stores almost none: it depends on continuous supply from the blood. A few minutes of severe hypoglycaemia suffice to cause damage.

Conversely, prolonged excess slowly damages vessels — the central mechanism of diabetic complications.

The pancreas: sensor and controller at once

The pancreas contains cell clusters, the islets of Langerhans, which continuously measure blood glucose and secrete the appropriate hormone.

   blood glucose   MEASURED by the islets of Langerhans
        |
        +---- too HIGH  ->  β cells  ->  INSULIN
        |
        +---- too LOW   ->  α cells  ->  GLUCAGON

One gland thus produces both antagonistic hormones. It is sensor and decision centre at once — hence its speed of response.

Insulin: bringing it down

It is the only glucose-lowering hormone in the body.

   INSULIN  ->  acts on:

   MUSCLE and FAT TISSUE  ->  glucose entry into the cells
   LIVER                  ->  storage as GLYCOGEN
                          ->  halting glucose production

The crucial point: without insulin, glucose cannot enter most cells. It stays in the blood, plentiful, while the cells lack fuel. This is the paradox of untreated diabetes — a body starving amid sugar.

Glucagon: bringing it up

   GLUCAGON  ->  acts mainly on the LIVER

   breakdown of glycogen into glucose       (immediate reserve)
   manufacture of glucose from other molecules

The liver is the real regulating organ: it stores about 100 grams of glycogen, mobilisable within minutes. This reserve covers roughly a day of fasting, after which other mechanisms take over.

After a meal: the full loop

   meal -> glucose absorption -> blood glucose RISES to 1.4 g/L
        |
        v
   the β cells detect it  ->  INSULIN secretion
        |
        v
   glucose entry into cells + liver storage
        |
        v
   blood glucose FALLS back towards 1 g/L
        |
        v
   the β cells detect it  ->  insulin secretion STOPS

This is a complete negative feedback: the correction stops itself once it has had its effect.

Summary

• Blood glucose must stay near 1 g/L; the brain depends on it continuously.
• The pancreas is both sensor and decision centre.
• Insulin (β cells) lowers it: cell entry, liver storage.
• It is the only glucose-lowering hormone.
• Glucagon (α cells) raises it: breakdown of liver glycogen.
• The liver stores about 100 g of glycogen, roughly a day's reserve.