Homeostasis and negative feedback

As you read this, your blood contains about 1 gram of glucose per litre and stays at 37 °C. These values barely vary, whatever you do — this is homeostasis.

The internal environment

Cells are not bathed in the outside world but in a buffer: blood and lymph, whose composition is held steady.

   temperature     37 °C          (tolerated variation: ± 1 °C)
   blood glucose   ~ 1 g/L        (0.7 to 1.1 g/L fasting)
   blood pH        7.40           (7.38 to 7.42: a tiny margin)
   blood calcium   ~ 100 mg/L

pH is the most telling: a margin of 0.04 units either way. Below 7.0 or above 7.8, life becomes impossible within minutes — enzymes, whose shape depends on pH, stop working.

Claude Bernard stated the principle as early as 1865: the constancy of the internal environment is the condition of free life. It is what lets a mammal stay active at -20 °C as at +40 °C, where a lizard must wait for the sun.

The regulation loop

Every biological regulation follows the same scheme, identical to a thermostat's:

   DISTURBANCE
        |
        v
   SENSOR          detects the gap from the set point
        |
        v
   CONTROLLER      compares and decides
        |
        v
   EFFECTOR        acts to correct
        |
        v
   RETURN to the normal value
        |
        +--------> the sensor detects the return  ->  the action STOPS

That last arrow is the essential one: the result of the action feeds back on its own cause. This is negative feedback.

Negative feedback

   variable TOO HIGH  ->  action that LOWERS it   ->  return to normal
   variable TOO LOW   ->  action that RAISES it   ->  return to normal

   value
      |     ___         ___
      |    /   \___    /   \___          oscillation around the set point
   ---+---/--------\--/--------\-----  <- set point
      |  /          \/          \
      +----------------------------> time

The system never holds a value exactly constant: it oscillates around it. That is the nature of feedback regulation, exactly like a radiator switching on and off around the set temperature.

The exception: positive feedback

Far rarer, it amplifies instead of correcting:

   childbirth: contractions -> oxytocin release
               -> stronger contractions -> more oxytocin -> ...

Such a runaway cannot last: an external event must stop it — here, the birth. This is why positive feedback is reserved for processes that must go all the way: childbirth, clotting, the action potential.

Summary

• Homeostasis keeps the constants of the internal environment stable.
• Benchmarks: 37 °C, ~1 g/L glucose, pH 7.40 — with very narrow margins.
• Every regulation follows sensor → controller → effector.
• Negative feedback brings the variable back to its set point.
• The value oscillates around the set point; it is never strictly fixed.
• Positive feedback, rare, amplifies: childbirth, clotting.