Homeostasis, Set Points and Feedback Loops

The active defense of the internal environment within narrow limits despite a fluctuating external world — named by Walter Cannon, foreshadowed by Claude Bernard's 'fixité du milieu intérieur'.

1) Sensor/receptor that measures the variable, 2) Integrator/control center that compares to a set point and generates an error signal, 3) Effector that changes the variable, 4) Feedback that alters what the sensor reads, closing the loop.

A loop in which the response opposes the disturbance, returning the variable toward its set point (e.g., baroreflex correcting a fall in blood pressure on standing).

Because the correcting signal is driven by the residual error — if the error reached zero, the correction would also fall to zero. Some error must remain to sustain the correction.

Gain = Correction ÷ Remaining error (written as a negative number by convention for negative feedback).

Correction = 40 mmHg, remaining error = 10 mmHg, so gain = 40/10 = 4 (i.e., −4).

High gain plus a delay in the loop — the system overcorrects before it 'sees' the result of its correction.

Feedback oscillation: normal chemoreceptor feedback combined with prolonged lung-to-brain circulation time in heart failure produces crescendo–decrescendo breathing.

Venous return ↓ → cardiac output and MAP ↓ → carotid baroreceptors fire less → medullary cardiovascular center withdraws vagal tone and increases sympathetic outflow → HR, contractility and vasoconstriction ↑ → pressure restored.

Pyrogens (IL-1, PGE₂ acting on the preoptic area) raise the thermal set point (e.g., to 39 °C). The loop works perfectly — it is simply defending a new target.