Homeostasis and negative feedback

What homeostasis is, and why it matters

• Homeostasis is the maintenance of a stable internal environment within narrow limits, despite changes in the external environment or in the animal's own activity.

• "Stable" does not mean constant. Conditions fluctuate within a narrow range around a set point, and the system continually corrects them.

• The purpose is that cell processes only work within narrow physical and chemical limits. This is the reason the whole system exists, and stating it is required:

  • Enzymes have an optimum temperature and pH. Outside a narrow range, reaction rates fall, and beyond it the enzyme's tertiary structure is disrupted and it is denatured.
  • Cells require a particular water potential in the surrounding fluid. Too high, and water enters by osmosis and the cell swells or bursts; too low, and water leaves and the cell shrinks, so its contents become too concentrated for reactions to proceed normally.
  • Respiring tissue requires a continuous oxygen supply and carbon dioxide removal, because accumulating CO₂ lowers pH.
  • Cells need glucose as a respiratory substrate at a reliable concentration — the brain in particular cannot store it and depends on a continuous supply.

• The overall advantage: an animal maintaining a stable internal environment can remain active across a wide range of external conditions, rather than being restricted to conditions that happen to suit its cells.

The components of a control system

• Every homeostatic system has the same four components. Identifying them is the fastest route into any answer.

• The set point — the value the system maintains, such as approximately 37 °C in humans.

• Receptors (sensors) — detect the current value and any deviation from the set point.

• The control centre — compares the detected value with the set point and determines the response. For most systems this is in the hypothalamus or the medulla oblongata.

• Effectors — the muscles or glands that carry out the response, restoring the value toward the set point.

• The pathway is always:

$$\text{stimulus} \rightarrow \text{receptor} \rightarrow \text{control centre} \rightarrow \text{effector} \rightarrow \text{response}$$

• The loop is what makes it self-correcting. Because the response feeds back to oppose the original stimulus, the correction shuts itself off as the value returns to the set point — no separate "stop" signal is needed.

• The signal from control centre to effector is carried by the nervous system (fast, short-lived) or by hormones in the blood (slower, longer-lasting), and many systems use both.

Negative feedback

• Negative feedback is the mechanism of all five systems. The response opposes the original change, returning the value toward the set point.

• The word negative refers to the direction of the response, not to anything harmful.

• The cycle:

  • A factor deviates from the set point.
  • Receptors detect the deviation.
  • The control centre triggers a response.
  • Effectors produce a change in the opposite direction to the deviation.
  • The value returns toward the set point.
  • As it does, the stimulus weakens, so the response is reduced — this is what stops the system overcorrecting.

• Two consequences worth understanding:

  • The value oscillates slightly around the set point rather than sitting exactly on it, because correction can only begin after a deviation has been detected.
  • The system is self-limiting: the response switches itself off as the value is restored, because the stimulus driving it disappears.

• Most systems have opposing effectors — one set that raises the value and another that lowers it. This gives much finer control than a single mechanism, because the system can correct in either direction and can oppose an overshoot.

Positive feedback — and why it is not homeostatic

• In positive feedback, the response amplifies the original change rather than opposing it, driving the value further from its starting value.
• It is not homeostatic, because it does not restore stability. It is useful where a process needs to be driven rapidly to completion:
  • Blood clotting, where activated platelets attract more platelets, so a clot forms quickly.
  • Childbirth, where oxytocin increases contractions, which stimulates more oxytocin release.
• Positive feedback must terminate through some external event — the clot forming, or the birth occurring — because unlike negative feedback it contains no self-limiting mechanism.

Selective advantage

• The adaptive advantage of homeostasis is one of the three routes to Excellence, so it is worth being able to argue properly.
• It widens the range of conditions in which an animal can function. An animal maintaining a stable internal environment can stay active in conditions that would otherwise slow or stop its cell processes — so it can forage, escape predators and reproduce across a wider range of temperatures, water availability and activity levels.
• It allows a higher and more constant metabolic rate, because enzymes operate near their optimum continuously rather than only when external conditions happen to suit.
• It permits activity at times competitors cannot manage — at night, in cold conditions, or during sustained exertion.
• The cost is energy. Maintaining a set point requires continuous expenditure — heat production, ion pumping, hormone synthesis. Homeostasis is therefore favoured where the benefit of remaining active exceeds that cost, which is why endotherms require far more food than ectotherms of similar size.