Explain how feedback mechanisms maintain homeostasis (a stable internal environment) in organisms, using examples such as temperature, glucose and water regulation (NYSSLS LS1, stability and change; systems and system models).

What this topic is asking

NYSSLS LS1 asks you to explain how living systems stay stable. The key crosscutting concept is stability and change: conditions inside an organism are kept within a narrow range even when the outside changes. On the Life Science: Biology Regents, this almost always comes as a cluster with a graph (glucose, temperature, hormone levels) where you identify the response and explain the feedback loop.

Dynamic equilibrium

Cells work best within narrow ranges of temperature, pH, water and nutrient concentration. Enzymes, for example, denature if it gets too hot or the pH shifts too far (see enzymes and metabolism). Maintaining the internal environment keeps these processes running.

Negative feedback

Most homeostatic control uses negative feedback. The loop has three parts:

1. a sensor (receptor) detects a change in a variable away from the set point;
2. a control center (often the brain or an endocrine gland) processes the signal;
3. an effector carries out a response that opposes the change, returning the variable toward the set point.

Because the response counteracts the original change, the system self-corrects. The word "negative" means the response is in the opposite direction to the disturbance.

Worked examples

Temperature

If body temperature rises, the body sweats (evaporation removes heat) and skin blood vessels widen (heat is lost); if it falls, the body shivers (muscle activity generates heat) and skin vessels narrow (heat is conserved). Both responses push temperature back toward about 37 degrees Celsius.

Blood glucose

After a meal, blood glucose rises. The pancreas releases insulin, which makes cells take up and store glucose, lowering it. Between meals, glucose falls and the pancreas releases glucagon, which makes the liver release stored glucose, raising it. The two hormones work in opposition to hold glucose steady. This is detailed in the nervous and endocrine systems.

Water balance

When the body is short of water, hormones make the kidneys reabsorb more water, producing less, more concentrated urine; when there is excess water, more dilute urine is produced. This keeps the body fluids at a stable concentration.

When homeostasis fails

If a feedback system breaks down, the internal environment moves out of its safe range and disease follows. In diabetes, for example, insulin is missing or ineffective, so blood glucose stays dangerously high. The exam may give data showing a variable that does not return to normal and ask you to explain that homeostasis has failed.

Try this

Q1. Define homeostasis. [2]

• Cue. The maintenance of a stable (relatively constant) internal environment in an organism despite changes in the external environment.

Q2. Explain why negative feedback is described as self-correcting. [2]

• Cue. The response produced is in the opposite direction to the change detected, so it reverses the disturbance and returns the variable toward its set point without outside intervention.