Homeostasis

Core Concepts

Set points and normal ranges

A set point is the target value a system defends (for example, human core body temperature, commonly taught as about 37 °C — verify against current texts). The body does not hold a variable at one exact number; it keeps it within a normal range that brackets the set point, hovering near the target like a thermostat. "Dynamic equilibrium" describes this constant give-and-take.

The three components of a feedback loop

1. Receptor (sensor): detects a change in the variable and reports it — for temperature, thermosensitive neurons in the skin and brain.
2. Control center: compares the incoming information with the set point and decides what to do. The hypothalamus plays this role for many regulated variables.
3. Effector: carries out the response — muscles (shivering), glands (sweat glands, pancreas), or organs.

For any regulation scenario, label these three parts first, then decide whether the response opposes or amplifies the stimulus.

Negative feedback: the dominant mechanism

In negative feedback, the response opposes the original stimulus, restoring the set point — the word "negative" refers to this counteracting effect, not to "bad." Most homeostatic systems use it:

• Thermoregulation: if core temperature rises, sweat output increases and skin vessels dilate to release heat; if it falls, shivering and vasoconstriction conserve or generate heat.
• Blood glucose: rising glucose after a meal triggers insulin, which moves glucose into cells; falling glucose triggers glucagon, which raises it (covered in the endocrine chapters).
• Blood pressure: baroreceptors sense pressure changes and adjust heart rate and vessel diameter to restore pressure.

Positive feedback: amplification with a purpose

In positive feedback, the response amplifies the stimulus, pushing the variable further from its set point. This is safe only when short-lived and ended by a specific event:

• Childbirth: pressure of the baby's head on the cervix triggers oxytocin, which strengthens contractions, which push harder — until the baby is born.
• Blood clotting: an initial platelet plug releases chemicals that activate more platelets, rapidly building the clot at the injury site.
• Milk ejection: suckling stimulates oxytocin, which causes milk release and more suckling.

The pattern: positive feedback drives a process to completion rather than maintaining a steady state.

Thermoregulation in endotherms and ectotherms

Endotherms (birds and mammals) generate body heat metabolically and defend a fairly constant internal temperature; their feedback loops are fast but energetically expensive. Ectotherms (fish, reptiles, amphibians, most invertebrates) gain heat from the environment and regulate temperature mostly through behavior (basking, seeking shade) — the same principle, achieved very differently.

Osmoregulation and fluid balance

Animals also defend water and salt balance (osmoregulation). Marine fish tend to lose water and gain salt; freshwater fish face the opposite problem. In mammals, the kidneys adjust water and salt excretion, driven by hormones (antidiuretic hormone and aldosterone). The loop: osmoreceptors (sensors), hypothalamus (control center), kidneys and drinking behavior (effectors).

When homeostasis breaks down

Diabetes is a failure of blood-glucose feedback: in type 1 the pancreas produces little or no insulin; in type 2 cells respond poorly to insulin — either way glucose stays high. Fever is not a broken loop but a raised set point: pyrogens make the control center defend a higher temperature, which helps fight infection. Heatstroke occurs when thermoregulation is overwhelmed and core temperature climbs out of control.

Common Confusions