Homeostasis and Feedback-System Components

Revision notes

Maintaining a workable internal environment

Homeostasis maintains internal conditions within suitable ranges despite internal and external changes. It is dynamic regulation, not perfectly constant values. Controlled variables include blood glucose, fluid concentration, blood pressure and blood-gas conditions. Appropriate ranges support enzyme function, membrane processes and organ performance.

An optimum is a condition at which a process works most effectively under the circumstances. A regulated variable may fluctuate around a reference level; some reference levels change with time, activity or physiological state. Avoid claiming that every variable has one unchanging numerical set point.

Components of a control loop

A stimulus is a change in a variable. Receptors or sensors detect it. A control centre receives and integrates information, compares it with the appropriate range and coordinates a response. Effectors, such as muscles, glands or transport epithelia, produce changes that alter the variable. Feedback occurs when the result influences the original control process.

In negative feedback, the response opposes the initiating deviation. For example, a rise in plasma osmolality can stimulate responses that conserve water. The response reduces the deviation, reducing the original stimulus. Negative does not mean harmful; it describes the direction of the feedback.

Positive feedback amplifies an initiating change until an external limiting event stops the process. It is useful in specific events such as childbirth or clotting, but is not the usual mechanism for keeping a variable near a target range.

Nervous and endocrine communication

Nervous signalling can provide rapid, targeted changes through impulses and synapses. Endocrine signalling uses hormones carried in blood to cells with suitable receptors. Both systems can act together; neither is exclusively responsible for every homeostatic process. The hypothalamus is an important point of integration.

Interpret a response graph

After a disturbance, negative feedback should move the variable towards its previous range, perhaps with delay or overshoot. A flat graph is not required. Repeated measurements reveal trends more reliably than one value. Distinguish a response variable, such as hormone concentration, from the controlled variable, such as blood glucose.

Worked application and data skills

A rise in plasma concentration is detected by hypothalamic osmoreceptors. Increased ADH release promotes water reabsorption by the kidneys, reducing water loss and helping restore concentration. Sensor: osmoreceptors. Integrating region: hypothalamus. Signal: ADH. Effector tissue: kidney collecting ducts. The concentration change, not ADH itself, is the controlled variable in this example.

Common misconceptions

Homeostasis allows controlled fluctuations. Negative feedback opposes a deviation, not necessarily lowers the variable. A hormone signal and the regulated variable are different.

Examination tips

Identify stimulus, sensor, control centre, effector, response and feedback in a named example.

Check your understanding

Five original questions. Use Tab to move between controls, arrow keys within a radio group, and Enter or Space on buttons. Feedback appears after Check Answer. Try Again clears the question. Attempts are not saved.

Sources and scope

Scope: Unit 1 C3.1. Original BTECRevision notes, schematics and questions. Independent resource; not endorsed by Pearson.

Official Pearson specification. Scientific review references are listed in the accompanying QA report.