Introduction to homeostasis: regulating internal conditions

Homeostasis and response • Homeostasis

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What is the immediate cause of change in urine concentration after drinking more water?

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Drinking more water reduces blood ADH levels, so the kidneys reabsorb less water and produce more dilute urine .

Key concepts

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Definition and scope of homeostasis

Homeostasis is the maintenance of a stable internal environment that supports enzyme activity and cell function. The term includes regulation at cell, tissue and whole-organism levels to keep variables near optimum values for health and function. The definition supplied in core resources states that homeostasis is the regulation of the internal environment of a cell or organism to maintain optimal conditions for function .

Components of a homeostatic system: receptor → control centre → effector

A receptor detects a specific change (stimulus) such as a rise in blood glucose or a fall in temperature. A control centre receives information from receptors, compares the value to a set point and initiates a response. An effector produces the response that brings the variable back toward the set point. This flow of information (receptor → control centre → effector) underlies reflexes and involuntary regulation in nervous and endocrine systems .

Negative feedback: cause → corrective effect

A change in a condition causes mechanisms that produce an opposite change to return the condition to normal. A rise in a variable triggers responses that reduce the variable; a fall triggers responses that increase it. Examples in core material show blood glucose and water regulation as negative feedback systems, where detection of deviation produces hormonal or organ responses that restore normal levels .

Key examples: blood glucose, water balance, body temperature

Blood glucose concentration changes cause hormone release from the pancreas: insulin lowers high glucose by converting it to glycogen; glucagon raises low glucose by converting glycogen back to glucose. Water content changes alter ADH release from the pituitary, which changes kidney reabsorption of water and urine concentration. Body temperature changes trigger vasodilation, vasoconstriction and sweating or shivering to alter heat loss and production. Each example follows detection → control → effect sequence and illustrates negative feedback .

Coordination methods and limiting factors

Nervous coordination uses fast electrical impulses for rapid responses; endocrine coordination uses hormones in the blood for slower, longer-lasting responses. Limitations include response speed (nervous faster than hormonal) and specificity (hormones travel systemically and affect target organs while nerve impulses act locally along defined pathways). Some homeostatic mechanisms fail under extreme conditions, leading to dysfunction such as hypothermia or hyperthermia when temperature regulation cannot keep up .

Key notes

Important points to keep in mind

Homeostasis maintains internal conditions near set points for optimal enzyme and cell function.

Homeostatic systems follow receptor → control centre → effector sequence.

Negative feedback produces corrective effects opposite to the initial change.

Blood glucose regulation uses insulin and glucagon released by the pancreas .

Water balance uses ADH from the pituitary to alter kidney reabsorption of water .

Temperature regulation uses vasodilation, vasoconstriction, sweating and shivering as cause → effect responses .

Nervous coordination provides rapid, short-lived responses; hormonal coordination provides slower, longer-lasting responses .

Homeostatic failure occurs when environmental changes exceed regulatory capacity (e.g., hypothermia, hyperthermia) .

Set points represent the normal ranges maintained by control centres.

All examples of homeostasis involve negative feedback to return conditions to normal .