homeostasis

Homeostasis is the property of a living organism to maintain stable internal conditions despite external changes. It encompasses regulation of variables such as body temperature, pH, electrolyte balance, fluid volume, glucose concentration, and blood pressure. The goal is to keep these variables within narrow ranges necessary for cellular function.

Mechanisms of homeostatic regulation involve sensors that detect deviations, a control center that compares the current state to a set point, and effectors that restore balance. Most control relies on negative feedback: a deviation triggers responses that oppose the initial change. Examples include shivering or sweating to adjust body temperature, insulin release to lower high blood glucose, and vasoconstriction to raise blood pressure.

There are also feedforward adjustments and, in specific contexts, positive feedback, though the latter is less common for maintaining stability. The nervous and endocrine systems are central to regulation, with the hypothalamus acting as a major integrator for temperature, hydration, and energy balance, and various glands and organs serving as effectors.

Allostasis describes achieving stability through change, with set points that may adjust in anticipation of demands. Chronic stress or disease can lead to allostatic load, contributing to maladaptive states.

Timescales vary: rapid reflexes occur within seconds; hormonal changes may take minutes to hours; longer-term adjustments include acclimatization and remodeling of physiology. Systems involved include cardiovascular, endocrine, nervous, renal, and respiratory. Understanding homeostasis provides a framework for physiology and medicine, illustrating how organisms maintain internal consistency amid fluctuating environments.