Homeostasis
Homeostasis is the maintenance of a relatively stable internal environment despite changes occurring within or outside the body. Physiological variables are not completely constant; they fluctuate within narrow limits that support normal cellular function.
The Internal Environment
The internal environment refers mainly to the extracellular fluid surrounding the cells. It includes plasma within blood vessels and interstitial fluid between cells. This fluid supplies oxygen, nutrients, hormones and electrolytes while receiving carbon dioxide and metabolic wastes.
Regulated Physiological Variables
Components of a Control System
The complete sequence is: Stimulus → Receptor → Control centre → Effector → Response.
Set Point and Normal Range
The set point is the desired value around which a variable is regulated. Most variables are maintained within a normal range rather than at one exact value. Body temperature, for example, varies with activity and time of day while remaining within safe limits.
Feedback Mechanisms
Negative feedback
The response opposes the original change and moves the variable towards normal. Examples include regulation of temperature, blood glucose, blood pressure and plasma osmolarity.
Positive feedback
The response amplifies the original stimulus until an event is completed. Examples include childbirth, blood clotting and the action-potential upstroke.
When body temperature rises, thermoreceptors detect the increase, the hypothalamus integrates the information, and sweating with cutaneous vasodilatation increases heat loss. This is a classic negative-feedback response.
Feed-Forward Control
Feed-forward control anticipates a change and initiates a response before the regulated variable is significantly disturbed. Examples include salivation at the sight of food and increases in heart rate and ventilation at the beginning of exercise.
Gain of a Control System
Gain describes how effectively a control system corrects a disturbance: Gain = Correction ÷ Remaining error. A high-gain system produces a strong correction and leaves only a small error.
Diabetes mellitus Normal regulation of blood glucose is impaired.
Shock Blood pressure and tissue perfusion cannot be maintained adequately.
Fever The hypothalamic temperature set point is raised.
Severe imbalance Disturbed pH, electrolytes or osmolarity may become life-threatening.
Identify the regulated variable → receptor → control centre → effector → response.
Negative feedback reverses; positive feedback amplifies; feed-forward control anticipates.