Homeostasis & Feedback

Homeostasis is the process by which the body maintains a stable internal environment despite changes in external conditions. It is essential for the optimal functioning of enzymes, ion channels, and metabolic pathways — all of which operate within narrow physiological ranges.

Every homeostatic control system has three components. The sensor (receptor) detects a deviation of a variable from its set point. The control centre (integrating centre) compares the sensory signal to the set point and determines the appropriate response. The effector carries out the corrective response. In biology, these three components are linked by two types of feedback loops.

Negative feedback is the dominant mechanism of homeostasis. When a variable deviates from the set point, the response acts to oppose the deviation and return the variable toward normal. Examples are pervasive: thermoregulation (rising core temperature → sweat, vasodilation → heat loss → temperature normalises); blood glucose regulation (rising glucose → insulin secretion → cellular glucose uptake → normalise glucose); blood pressure regulation (rising BP → baroreceptor activation → cardiac inhibition and vasodilation → BP falls); blood calcium regulation (falling Ca2+ → PTH secretion → increased osteoclast activity, renal reabsorption, and vitamin D activation → Ca2+ rises).

Positive feedback amplifies a response rather than correcting it. The effector output drives the variable further from the set point, creating a self-amplifying cycle. Positive feedback operates in situations requiring a rapid, decisive, and time-limited response. Parturition (childbirth) is the classic example: uterine contractions cause oxytocin release from the posterior pituitary; oxytocin intensifies contractions, causing more oxytocin release — the cycle escalates until birth terminates it. Blood coagulation is another example: platelet activation and fibrin deposition recruit further coagulation factors. The LH surge at ovulation is triggered by rising oestrogen — a case of positive feedback within an overall negative feedback system.

Thermoregulation illustrates the integration of multiple effector systems around a single set point (~37°C core temperature). The primary thermostat is the hypothalamic preoptic area. When core temperature rises above the set point, the hypothalamus activates sweat glands (evaporative cooling), cutaneous vasodilation (heat loss via radiation and convection), and behavioural responses (seeking cool environments). When temperature falls, vasoconstriction (reduces cutaneous heat loss), shivering (skeletal muscle thermogenesis), non-shivering thermogenesis (brown adipose tissue uncoupling via UCP1 — catecholamine-driven), and piloerection (limited in humans) are activated. Fever occurs when pyrogens (IL-1, IL-6, TNF, prostaglandin E2 from hypothalamic cyclo-oxygenase) raise the hypothalamic set point; the body then responds as if hypothermic — shivering and vasoconstriction — to achieve the new, higher set point.

Fluid balance homeostasis integrates renal, cardiovascular, and endocrine systems. Total body water (~60% of body weight) is partitioned into intracellular (two-thirds) and extracellular (one-third) compartments. The extracellular fluid (ECF) is further divided into interstitial fluid (~75%) and plasma (~25%). Plasma osmolality (~285–295 mOsm/kg) is tightly regulated by ADH (vasopressin): hyperosmolality detected by hypothalamic osmoreceptors stimulates ADH release → AQP2 insertion in renal collecting duct → water retention → osmolality normalises. Volume is regulated primarily by the RAAS and ANP, which adjust renal sodium handling. In severe volume depletion, baroreceptor-mediated ADH release overrides osmotic regulation, causing dilutional hyponatraemia.