Homeostasis

The principle: negative feedback

A homeostatic control system has three components:

The key word is negative: the effector's response is in the opposite direction to the deviation. A rise in body temperature triggers cooling mechanisms; a fall triggers warming. This oppositional response is what makes the system stable.

Negative feedback contrasts with positive feedback, which amplifies a change. Positive feedback is useful in controlled situations — the release of oxytocin during childbirth, or the opening of ion channels during an action potential — but cannot maintain a stable variable and is always terminated by an external event.

The four key regulated variables

Blood pH regulation links directly to acid-base chemistry. CO₂ produced by cellular respiration dissolves in blood to form carbonic acid (CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺). Rising CO₂ lowers pH; the lungs compensate by increasing ventilation rate to exhale more CO₂. The kidneys regulate HCO₃⁻ excretion on a longer timescale. See our acids and bases guide for the acid-base equilibrium chemistry.

Blood glucose regulation in detail

Blood glucose is maintained between approximately 4 and 7 mmol/L by two antagonistic hormones produced by the islets of Langerhans in the pancreas:

• Insulin (from beta cells) — secreted when blood glucose is high. Actions: (1) increases glucose uptake by liver, muscle and adipose cells via GLUT4 transporter insertion; (2) promotes glycogenesis (glucose → glycogen) in the liver; (3) promotes lipogenesis in adipose tissue; (4) inhibits gluconeogenesis.
• Glucagon (from alpha cells) — secreted when blood glucose is low. Actions: (1) promotes glycogenolysis (glycogen → glucose) in the liver; (2) promotes gluconeogenesis from amino acids and lactate; (3) promotes lipolysis in adipose tissue.

Insulin and glucagon are enzyme-linked signals. Insulin binding to its receptor activates a protein kinase cascade — a chain of enzyme activations that ultimately moves GLUT4 glucose transporters to the cell surface. See our enzymes guide for how enzyme-controlled signalling cascades work.

Water homeostasis (osmoregulation) involves a related negative feedback loop. Osmoreceptors in the hypothalamus detect rising plasma osmolarity (dehydration). The posterior pituitary releases ADH (antidiuretic hormone), which increases water reabsorption in the kidney's collecting duct. More water reabsorbed → osmolarity falls → ADH release decreases. For the membrane transport mechanisms involved, see our osmosis glossary entry.

When homeostasis fails

Homeostatic failure underlies many common diseases:

• Diabetes mellitus — in type 1, autoimmune destruction of beta cells eliminates insulin production. In type 2, insulin resistance in target cells means insulin's signal is insufficient to control blood glucose. Both result in chronic hyperglycaemia and its associated tissue damage.
• Fever — infection raises the hypothalamic set point (via prostaglandin E2 acting on the hypothalamus). This is a controlled disruption of thermoregulation, not a failure — pathogens are less effective at elevated temperatures, and immune cells work faster. Antipyretics (aspirin, paracetamol) lower the set point back to normal.
• Acidosis and alkalosis — blood pH outside 7.35–7.45 disrupts enzyme function (see our enzymes guide) and can be rapidly fatal. Respiratory acidosis occurs when the lungs cannot remove CO₂; metabolic acidosis when H⁺ production exceeds the buffering capacity of the blood.

The link to protein structure is direct: enzymes denature at temperatures above their optimum and at extreme pH. All metabolic processes — including those described in our cellular respiration guide and protein synthesis guide — depend on the narrow stability range that homeostasis maintains.