When Balance Breaks Down

Homeostatic systems usually correct small disturbances before you notice them. Illness can develop when regulation is impaired, when a disturbance exceeds available capacity or when a shifted or prolonged response becomes harmful. These patterns can overlap and change as an illness progresses.

There is no single way in which balance breaks down, but three patterns provide a useful starting map. First, part of the regulatory system may be impaired. Second, the regulatory responses may initially work but be outpaced by the size or duration of the disturbance. Third, the regulated target or compensatory response may change. The response may be useful at first but harmful if it persists. These are broad patterns rather than separate boxes. More than one may occur during the same illness. Regulation is impaired Sometimes the body loses part of its regulatory capacity. In type 1 diabetes, the immune system attacks and destroys insulin producing beta cells in the pancreas. Once this function is lost, the body makes little or no insulin. Blood glucose can then remain high because the normal insulin response is no longer available. Daily insulin treatment replaces the missing hormone, although it does not reverse the autoimmune process. Demand exceeds capacity Sometimes the regulatory responses are active but cannot keep pace with the disturbance. During severe heat exposure, sweating and increased blood flow to the skin may not remove heat quickly enough. During prolonged vomiting, diarrhoea or heavy sweating, fluid loss may exceed drinking, thirst responses and the kidneys' ability to conserve water and salt. An exceeded system may later become damaged. Severe dehydration can reduce blood flow to the kidneys and cause acute kidney injury. The pattern can therefore change as illness progresses. The target or compensation shifts Sometimes the regulated target or compensatory response changes. During fever, immune and inflammatory signals raise the temperature set point in the hypothalamus. Temperature regulation continues around that higher target. This is different from uncontrolled overheating. Compensation may also become maladaptive. For example, narrowing of a renal artery can reduce blood flow to a kidney. The kidney then activates a hormone pathway that narrows blood vessels and retains salt and water. This response may support local perfusion but raise blood pressure across the body. This renal mechanism is one possible cause of hypertension, not the explanation for every person with high blood pressure. Why this reframes illness These patterns help explain why the same measured abnormality can have different causes. High blood glucose may result from little or no insulin, reduced response to insulin, another illness or a medicine. Dehydration may result from vomiting, diarrhoea, inadequate intake or excessive urinary loss. High blood pressure may reflect several interacting long term influences or a specific secondary cause. Clinicians therefore consider the cause, time course, severity, compensatory responses and evidence of organ injury. The homeostasis model supports that reasoning, but it does not establish the diagnosis by itself.

Homeostatic problems may arise when regulation is impaired, demand exceeds available capacity or a shifted or prolonged response becomes maladaptive. One pattern may develop into another as illness progresses.