When Balance Breaks Down

Homeostasis can fail in more than one way. A control mechanism may be damaged, a disturbance may overwhelm an intact response, or a useful compensatory response may become harmful if it persists. These patterns can overlap as an illness develops.

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. This is an autoimmune process. Auto means self, and here the immune system attacks the body's own cells. 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 damage process. High blood glucose can also arise through other mechanisms. In type 2 diabetes, the body often responds less effectively to insulin and insulin production may become insufficient. Demand exceeds capacity Sometimes the regulatory responses are active but cannot keep pace with the disturbance. During severe heat exposure for instance, 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, meaning a sudden drop in kidney function. 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 a part of the brain called the hypothalamus. Temperature regulation continues around that higher target. This is different from hyperthermia, where body temperature rises without a raised set point. A compensatory response that helps at first but later contributes to harm is sometimes called a maladaptive response . If an artery supplying a kidney becomes narrow enough, less blood reaches that kidney. The kidney may interpret this as a sign that blood pressure is too low. It releases renin , a hormone that starts a pathway which narrows blood vessels around the body and makes the body retain more salt and water. If this continues, it can contribute to high blood pressure. This is one example of secondary hypertension , meaning high blood pressure caused by another medical problem. Most high blood pressure has other causes. These examples also show why the same abnormal measurement can have different causes. High blood glucose, dehydration or high blood pressure can each arise through more than one mechanism. Clinicians therefore consider what caused the change, how quickly it developed, how severe it is and whether organs are being affected.

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.