Explain how disease, malfunction of feedback systems and environmental factors disrupt homeostasis, using named examples such as diabetes

What this dot point is asking

WACE wants you to connect the homeostasis model to disease. A disease is any condition that disrupts the normal functioning of the body. When a homeostatic control mechanism cannot keep a variable within its tolerance limits, the internal environment changes too much and cells stop working properly. Your job is to explain a cause, the variable affected, and the consequences.

Categories of disease

You should be able to classify disease, because the cause shapes the disruption.

• Infectious (communicable) disease is caused by pathogens (bacteria, viruses, fungi, protozoa) and can spread between people. It links directly to the immune system topic.
• Non-infectious (non-communicable) disease is not spread by pathogens. It includes genetic disorders, nutritional disorders, degenerative diseases, and lifestyle-related conditions.

Many diseases have several contributing factors. Type 2 diabetes, for example, has genetic, dietary and lifestyle components.

How feedback systems fail

A negative feedback loop can break at any stage of the stimulus-response pathway. The receptor may fail to detect the change; the control centre may misjudge the set point; the effector gland or organ may not produce enough hormone, or the target cells may not respond to it. When any link fails, the corrective response is too weak or absent, the variable drifts past its tolerance limits, and cells are harmed. If the variable stays outside the limits, the damage can become life threatening.

Diabetes mellitus: the central example

Diabetes mellitus is a failure of blood glucose homeostasis and is the example WACE expects you to explain in detail. Recall from the endocrine topic that insulin (from beta cells) lowers blood glucose and glucagon raises it.

Type 1 diabetes is an autoimmune condition. The body's own immune system destroys the insulin-producing beta cells in the pancreas, so little or no insulin is made. This is an effector failure: the gland cannot produce the hormone. It usually begins early in life and is managed with insulin injections.

Type 2 diabetes is when the body still makes insulin but the target cells become resistant to it (insulin resistance), so they no longer take up glucose efficiently. This is a target-cell failure. It is strongly linked to lifestyle and diet and usually develops later in life, though it is managed first through diet, exercise and sometimes medication.

In both types, blood glucose rises and stays above the tolerance limit (hyperglycaemia). Glucose appears in the urine and draws water with it, causing thirst, frequent urination and dehydration. Cells cannot take up enough glucose for energy, so the body breaks down fat and protein, causing weight loss and tiredness. Over the long term, high blood glucose damages blood vessels and nerves.

Environmental and lifestyle factors

Homeostasis can also be overwhelmed by the external environment. Extreme heat can exceed the cooling capacity of thermoregulation, causing hyperthermia; extreme cold can cause hypothermia. Poor diet, lack of exercise, smoking and excess alcohol increase the risk of conditions such as cardiovascular disease and type 2 diabetes. These factors do not break a single feedback step so much as push the system beyond what it can correct.

How this maps to the exam

Expect to be given a disorder and asked to explain which variable is disrupted, where the feedback loop fails, and the resulting symptoms. Diabetes is the most common context, so be ready to compare type 1 (no insulin produced) and type 2 (cells resist insulin). You may also be asked to classify a disease as infectious or non-infectious and to suggest management strategies.