Disruption of homeostasis: WACE Year 12 Human Biology Unit 3
“Explain how disease, malfunction of feedback systems and environmental factors disrupt homeostasis, using named examples such as diabetes”
Homeostasis is disrupted when a feedback system fails or is overwhelmed by pathogens, genetic faults, lifestyle or environment, so a regulated variable moves outside its tolerance limits and cells are damaged; diabetes mellitus, where blood glucose control fails, is the key WACE example.
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.
Tolerance limits are the upper and lower values of a regulated variable within which cells continue to function normally. Inside the limits, homeostasis returns the variable toward the set point. Beyond the limits, cells are damaged and disease follows.
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.
Tracing the feedback failure
After a meal, blood glucose rises above the set point (stimulus). Normally beta cells in the islets of Langerhans would detect this and secrete insulin, which makes liver and muscle cells take up glucose and store it as glycogen, lowering blood glucose. In type 1 diabetes the immune system has destroyed the beta cells, so no insulin is released. The effector step of the negative feedback loop is missing, the corrective response does not occur, and blood glucose remains above the tolerance limit. Injected insulin replaces the missing hormone and restores the loop.
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.
Disease disrupts homeostasis when a regulated variable moves outside its tolerance limits and cannot be corrected. Identify the variable (such as blood glucose), the failed step (receptor, control centre, effector or target cell), and the consequence for cells.
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.
A frequent error is treating both types of diabetes the same. In type 1 the beta cells are destroyed so little or no insulin is produced; in type 2 insulin is still made but the target cells resist it. State the failed step precisely. Also avoid vague answers like "the body cannot control sugar"; name the hormone (insulin), the gland (pancreas, islets of Langerhans) and the tolerance limit that is exceeded.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style6 marksType 1 and type 2 diabetes mellitus both disrupt the regulation of blood glucose. Compare the causes of the two types and explain why a person with untreated type 1 diabetes shows glucose in the urine and high blood glucose after a meal.Show worked answer →
A 6 mark response needs the cause of each type, the contrast, and the link to the symptoms.
- Type 1
- The beta cells of the islets of Langerhans are destroyed, usually by an autoimmune response, so little or no insulin is produced. It often appears in childhood.
- Type 2
- Insulin is produced but body cells become resistant to it (reduced response), so glucose is not taken up effectively. It is associated with older age, body weight and lifestyle and develops gradually.
- Why glucose appears in urine
- Without effective insulin, the liver and body cells do not take up and store glucose after a meal, so blood glucose stays very high. When it exceeds the kidney threshold, not all glucose is reabsorbed in the nephron, so glucose passes into the urine.
Markers reward the destroyed-cells versus resistance contrast and the link from high blood glucose to exceeding the kidney threshold.
WACE 2023-style3 marksExplain why a failure of negative feedback can move a variable outside its tolerance limits, using one named example.Show worked answer →
A 3 mark explain answer needs the role of feedback plus a worked example.
- Role of feedback
- Negative feedback normally detects a change and triggers a response that reverses it, keeping the variable inside tolerance limits.
- When it fails
- If the receptor, control centre, hormone or effector fails, the correcting response does not occur, so the variable keeps moving away from the set point.
- Example
- In untreated type 1 diabetes, the absence of insulin means blood glucose is not lowered after a meal, so it rises beyond tolerance limits and can damage tissues.
Markers reward the link between a broken loop component and the variable leaving its tolerance range.