HomeostasisCambridge IGCSE Biology: Revision notes
Section 1
What is homeostasis and why is it important?
Homeostasis is the maintenance of a constant internal environment within the body, despite changes in external conditions. This involves keeping conditions such as blood glucose concentration, body temperature, and water potential relatively stable.
Homeostasis is essential because:
- Enzymes work optimally within narrow temperature and pH ranges
- Cells require a stable osmotic environment to function
- Blood glucose must be kept within a narrow range to provide consistent energy
- Body temperature affects metabolic rates and enzyme activity
All homeostatic systems operate through negative feedback, which counteracts any change away from the set point and restores the system to its original state.
Examiners always ask you to 'explain' negative feedback with reference to the set point. You must state: detection of change → response triggered → change reversed → return to set point.
Think of a room thermostat: if temperature drops below the set point, heating switches on; once the set point is reached, heating switches off. This is negative feedback.
Section 2
How is blood glucose concentration controlled?
Blood glucose concentration is regulated by the liver and two hormones: insulin and glucagon.
Normal blood glucose level: approximately 90 mg/100 cm³
When blood glucose is too high:
- The pancreas (beta cells) detects high glucose
- Insulin is secreted into the blood
- Insulin causes:
- Glucose uptake by cells (especially muscles and fat cells)
- Conversion of glucose to glycogen in the liver
- Conversion of glucose to fat for storage
- Blood glucose concentration decreases back to normal
When blood glucose is too low:
- The pancreas (alpha cells) detects low glucose
- Glucagon is secreted into the blood
- Glucagon causes:
- Breakdown of glycogen (glycogenolysis) in the liver
- Conversion of other substances to glucose (gluconeogenesis)
- Blood glucose concentration increases back to normal
The liver's role:
- Stores glucose as glycogen when blood glucose is high
- Releases glucose when blood glucose is low
- Acts as the main organ for maintaining blood glucose homeostasis
| Hormone | Secreted by | Stimulus | Effect on blood glucose |
|---|---|---|---|
| Insulin | Pancreas (beta cells) | High blood glucose | Decreases |
| Glucagon | Pancreas (alpha cells) | Low blood glucose | Increases |
Students often say insulin 'removes' glucose or makes it disappear. Instead, state that insulin 'increases uptake by cells' or 'increases storage as glycogen'—the glucose is still there, just moved into cells or stored.
After a meal: blood glucose rises → pancreas detects this → beta cells release insulin → insulin allows muscle cells to take up glucose → glucose is used for respiration or stored as glycogen → blood glucose returns to 90 mg/100 cm³.
Section 3
What is Type 1 diabetes and how is it treated?
Type 1 diabetes is a condition in which the pancreas cannot produce sufficient insulin (or any insulin at all) due to damage to or destruction of beta cells. This means blood glucose cannot be controlled effectively and becomes dangerously high.
Symptoms:
- High blood glucose that cannot be lowered
- Glucose appears in urine
- Increased thirst and frequent urination
- Fatigue and weight loss
Treatment of Type 1 diabetes:
-
Insulin injections (or insulin pump): Regular insulin is administered to:
- Allow glucose uptake by cells
- Restore blood glucose control
- Prevent the complications of high blood glucose
-
Dietary management:
- Avoid high-sugar foods
- Monitor carbohydrate intake
- Maintain consistent meal timing to match insulin doses
-
Regular monitoring:
- Blood glucose testing
- HbA1c tests (measures average glucose over weeks/months)
Note: Type 1 diabetes is an autoimmune condition and cannot be prevented; Type 2 (associated with obesity and age) can sometimes be managed through diet and lifestyle alone.
Exam questions often ask you to 'outline' treatment—provide a brief description of the main methods (insulin injections, diet, monitoring) without excessive detail.
Section 4
Which skin structures are involved in temperature regulation?
The skin contains several structures that work together to maintain a constant body temperature:
Structural components:
- Hairs: Trap insulating air close to the skin surface; reduce heat loss
- Hair erector muscles (arrector pili): Contract in response to cold; cause hairs to stand upright, trapping more air
- Sweat glands: Produce sweat that evaporates from the skin surface, cooling the body
- Receptors (temperature receptors): Detect changes in skin temperature and send signals to the brain
- Sensory neurones: Transmit temperature information from skin receptors to the brain
- Blood vessels (arterioles): Control heat loss through vasodilation (widening) and vasoconstriction (narrowing)
- Fatty tissue (subcutaneous fat/adipose tissue): Acts as insulation to reduce heat loss
Location and function summary:
- Receptors and sensory neurones detect temperature changes
- Blood vessels and sweat glands respond to control heat loss
- Hair, muscles, and fatty tissue provide insulation
- All structures are coordinated by the hypothalamus in the brain
When describing skin structures, always link them to their function in temperature regulation. Examiners want to see that you understand how each structure contributes to homeostasis.
Section 5
How does the body maintain constant temperature through physiological responses?
Body temperature is maintained around 37°C through a combination of physiological and behavioural responses, coordinated by the hypothalamus in the brain.
When body temperature is too high (above set point):
- Temperature receptors in skin and hypothalamus detect the increase
- The hypothalamus triggers:
- Vasodilation: Arterioles supplying skin capillaries widen → more blood flows to the skin surface → more heat is radiated and lost to the environment
- Increased sweating: Sweat glands produce more sweat → evaporation cools the skin
- Heat loss increases → body temperature returns to 37°C
When body temperature is too low (below set point):
- Temperature receptors in skin and hypothalamus detect the decrease
- The hypothalamus triggers:
- Vasoconstriction: Arterioles supplying skin capillaries narrow → less blood flows to the skin surface → less heat is radiated
- Shivering: Muscles contract involuntarily, generating heat through respiration
- Hair erection: Hair erector muscles contract, trapping insulating air close to the skin
- Heat loss is reduced and heat production increases → body temperature returns to 37°C
Role of the brain:
- The hypothalamus acts as the body's thermostat
- It detects changes in core body temperature
- It coordinates responses through the nervous system
- It maintains the set point at approximately 37°C
| Response | Stimulus | Mechanism | Effect |
|---|---|---|---|
| Vasodilation | High temperature | Arterioles widen | ↑ Heat loss |
| Vasoconstriction | Low temperature | Arterioles narrow | ↓ Heat loss |
| Sweating | High temperature | Sweat glands active | ↑ Heat loss (evaporation) |
| Shivering | Low temperature | Muscle contractions | ↑ Heat production |
| Hair erection | Low temperature | Erector muscles contract | ↑ Insulation |
The hypothalamus is like a home thermostat: it detects temperature deviation from the set point and automatically triggers heating (when cold) or cooling (when hot) to restore the set point.
Students often say sweat 'evaporates heat' or sweating 'removes heat directly.' In fact, sweat absorbs heat energy during evaporation, cooling the body—the evaporation process requires energy from the skin.
Must Know
- Homeostasis is the maintenance of a constant internal environment; all homeostatic control uses negative feedback to detect a change, trigger a response, and return the system to the set point.
- Insulin (from pancreas beta cells) decreases blood glucose by increasing cellular uptake and glycogen storage; glucagon (from alpha cells) increases blood glucose by promoting glycogen breakdown and gluconeogenesis.
- The liver stores glucose as glycogen when blood glucose is high and releases glucose when it is low; blood glucose set point is approximately 90 mg/100 cm³.
- Type 1 diabetes results from insufficient insulin production; treatment involves insulin injections, dietary management, and regular blood glucose monitoring.
- Skin structures involved in temperature regulation: hairs (insulation), hair erector muscles (trap air), sweat glands (cooling), temperature receptors (detection), sensory neurones (signal transmission), arterioles (vasodilation/vasoconstriction), and fatty tissue (insulation).
- Body temperature is maintained at 37°C by the hypothalamus; when too hot, vasodilation and sweating increase heat loss; when too cold, vasoconstriction, shivering, and hair erection decrease heat loss and increase heat production.
That's the notes covered.
Carry on to the next subtopic.