Homeostasis and Blood Glucose Control
Homeostasis is the active regulation of an animal's internal environment, such as temperature and blood sugar, to maintain stable conditions necessary for enzyme function and cell survival. This topic covers the negative feedback mechanisms that control these vital internal states.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Homeostasis is the maintenance of a constant internal environment, crucial for optimal enzyme activity and metabolic processes in multicellular organisms.
- Negative feedback is the core control mechanism. A change from a set point triggers a response that counteracts the original change, restoring balance.
- Blood glucose is regulated by two pancreatic hormones: insulin lowers high blood glucose by promoting its uptake into cells and conversion to glycogen in the liver; glucagon raises low blood glucose by stimulating the liver to convert stored glycogen back into glucose.
- Type 1 diabetes is an autoimmune condition where the pancreas produces little or no insulin, treated with insulin injections. Type 2 diabetes occurs when body cells become resistant to insulin's effects, often linked to lifestyle factors.
- Water balance is regulated by Antidiuretic Hormone (ADH). High ADH, released by the pituitary gland when the blood is too concentrated, increases water reabsorption in the kidneys, producing concentrated urine.
- Body temperature is regulated by the brain. To cool down, the body uses vasodilation (widening of skin arterioles) and sweating. To warm up, it uses vasoconstriction (narrowing of skin arterioles) and shivering.
Definitions
- Homeostasis
- The regulation of conditions inside the body and cells to maintain a stable, constant internal environment in response to both internal and external changes.
- Negative Feedback
- A control mechanism where a stimulus causes a response that opposes or counteracts the original stimulus, bringing the system back towards a set point.
- Insulin
- A hormone produced by the pancreas that lowers blood glucose concentration by increasing glucose uptake by cells and its conversion to glycogen in the liver and muscles.
- Glucagon
- A hormone produced by the pancreas that raises blood glucose concentration by stimulating the breakdown of glycogen into glucose in the liver.
- ADH (Antidiuretic Hormone)
- A hormone released from the pituitary gland that controls the water content of the blood by increasing the permeability of the kidney collecting ducts to water.
Worked example
A person is lost in a hot desert for a day with no food or water. Which row correctly describes the likely state of their hormone levels upon being rescued?
- 1
Step 1:
Analyse the effect of 'no food'.
Lack of food intake, especially carbohydrates, for a prolonged period will cause blood glucose levels to fall.
- 2
Step 2:
Determine the hormonal response to low blood glucose.
To counteract falling blood sugar, the pancreas releases glucagon.
Glucagon stimulates the liver to convert its glycogen stores into glucose and release it into the blood.
Insulin levels would be very low.
- 3
Step 3:
Analyse the effect of 'hot desert' and 'no water'.
Sweating in the heat combined with no water intake leads to dehydration.
This makes the blood plasma more concentrated (higher water potential gradient).
- 4
Step 4:
Determine the hormonal response to dehydration.
The hypothalamus in the brain detects the concentrated blood and stimulates the pituitary gland to release high levels of Antidiuretic Hormone (ADH).
- 5
Step 5:
Conclude the overall hormonal state.
The person will have high levels of glucagon to manage blood sugar and high levels of ADH to conserve water.
Insulin levels will be low.
Answer: High glucagon, low insulin, high ADH.
Common mistakes
- ×Confusing the roles of insulin and glucagon. Remember: INsulin puts glucose INto the cells; glucaGON makes glucose 'gone' from the liver.
- ×Mixing up Type 1 and Type 2 diabetes. Type 1 is an insulin production problem (autoimmune). Type 2 is an insulin sensitivity problem (resistance).
- ×Failing to fully explain a negative feedback loop. A complete answer must state the initial change, the detection, the corrective response, and how that response brings the level back to normal, thus switching off the response.
- ×Confusing vasodilation with vasoconstriction. Vasodilation is for cooling (more blood to skin surface radiates heat away). Vasoconstriction is for warming (less blood to skin surface conserves heat).
No-calculator tips
- ✓Think in terms of 'cause and effect' chains for any homeostasis question. For example: High blood sugar → Pancreas detects → Insulin released → Liver takes up glucose → Blood sugar falls.
- ✓Use hormone names to remember their function. 'Anti-diuretic' means 'against urination'. Therefore, high ADH leads to less urine, meaning more water is retained by the body.
- ✓For scenarios involving multiple factors (e.g., exercise and eating), consider the effect of each factor on the internal environment separately before combining them to find the net result.