HomeostasisIB MYP Biology: Revision notes
Section 1
Homeostasis and negative feedback
Homeostasis is the maintenance of a constant internal environment, such as body temperature and blood glucose concentration, even when conditions outside change.
It works by negative feedback:
- A change from the normal level is detected.
- A response is triggered.
- The response reverses the change and brings the level back to normal.
Because the response always opposes the change, the level stays within a narrow range.
Negative feedback = the response opposes the change. Write 'reverses the change and returns it to normal'.
Section 2
Controlling body temperature
Core body temperature is kept close to 37 °C.
When too hot:
- Sweating: sweat evaporates from the skin and takes heat energy with it, which cools the body.
- Vasodilation: blood vessels in the skin widen, so more blood flows through the skin and more heat is lost to the surroundings.
When too cold:
- Vasoconstriction: blood vessels in the skin narrow, so less blood flows through the skin and less heat is lost.
- Shivering: muscles contract rapidly, and the extra respiration releases heat.
- Sweating stops.
Blood vessels do not move up or down towards the skin. They widen (vasodilation) or narrow (vasoconstriction).
Section 3
Controlling blood glucose
Blood glucose must stay at a steady concentration. The pancreas monitors it and releases two hormones that have opposite effects.
- Glucose too high (after a meal): the pancreas releases insulin. Insulin makes the liver and muscles take up glucose and store it as glycogen, so blood glucose falls.
- Glucose too low (between meals or exercise): the pancreas releases glucagon. Glucagon makes the liver convert glycogen to glucose and release it into the blood, so blood glucose rises.
This is negative feedback: each hormone reverses the change.
Section 4
Diabetes
Diabetes is a condition in which blood glucose cannot be controlled properly.
- Type 1: the pancreas does not make enough insulin. It usually starts in childhood or youth. It is treated with insulin injections and by monitoring diet and blood glucose.
- Type 2: body cells stop responding properly to insulin. It is linked to obesity, a diet high in sugar and lack of exercise. It is often treated with a healthier diet, exercise and medication.
Without treatment, blood glucose stays too high, which damages the body.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Homeostasis
- A cyclist is racing up a mountain road in Colombia. At dawn the air is cold, but by midday it is hot and she is working hard. Throughout the race her core body temperature stays close to 37 °C.Explain how vasodilation helps to cool her at midday.2 marks
- After eating a large bowl of rice, a student's blood glucose concentration rises above its normal level. Several hours later, without any more food, it begins to fall below normal.Explain why the control of blood glucose is an example of negative feedback.2 marks
- A clinic compares the blood glucose concentration of a healthy person with that of a patient who has type 1 diabetes and has not yet been treated. Each person drinks a sugary drink, and their blood glucose is measured before the drink and again after one hour and after two hours. The healthy person has 5.0 mmol dm⁻³ before the drink, 7.5 mmol dm⁻³ after one hour and 5.2 mmol dm⁻³ after two hours. The patient has 9.0 mmol dm⁻³ before the drink, 15.0 mmol dm⁻³ after one hour and 14.5 mmol dm⁻³ after two hours.Describe three differences between the results for the healthy person and the patient.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).