Written for this site from the course textbook and class slides. Use the page numbers to read the book's fuller account. Image credits are on slide 18.
Homeostasis is maintaining the internal environment within preset limits, despite changes in the external environment. In humans, the controlled variables include body temperature, blood pH, blood glucose concentration and blood osmotic concentration.
Watch out: homeostasis is about the internal environment. It does not mean moving to somewhere more comfortable.
A receptor detects a change, a coordinator compares it with the set point, and an effector acts in the opposite direction. Negative feedback returns a variable to the set point from above and from below. Positive feedback would move it further away, which is why homeostasis does not use it.
Blood glucose is normally about 90 mg per 100 cm³. The brain has no glycogen store, so it depends on a steady supply from the blood.
| Glucose too high | Glucose too low | |
|---|---|---|
| Detected by | Beta cells in the islets of Langerhans | Alpha cells in the islets of Langerhans |
| Hormone | Insulin | Glucagon |
| Target cells | Liver, muscle and other tissues | Liver |
| Effect | Glucose uptake; storage as glycogen; more used in respiration | Glycogen broken down; glucose released |
Watch out: glucagon is a hormone, and glycogen is the storage product. The pancreas itself detects the change, so the hypothalamus is not involved.
| Type 1 | Type 2 | |
|---|---|---|
| Physiological change | The immune system destroys beta cells, so little or no insulin is produced | Insulin is produced, but target cells do not respond well to it |
| Risk factors | Genes, and possibly exposure to some viruses | Overweight, inactivity, age, family history |
| Prevention | None known at present | Healthy diet, regular exercise, healthy weight |
| Treatment | Insulin, with regular monitoring of blood glucose | Diet and exercise, sometimes medicine |
In either type, blood glucose stays raised and glucose may appear in urine. Long-term effects can include damage to the kidneys, eyes, nerves and blood vessels.
Watch out: do not say that type 2 is simply caused by a high-sugar diet. Such a diet is linked with it, and is one of several risk factors.
Peripheral thermoreceptors in the skin detect temperature changes. The hypothalamus compares these with the set point and signals to effectors, including muscle and adipose tissue.
Hormones are involved too. The hypothalamus secretes a releasing hormone, the pituitary gland secretes thyroid-stimulating hormone, and the thyroid gland secretes thyroxin. Thyroxin raises oxygen consumption and metabolic rate, which generates heat.
| Too hot | Too cold | |
|---|---|---|
| Skin arterioles | Vasodilation: more blood to the skin | Vasoconstriction: less blood to the skin |
| Sweat glands | More sweat, which evaporates | Less sweat |
| Hair | Muscles relax; hairs lie flat | Muscles contract; hair erection traps air |
| Muscle | No shivering | Shivering generates heat |
| Brown adipose tissue | Little activity | Uncoupled respiration releases heat |
Sweat cools the skin because evaporation breaks hydrogen bonds, which takes heat energy. Uncoupled respiration in brown adipose tissue releases energy as heat rather than storing it in ATP, which is important for newborn babies. Birds and mammals also use behaviour to regulate temperature.
Watch out: blood vessels do not move up or down in the skin. Arterioles widen or narrow, which changes how much blood flows through the surface capillaries.
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Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
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A printable worksheet covering all six statements, from quick recall to data questions and longer answers. It has 48 marks and takes about 50 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
Questions built on realistic experimental data, in the style of Paper 1B and Paper 2. Work them on paper and show your working. 19 marks in total. Your teacher has the mark scheme.
In an oral glucose tolerance test, a person who has not eaten overnight drinks a solution containing 75 g of glucose, and blood glucose is measured for three hours. The graph shows mean results for 20 healthy adults and 20 adults with type 2 diabetes.
Organisers of four city marathons recorded the percentage of runners treated for heat illness. Weather at the start of each race is shown.
Brown-throated three-toed sloths (Bradypus variegatus) live in trees in Central and South America and eat leaves, which give little energy. The body temperature of sloths was measured at different air temperatures.
You will need: Two boiling tubes, paper towel, thermometers or probes, warm water at about 40 C
Safety: Water no hotter than 45 C. Wipe up spills.
Carry this out with your teacher's supervision and follow your school's lab rules.
Isolating insulin in 1921 turned type 1 diabetes from a fatal disease into a treatable one, saving millions of lives.
The insulin work relied on experiments on dogs. Consider how the benefits of knowledge should be weighed against the way it was gained.
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
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