IB DP Biology: Homeostasis: feedback, glucose, thermoregulation and the kidney (DP Biology) – Revision Notes

These notes condense section D3.3 (Homeostasis) of the IB Biology guide, first assessment 2025. Statements D3.3.1 to D3.3.6 are SL and HL; D3.3.7 to D3.3.11 are HL only. For full explanations and worked examples use the study guide, then try the practice questions. Course hub: IB DP Biology. Checklist: printable checklist. To find weak areas first, take the free 10-minute diagnostics.

Key definitions

Term	Meaning
Homeostasis (D3.3.1)	Keeping the internal environment within preset limits despite external change
Homeostatic variables	Body temperature, blood pH, blood glucose concentration, blood osmotic concentration
Set point	The target value of a variable
Negative feedback (D3.3.2)	The response reverses the change, returning the variable to the set point from above or below
Positive feedback	The response amplifies the change, moving the variable away from the set point
Excretion (D3.3.7)	Removal of metabolic waste from the body (HL only)
Osmoregulation (D3.3.7)	Regulation of osmotic concentration, in osmol L⁻¹ (HL only)
Ultrafiltration	Pressure filtration of plasma into Bowman’s capsule (HL only)

Feedback loop in steps

1. A change moves the variable away from the set point.
2. A receptor detects the change.
3. A control centre compares the value with the set point.
4. An effector produces a response that reverses the change.
5. The variable returns towards the set point and the response weakens.

Why negative, not positive: positive feedback would push the variable further from the set point. Negative feedback corrects change in either direction.

Blood glucose (D3.3.3, D3.3.4)

	Insulin	Glucagon
Made by	Beta cells of the pancreas	Alpha cells of the pancreas
Released when	Blood glucose is high	Blood glucose is low
Transport	In blood plasma	In blood plasma
Target cells	Liver, muscle, fat	Mainly liver
Effect	More glucose uptake; glucose converted to glycogen	Glycogen broken down; glucose released
Result	Blood glucose falls	Blood glucose rises

Diabetes

	Type 1	Type 2
Change	Beta cells destroyed (autoimmune); little or no insulin	Target cells less responsive to insulin
Risk factors	Genetic predisposition	Excess body fat, inactivity, diet, age, family history
Prevention	None known	Healthy weight, activity, balanced diet
Treatment	Insulin, glucose monitoring, diet	Diet and exercise; medicines; sometimes insulin

Method for a glucose graph: state the starting value, the peak and its time, the return time. Quote values with units (mmol L⁻¹). Percentage rise = (peak − start) / start × 100.

Reminder: start 4.8, peak 7.6. Rise = 2.8, and 2.8 / 4.8 × 100 = 58.3%.

Thermoregulation (D3.3.5, D3.3.6)

• Core temperature is near 37 °C.
• Receptors: peripheral thermoreceptors in the skin; the hypothalamus monitors blood temperature.
• Control centre: the hypothalamus.
• Hormonal route in the cold: hypothalamus signals the pituitary gland, which stimulates the thyroid; thyroxin raises metabolic rate, so more heat is released.
• Effectors: skeletal muscle (shivering) and adipose tissue (brown adipose tissue heat; subcutaneous fat insulates).

Mechanism	Hot or cold	Action
Vasodilation	Hot	Skin arterioles widen; more heat lost
Vasoconstriction	Cold	Skin arterioles narrow; heat retained
Sweating	Hot	Evaporation removes heat
Shivering	Cold	Muscle contraction releases heat
Brown adipose tissue	Cold	Uncoupled respiration: protons leak across the inner mitochondrial membrane, energy released as heat, not ATP
Hair erection	Cold	Traps insulating air; limited use in humans

Birds and mammals use physiological and behavioural means. Only the human details are required.

The kidney (HL only: D3.3.7 to D3.3.10)

Ultrafiltration (D3.3.8)

• High pressure in the glomerulus pushes plasma through the capillary wall, the basement membrane and the slits between podocytes.
• Small solutes pass: water, glucose, amino acids, urea, ions.
• Blood cells and large plasma proteins stay in the blood.

Proximal convoluted tubule (D3.3.8)

• Reabsorbs glucose, amino acids, vitamins, mineral ions and much of the water.
• Adaptations: microvilli and many mitochondria.
• Toxins and unwanted solutes remain in the filtrate and leave in urine.

Loop of Henle (D3.3.9)

• The ascending limb actively transports sodium ions out into the medulla.
• This maintains a high osmotic concentration in the medulla.
• The gradient lets water leave the collecting duct by osmosis.

Collecting duct and ADH (D3.3.10)

	Blood too concentrated	Blood too dilute
Detected by	Osmoreceptors in the hypothalamus	Osmoreceptors in the hypothalamus
ADH from pituitary	More	Less
Aquaporins	Vesicles fuse with the membrane; more in membrane	Taken back into vesicles; fewer in membrane
Water movement out of duct	Increased	Reduced
Urine produced	Little, and strong	Plenty, and weak

Blood supply to organs (HL only: D3.3.11)

Smooth muscle in arteriole walls constricts or relaxes to redirect blood.

Condition	Skeletal muscle	Gut	Brain	Kidneys
Vigorous exercise	Much more	Less	About constant	Less
Wakeful rest	Low	Larger share	Maintained	Large share
Sleep	Lowest	Related to digestion	Maintained	Maintained

Model chains to learn

Write each chain in this order: stimulus, receptor, control centre, effector, response, return to set point.

High blood glucose. Glucose rises after a meal. Beta cells in the pancreas detect it and secrete insulin. Insulin travels in the plasma to liver, muscle and fat cells, which take up glucose. The liver and muscle store it as glycogen. Blood glucose falls, so insulin secretion falls.

Low blood glucose. Glucose falls between meals. Alpha cells secrete glucagon. It travels in the plasma to liver cells, which break glycogen down to glucose and release it. Blood glucose rises, so glucagon secretion falls.

Cold. Peripheral thermoreceptors and the hypothalamus detect a fall in temperature. The hypothalamus acts as the control centre. Effectors: skin arterioles constrict, muscle shivers, brown adipose tissue releases heat, thyroxin raises metabolic rate. Temperature rises towards 37 °C and the responses weaken.

Dehydration (HL only). Blood osmotic concentration rises. Osmoreceptors in the hypothalamus detect it. The pituitary gland secretes more ADH. Aquaporin-carrying vesicles fuse with collecting duct membranes. More water is reabsorbed, giving less, more concentrated urine. Blood becomes more dilute.

Worked reminders

Rate from a graph. Glucose falls from 9.0 to 5.4 mmol L⁻¹ between 60 and 120 minutes. Rate = (9.0 − 5.4) / 60 = 0.06 mmol L⁻¹ min⁻¹.

Heat production ratio. 240 W while shivering and 80 W at rest gives 240 / 80 = 3, an increase of 200%.

Reabsorption (HL only). Filtrate 180 L per day, urine 2.0 L per day. Reabsorbed = 178 L, so 178 / 180 × 100 = 98.9%.

Concentration factor (HL only). Urea 0.25 g L⁻¹ in plasma and 20 g L⁻¹ in urine: 20 / 0.25 = 80 times.

Must-know distinctions

• Excretion removes waste. Osmoregulation controls osmotic concentration.
• Insulin lowers blood glucose. Glucagon raises it.
• Type 1 is loss of insulin production. Type 2 is reduced response to insulin.
• Vasoconstriction conserves heat. Vasodilation loses heat.
• Ultrafiltration separates solutes by size under pressure, whatever their usefulness. Reabsorption picks out the useful substances.
• ADH alters where aquaporins are, not how many exist.

Quick self-test

1. List the four homeostatic variables the guide names for humans.
2. Why is negative feedback suited to homeostasis?
3. Which cells secrete insulin?
4. Name the main target organ of glucagon.
5. Give one physiological difference between type 1 and type 2 diabetes.
6. Which hormone raises metabolic rate as part of the response to cold?
7. What happens in brown adipose tissue to release heat?
8. Blood glucose rises from 5.2 to 8.4 mmol L⁻¹. Calculate the percentage rise.
9. (HL only) A kidney filters 150 mL per minute and 1.0 mL per minute leaves as urine. What percentage is reabsorbed?
10. (HL only) State what happens to aquaporins in the collecting duct when ADH secretion falls.
11. (HL only) Convert 900 milliosmol L⁻¹ to osmol L⁻¹.

Answers

1. Body temperature, blood pH, blood glucose concentration, blood osmotic concentration.
2. It reverses a change in either direction, returning the variable to the set point.
3. Beta cells in the pancreas.
4. The liver.
5. Type 1: beta cells destroyed, little insulin. Type 2: target cells respond poorly to insulin.
6. Thyroxin.
7. Uncoupled respiration: protons leak across the inner mitochondrial membrane, so energy is released as heat.
8. (8.4 − 5.2) / 5.2 × 100 = 61.5%.
9. (150 − 1.0) / 150 × 100 = 99.3%.
10. Aquaporins are moved from the cell membrane back into vesicles; the membrane is less permeable to water.
11. 0.9 osmol L⁻¹.

Where marks are usually lost

• Writing “hormones are released and the glucose goes down” with no named cells or target tissue.
• Leaving out the return to the set point when describing negative feedback.
• Mixing up the two diabetes types or saying type 1 is caused by diet.
• Describing brown adipose tissue as “burning fat” without uncoupled respiration.
• Using “the skin vessels move up” for vasodilation.
• Treating ultrafiltration as selective by shape instead of by size and pressure.
• Forgetting that ADH changes aquaporin location.
• Writing osmotic concentration units as mmol L⁻¹ or leaving the units out.
• Describing a hormone as acting on “the body” instead of on target cells that have its receptor.
• Giving only the cold responses when a question about a hot day asks for sweating and vasodilation.
• Stating a percentage change without the working: write the difference, divide by the starting value, then multiply by 100.
• Calling the loop of Henle the site of ultrafiltration. Filtration happens between the glomerulus and Bowman’s capsule.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Section D3.3, Homeostasis.