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

This guide teaches section D3.3 (Homeostasis) of the IB Biology guide, first assessment 2025. It covers statements D3.3.1 to D3.3.6 for SL and HL, plus D3.3.7 to D3.3.11 (the kidney, osmoregulation and changes in blood supply), which are HL only. For condensed recall use the revision notes, then test yourself with the practice questions. Course hub: IB DP Biology. Checklist: printable checklist. To find weak areas first, take the free 10-minute diagnostics.

What this unit covers

Guide section	What you must be able to do	Level
D3.3.1	Define homeostasis; name the four human variables (temperature, pH, glucose, osmotic concentration)	SL and HL
D3.3.2	Explain negative feedback and why it is used, not positive feedback	SL and HL
D3.3.3	Explain blood glucose control by insulin and glucagon	SL and HL
D3.3.4	Explain the changes behind type 1 and type 2 diabetes, with risk factors, prevention and treatment	SL and HL
D3.3.5	Explain thermoregulation as negative feedback, including thyroxin, muscle and adipose tissue	SL and HL
D3.3.6	Describe the human mechanisms: vasodilation, vasoconstriction, shivering, sweating, brown adipose tissue, hair erection	SL and HL
D3.3.7	Distinguish excretion from osmoregulation; use osmol L⁻¹	HL only
D3.3.8	Explain ultrafiltration (glomerulus to capsule) and reabsorption in the proximal tubule	HL only
D3.3.9	Explain the loop of Henle’s role in the medulla	HL only
D3.3.10	Explain osmoregulation in the collecting duct: osmoreceptors, ADH, aquaporins	HL only
D3.3.11	Explain changes in blood supply to muscles, gut, brain and kidneys	HL only

D3.3.1 Homeostasis and the internal environment

Homeostasis keeps the conditions inside an organism within preset limits, even when the external environment changes. In humans the guide names four variables: core temperature, blood pH, blood osmotic concentration and the concentration of glucose in the blood.

Cell processes depend on stable conditions. Enzyme activity falls away from its optimum temperature and pH. Glucose is the fuel for respiration. If blood becomes too dilute, cells take in water by osmosis and swell. If it becomes too concentrated, cells lose water.

D3.3.2 Negative feedback

A feedback loop has a receptor, a control centre that compares the variable with the set point, and an effector. In negative feedback the effector’s action reverses the change that started it. That brings the variable back towards the set point.

Negative feedback works from both directions. A rise above the set point triggers a response that lowers the variable. A fall below it triggers a different response that raises it.

Positive feedback amplifies a change and drives the variable further from the set point. Used for temperature, a small rise would cause more heating. That is why homeostasis uses negative feedback.

D3.3.3 Blood glucose and hormones

The pancreas contains endocrine cells that detect blood glucose and secrete hormones directly into the blood.

• Beta cells release insulin when blood glucose is high, for example after a meal.
• Alpha cells release glucagon when blood glucose is low, for example between meals.

Both hormones travel in the blood plasma to target cells, which have the receptors for them.

• Insulin acts on liver, muscle and fat cells. It increases glucose uptake and stimulates the liver and muscle to convert glucose to glycogen. Blood glucose falls.
• Glucagon acts mainly on liver cells. It stimulates the breakdown of glycogen to glucose, which is released into the blood. Blood glucose rises.

As blood glucose returns to the set point, the stimulus for the hormone falls and its secretion drops. That is negative feedback.

Worked example 1. After a meal, a person’s blood glucose is 4.8 mmol L⁻¹ before eating, peaks at 7.6 at 45 minutes and is 5.0 at 150 minutes.

• Percentage rise to the peak: (7.6 − 4.8) / 4.8 × 100 = 58.3%.
• Mean rate of fall from 45 to 150 min: (7.6 − 5.0) / 105 = 0.025 mmol L⁻¹ min⁻¹ (2 significant figures).
• The peak triggers insulin secretion; the return to near the starting value shows negative feedback.

D3.3.4 Diabetes: types 1 and 2

In diabetes, glucose control fails and blood glucose stays high.

Feature	Type 1	Type 2
Physiological change	Beta cells are destroyed (an autoimmune process), so little or no insulin is made	Target cells respond poorly to insulin (reduced sensitivity); beta cells may later fail
Risk factors	Genetic predisposition; not linked to lifestyle	Excess body fat, inactivity, diet, age, family history
Prevention	No known way to prevent it	Healthy body weight, regular activity, balanced diet
Treatment	Insulin by injection or pump, with glucose monitoring and attention to diet	Diet and exercise first; medicines that raise sensitivity or insulin output; insulin in some cases

Worked example 2 (glucose tolerance data). Glucose in mmol L⁻¹ was measured at 0, 30, 60 and 120 minutes after a glucose drink.

Time / min	0	30	60	120
Person without diabetes	4.9	7.4	6.2	5.0
Person X	8.1	12.6	14.0	12.2

• Rise to the peak in the control: 7.4 − 4.9 = 2.5, which is 51.0% of the starting value.
• Rise to the peak in Person X: 14.0 − 8.1 = 5.9, which is 72.8%.
• At 120 min Person X is 12.2 − 5.0 = 7.2 higher than the control, or 2.44 times as high.
• Person X starts high, peaks later and stays high, so glucose is not being cleared. The data cannot show which type.

D3.3.5 Thermoregulation as negative feedback

Humans keep core body temperature near 37 °C. Two groups of receptors detect change. Peripheral thermoreceptors in the skin detect surface temperature. The hypothalamus also monitors the temperature of the blood passing through it, and acts as the control centre.

Effectors include skeletal muscle, skin and adipose tissue. In the cold, the hypothalamus signals the pituitary gland, which releases a hormone that stimulates the thyroid gland. The thyroid releases thyroxin, which raises the metabolic rate of many cells, so more heat is produced. When temperature returns to the set point, the signals weaken.

• Muscle is an effector because shivering produces heat.
• Adipose tissue is an effector in two ways. Brown adipose tissue releases heat (see below). Subcutaneous fat also insulates.

D3.3.6 Human thermoregulation mechanisms

Birds and mammals use physiological and behavioural means to regulate temperature. Behaviour includes seeking shade and changing clothing. You need the physiological detail for humans only.

Mechanism	Condition	How it works
Vasodilation	Hot	Skin arterioles widen, more blood flows near the surface, more heat is lost to the surroundings
Vasoconstriction	Cold	Skin arterioles narrow, less blood flows near the surface, heat is retained
Sweating	Hot	Sweat evaporates from the skin, taking heat from the body
Shivering	Cold	Rapid involuntary muscle contractions; respiration in the muscle releases heat
Uncoupled respiration in brown adipose tissue	Cold	Protons leak back across the inner mitochondrial membrane without making ATP, so the energy is released as heat
Hair erection	Cold	Hairs rise and trap a layer of air that insulates; of limited use in humans

Worked example 3. A person’s metabolic heat production is 90 W at rest and 270 W during intense shivering. The ratio is 270 / 90 = 3, an increase of (270 − 90) / 90 × 100 = 200%.

D3.3.7 Excretion and osmoregulation (HL only)

Excretion is getting rid of metabolic waste from the body. Urea, which the liver makes when it breaks down surplus amino acids, is excreted by the kidney. Osmoregulation is the regulation of osmotic concentration, which is the total concentration of dissolved solutes. Its unit is osmol L⁻¹ (osmoles per litre). The kidney does both jobs, but they are different.

Conversion: 1200 milliosmol L⁻¹ is 1.2 osmol L⁻¹. Blood plasma is taken as 0.29 osmol L⁻¹ in the examples here.

D3.3.8 Ultrafiltration and the proximal convoluted tubule (HL only)

Ultrafiltration takes place between the glomerulus (a knot of capillaries) and Bowman’s capsule. The blood pressure in the glomerulus is high, partly because the vessel leaving it is narrower than the vessel entering. This forces plasma out through three layers: the pores of the capillary wall, a basement membrane and the slits between podocytes of the capsule. Water and small solutes pass through: glucose, amino acids, urea, ions. Blood cells and large plasma proteins stay in the blood. The result is the filtrate.

In the proximal convoluted tubule the cells reabsorb useful substances, so that toxins and unwanted solutes stay in the filtrate. Glucose, amino acids, vitamins, mineral ions and much of the water are reabsorbed. The cells have microvilli for surface area and many mitochondria for active transport. Normally all the glucose is reabsorbed.

Worked example 4. A kidney forms 125 mL of filtrate per minute: 125 × 1440 = 180 000 mL, or 180 L per day. A person excretes 1.5 L of urine per day.

• Water reabsorbed: 180 − 1.5 = 178.5 L, which is 178.5 / 180 × 100 = 99.2%.
• Urea is 0.30 g L⁻¹ in plasma and 18 g L⁻¹ in urine, so urine is 18 / 0.30 = 60 times more concentrated. Water has been reabsorbed from the tubule while urea mostly stays in it.

D3.3.9 The loop of Henle (HL only)

The ascending limb of the loop of Henle actively transports sodium ions out of the filtrate into the tissue of the medulla. This keeps the medulla at a high osmotic concentration, which rises with depth. That gradient lets water leave the collecting ducts by osmosis, so urine can become more concentrated than blood.

D3.3.10 Osmoregulation in the collecting duct (HL only)

Osmoreceptors in the hypothalamus monitor the osmotic concentration of blood. The pituitary gland secretes antidiuretic hormone (ADH) at a rate that depends on their signals.

• Blood too concentrated: more ADH is secreted. ADH binds to receptors on collecting duct cells. Vesicles carrying aquaporins (water channels) fuse with the cell membrane. The membrane becomes more permeable to water, so the urine volume is small and the urine is concentrated, because more water returns to the blood.
• Blood too dilute: less ADH is secreted. Aquaporins are taken back into vesicles by endocytosis. The membrane is less permeable, less water is reabsorbed and a large volume of dilute urine is produced.

Worked example 5. Maximum urine concentration is 1200 milliosmol L⁻¹, or 1.2 osmol L⁻¹. Plasma is 0.29 osmol L⁻¹. The ratio is 1.2 / 0.29 = 4.1.

D3.3.11 Blood supply to organs (HL only)

Blood flow is redirected by arterioles. Smooth muscle in their walls contracts (vasoconstriction) or relaxes (vasodilation), so the same cardiac output is shared differently as activity changes.

• Vigorous exercise: blood flow to skeletal muscle rises sharply for oxygen and glucose supply and to remove carbon dioxide. Flow to the gut and kidneys falls. The brain’s supply is kept roughly constant.
• Wakeful rest: flow to muscle is low, and the gut and kidneys receive a larger share.
• Sleep: total demand is lower; the brain keeps its supply and muscle flow is lowest.

Worked example 6. Illustrative data: cardiac output is 5.0 L min⁻¹ at rest and 15 L min⁻¹ in exercise. Muscle gets 20% at rest and 75% in exercise.

• At rest: 5.0 × 0.20 = 1.0 L min⁻¹. In exercise: 15 × 0.75 = 11.25 L min⁻¹, or 11.25 times as much.
• The gut gets 25% at rest and 5% in exercise: 1.25 L min⁻¹ falls to 0.75 L min⁻¹, a 40% fall in absolute flow.

Common errors

• Writing “hormones travel to the organ” without naming target cells with receptors.
• Saying vasodilation “moves vessels closer to the skin”. The arterioles widen; they do not move.
• Confusing excretion with osmoregulation.
• Saying ADH changes the number of aquaporins made. It changes their location.
• Saying water is pumped out of the collecting duct. Water moves by osmosis.

Official syllabus

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