IB Biology · Theme D: Continuity and change · SL and HL
D3.3 Homeostasis
A one-page summary of D3.3 Homeostasis, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How are constant internal conditions maintained in humans?
What are the benefits to organisms of maintaining constant internal conditions?
What D3.3 covers
Keeping conditions constant
- D3.3.1The internal environment is held within preset limits
- D3.3.2A negative feedback loop has a receptor, a coordinator and an effector
- D3.3.2Negative feedback keeps a system stable
Blood glucose and diabetes
- D3.3.3Two kinds of islet cell make antagonistic hormones
- D3.3.3Insulin lowers blood glucose and glucagon raises it
- D3.3.4Type 1 and type 2 diabetes have different causes
- D3.3.4Risk factors, prevention and treatment differ
Thermoregulation
- D3.3.5Body temperature is controlled by negative feedback
- D3.3.6When the body is too hot, it loses heat
- D3.3.6When the body is too cold, it keeps and makes heat
- D3.3.6Brown adipose tissue makes heat by uncoupled respiration
HL — the kidney and blood supply
- D3.3.7 · HLExcretion and osmoregulation are different jobs
- D3.3.8–10 · HLThe nephron does its work in three stages
- D3.3.8 · HLStage 1: ultrafiltration removes small solutes from the blood plasma
- D3.3.8 · HLStage 2: the proximal convoluted tubule reabsorbs useful substances
- D3.3.9 · HLStage 3: the loop of Henle builds a concentrated medulla
- D3.3.10 · HLStage 3: osmoreceptors and ADH keep the blood’s osmotic concentration steady
- D3.3.10 · HLStage 3: ADH switches aquaporins between vesicles and the membrane
- D3.3.11 · HLBlood is redirected to the organs that are most active
D3.3 Homeostasis: summary
Homeostasis and feedback
- Homeostasis keeps variables within preset limits around a set point: temperature, pH, glucose, osmotic concentration. Negative feedback (receptor, coordinator, effector) corrects from above and below; positive feedback would run away.
Blood glucose
- Islet β cells secrete insulin: liver and muscle cells take up glucose, store glycogen, glucose falls. α cells secrete glucagon: liver breaks down glycogen, glucose rises. Range about 4 to 6 mmol dm−3.
Diabetes
- Type 1: β cells destroyed, no insulin, treated with insulin. Type 2: insulin resistance; risks are obesity, inactivity, diet, age and family history; largely preventable with diet, exercise and healthy body mass.
Thermoregulation
- Skin thermoreceptors, hypothalamus, pituitary, thyroxin. Hot: vasodilation, sweating. Cold: vasoconstriction, hair erection, shivering, brown fat (uncoupled respiration makes heat, not ATP).
HL · The nephron
- Excretion (waste, urea) is not osmoregulation (osmol L−1). Ultrafiltration at the glomerulus is by size only; the PCT reabsorbs all glucose and amino acids, most water. Loop of Henle pumps Na+ into the medulla.
HL · ADH and blood supply
- High blood osmotic concentration: osmoreceptors, more ADH, aquaporins move into the membrane, concentrated urine. Blood is redirected by vasodilation and vasoconstriction: muscle up, gut and kidneys down, brain steady.
Key terms
- Homeostasis
- Maintenance of the internal environment within preset limits, despite changes outside.
- Set point
- The target value of a variable that the body works to maintain.
- Negative feedback
- A response that reverses the change that triggered it, returning the variable to its set point.
- Positive feedback
- A response that amplifies the change that triggered it, moving the variable further away.
- Islets of Langerhans
- Clusters of endocrine cells in the pancreas: β cells make insulin, α cells make glucagon.
- Insulin
- A hormone that lowers blood glucose by making cells take up glucose and store it as glycogen.
- Glucagon
- A hormone that raises blood glucose by making liver cells break down glycogen.
- Type 1 diabetes
- An autoimmune destruction of β cells, so little or no insulin is made.
- Type 2 diabetes
- A poor response of target cells to insulin (insulin resistance), so glucose stays in the blood.
- Thermoreceptor
- A receptor that detects temperature; peripheral thermoreceptors are in the skin.
- Hypothalamus
- The part of the brain that compares body temperature with the set point.
- Thyroxin
- A hormone that raises the metabolic rate and so the heat production of cells.
- Vasodilation
- Widening of skin arterioles, so more heat is lost.
- Vasoconstriction
- Narrowing of skin arterioles, so less heat is lost.
- Sweating
- Release of sweat, which cools the skin as it evaporates.
- Shivering
- Rapid involuntary contractions of skeletal muscle that release heat.
- Hair erection
- Erector muscles pull hairs upright, trapping insulating air.
- Uncoupled respiration
- Protons return through a channel that bypasses ATP synthase, so energy is released as heat.
- Excretion
- Removal of metabolic waste products from the body, for example urea.
- Osmoregulation
- Regulation of the osmotic concentration of the body fluids.
- Osmotic concentration
- The concentration of dissolved solute particles, measured in osmol L−1.
- Ultrafiltration
- Pressure filtration of small molecules from blood plasma into Bowman’s capsule.
- Selective reabsorption
- Return of useful substances from the filtrate to the blood in the proximal convoluted tubule.
- Loop of Henle
- The part of the nephron whose ascending limb pumps sodium ions into the medulla.
- Osmoreceptor
- A receptor in the hypothalamus that monitors the osmotic concentration of the blood.
- ADH
- Antidiuretic hormone from the pituitary gland: it increases water reabsorption in the collecting ducts.
- Aquaporin
- A water channel protein, moved between vesicles and the cell membrane under the control of ADH.
Sample exam questions
Three of the 45 multiple-choice questions for D3.3. Try each one before opening the answer.
Question 1. Which of the following best defines homeostasis?
- Growth of an organism in response to a constant external environment
- Maintenance of the internal environment within preset limits despite external fluctuation
- The process by which an organism reproduces to maintain population size
- A permanent, one-way change in an organism's internal chemistry
Show the answer
Answer: B. Homeostasis is specifically the maintenance of internal conditions within narrow preset limits even as external conditions change.
Question 2. Which list contains only variables that are kept within preset limits by homeostasis in humans?
- Body mass, blood pH and height
- Blood glucose concentration, body mass and muscle size
- Heart size, body temperature and number of red blood cells made each day
- Body temperature, blood pH and blood osmotic concentration
Show the answer
Answer: D. Body temperature, blood pH, blood glucose concentration and blood osmotic concentration are each held near a set point by negative feedback. Body mass, height and organ size are not regulated in this way.
Question 3. After drinking a large volume of water, a person produces a large volume of dilute urine. Which process does this mainly show?
- Excretion, because water is a waste product of metabolism
- Ultrafiltration only, because more fluid is forced out of the glomerulus
- Osmoregulation, because extra water is removed to bring the blood’s osmotic concentration back to its set point
- Excretion, because urea is removed in the urine
Show the answer
Answer: C. The extra water lowered the osmotic concentration of the blood. Removing it in a large volume of dilute urine returns the blood to its set point, which is osmoregulation. Urea is removed whatever the volume of urine, so the change in volume is not explained by excretion.
Linking questions
Questions that connect D3.3 to other parts of the course, the kind that come up in Paper 2.
- Enzymes work only in a narrow range of temperature and pH (C1.1.8). What biological systems are sensitive to temperature changes, and why does body temperature need to be regulated (D3.3.1, D3.3.5)? (see C1.1)
- Hormonal and nervous signalling integrate organs (C3.1.3), and the hypothalamus and pituitary gland control the endocrine system (C3.1.13). Compare the feedback control of blood glucose (D3.3.3) with the feedback control of heart rate (C3.1.14). (see C3.1)
- Blood osmotic concentration is a homeostatic variable (D3.3.1). Using osmosis (B2.1.5), explain what would happen to body cells if the osmotic concentration of the blood fell too far below its set point. (see B2.1)
- For what reasons do organisms need to distribute materials and energy? Use the transport of insulin and glucagon in the blood (D3.3.3) and the changes in blood flow through skin arterioles (D3.3.6) in your answer, with reference to the structure of arteries and capillaries (B3.2.1, B3.2.2). (see B3.2)
Practise D3.3
45 quiz questions9 data questions6 exam questionsmarkschemes included
Study notes, every question and full markschemes for D3.3 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.