Homeostasis — IB Diploma Biology HL
IB Biology D3.3: homeostasis and negative feedback, blood glucose, diabetes, thermoregulation, and HL kidney function, ADH and blood supply.
Revision notes
7 short notes, in the order of the specification. Each one in short:
Homeostasis is the maintenance of the internal environment of an organism: variables are kept within preset limits despite fluctuations in the external environment. In humans these include body temperature, blood pH, blood glucose concentration and blood osmotic concentration. Negative feedback returns a variable to its set point whether it rises above or falls below it.
Blood glucose regulation is an example of hormones in homeostasis. Pancreatic endocrine cells secrete insulin when blood glucose is high and glucagon when it is low, and the blood carries them to target cells: insulin makes liver and muscle cells absorb glucose and store it as glycogen; glucagon makes liver cells break glycogen down to glucose.
Diabetes is a condition in which blood glucose is not kept within its normal limits. In type 1 diabetes the immune system destroys the β cells, so little or no insulin is secreted. In type 2 diabetes target cells become resistant to insulin. Type 1 is treated with insulin; type 2 is largely prevented and managed by diet and exercise.
Thermoregulation is the control of body temperature by negative feedback. Peripheral thermoreceptors send information to the hypothalamus, which controls effectors directly by nerves and through the pituitary gland and thyroxin. When cold, vasoconstriction, shivering, hair erection and uncoupled respiration in brown adipose tissue conserve or generate heat; when hot, vasodilation and sweating increase heat loss.
The kidney carries out both excretion, the removal of metabolic wastes such as urea, and osmoregulation, the regulation of osmotic concentration, measured in osmol L⁻¹. Ultrafiltration in the glomerulus and Bowman's capsule removes small solutes and water from blood plasma; the proximal convoluted tubule then reabsorbs useful substances, leaving toxins and other unwanted solutes to be excreted in urine.
The loop of Henle actively transports sodium ions out of the ascending limb, maintaining a high osmotic concentration in the medulla so that water can be reabsorbed from the collecting ducts. Osmoreceptors in the hypothalamus control antidiuretic hormone (ADH) secretion by the pituitary; ADH moves aquaporins from intracellular vesicles into collecting duct cell membranes, increasing water reabsorption.
The blood supply to organs changes with activity because arterioles dilate or constrict to redirect blood where it is needed. During vigorous physical activity cardiac output rises and most extra blood goes to skeletal muscles, while flow to the gut and kidneys falls. Blood flow to the brain stays roughly constant during sleep, wakeful rest and exercise.
Exam questions
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Quick check questions
Name four homeostatic variables in humans.
Show answer
Body temperature, blood pH, blood glucose concentration and blood osmotic concentration.
Which pancreatic cells secrete glucagon?
Show answer
α (alpha) cells of the islets of Langerhans.
State the physiological cause of type 1 diabetes.
Show answer
Autoimmune destruction of the β cells, so little or no insulin is secreted.
How does brown adipose tissue generate heat?
Show answer
Uncoupled respiration: energy is released as heat instead of being used to make ATP.
HL only What does ADH do to collecting duct cells?
Show answer
Moves aquaporins from intracellular vesicles into the cell membrane, increasing water reabsorption.
Frequently asked questions
Why does homeostasis use negative feedback rather than positive feedback?
Negative feedback reverses any change from the set point, so it returns a variable to normal whether it rises above or falls below it. Positive feedback amplifies a change away from the set point, which would make an imbalance worse. That is why positive feedback only controls processes that must finish quickly, such as childbirth.
How do insulin and glucagon control blood glucose?
When blood glucose rises, β cells in the pancreatic islets secrete insulin, which makes liver and muscle cells absorb glucose and store it as glycogen. When blood glucose falls, α cells secrete glucagon, which makes liver cells break glycogen down and release glucose. Both hormones travel to their target cells in the blood.
What is the difference between type 1 and type 2 diabetes?
In type 1 diabetes the immune system destroys the insulin-secreting β cells, so little or no insulin is made and insulin must be injected. In type 2 diabetes insulin is still produced but target cells become resistant to it. Type 2 is linked to obesity and inactivity and is managed mainly by diet and exercise.
How does the body respond when it gets too cold?
Thermoreceptors signal to the hypothalamus, which triggers vasoconstriction of skin arterioles, shivering of skeletal muscles, hair erection and uncoupled respiration in brown adipose tissue to release heat. Through the pituitary gland it also increases thyroxin secretion, which raises metabolic rate. Behavioural responses, such as adding clothes, also help.
How does ADH control the amount of water in the blood?
HL only Osmoreceptors in the hypothalamus detect a high blood osmotic concentration, so the pituitary secretes more ADH. ADH causes aquaporins to move from intracellular vesicles into the membranes of collecting duct cells, so more water is reabsorbed by osmosis and urine becomes concentrated. Low osmotic concentration reduces ADH secretion.
Written and checked against the IB Biology HL specification · Updated October 2026