Body temperature, blood sugar, water balance. Setpoints, feedback loops, the engineering of steady state.
Every sub-topic below feeds at least one of these questions.
How are constant internal conditions maintained in humans?
What are the benefits to organisms of maintaining constant internal conditions?
The required syllabus content for D3.3, in order. Each card is one lesson-sized checkpoint.
Variables are kept within preset limits, despite fluctuations in external environment.
Negative feedback loops in homeostasis
Regulation of blood glucose as an example of the role of hormones in homeostasis
Physiological changes that form the basis of type 1 and type 2 diabetes
Thermoregulation as an example of negative feedback control
Thermoregulation mechanisms in humans
Homeostasis maintains stable internal conditions — body temperature, blood glucose, water balance, pH, gas levels — despite changes in the external environment. The mechanism is almost always negative feedback.
Every homeostatic loop has the same components:
Endotherms (mammals, birds) generate heat from metabolism and regulate it actively. The hypothalamus is the master thermostat.
Vasodilation of skin arterioles → more blood to surface → more heat radiated. Sweating → evaporation removes heat. Reduced metabolic rate. Behavioural: seek shade, reduce activity.
Vasoconstriction of skin arterioles → less blood to surface → less heat lost. Shivering generates heat from muscle activity. Hair erection (piloerection) traps air. Increased metabolic rate. Behavioural: seek warmth, curl up.
The pancreas senses blood glucose directly. Two hormones with opposite effects keep glucose around ~5 mmol/L.
Insulin binds receptors on body cells → glucose uptake increased → liver converts glucose to glycogen → blood glucose falls.
Glucagon binds liver cells → glycogen broken down to glucose → glucose released into blood → blood glucose rises.
Two main types of diabetes — both result in elevated blood glucose. Different causes, different treatments.
Body's own immune system attacks the insulin-producing β-cells. No insulin produced. Onset usually in childhood/adolescence. Lifelong insulin injection required.
Body cells become less responsive to insulin; over time β-cells may exhaust. Associated with obesity, inactivity, advancing age, genetic predisposition. Initial management: diet, exercise, oral medication. May progress to needing insulin.
Kidneys filter blood, reabsorb essential substances, and excrete waste. Antidiuretic hormone (ADH) tunes the system.
Each kidney contains millions of nephrons. In each nephron:
When you're dehydrated, the hypothalamus detects high blood osmolarity. The posterior pituitary releases ADH → collecting ducts become more permeable to water → more water reabsorbed → urine is concentrated. When you're well-hydrated, ADH falls → urine is dilute.
An extra 5 sub-topics for HL — same syllabus, deeper mechanism.
Role of the kidney in osmoregulation and excretion
Role of the glomerulus, Bowman’s capsule and proximal convoluted tubule in excretion
Role of the loop of Henle
Osmoregulation by water reabsorption in the collecting ducts
Changes in blood supply to organs in response to changes in activity
If you can't define one of these in a sentence, that's where to revise next.
“For what reasons do organisms need to distribute materials and energy?”
“What biological systems are sensitive to temperature changes?”