Excretion, Homeostasis and Coordination
What this domain tests
Excretion (kidney, liver), homeostasis (osmoregulation, thermoregulation, blood glucose) and coordination (nervous and endocrine systems, tropisms). Questions often ask you to trace a feedback loop or order a reflex arc.
Core rules
- The nephron: ultrafiltration in the glomerulus, selective reabsorption of glucose, some salts and most water in the proximal tubule, then further water reabsorption in the collecting duct under ADH control.
- ADH is made in the hypothalamus and released from the posterior pituitary; it increases the permeability of the collecting duct to water, so urine becomes more concentrated and smaller in volume.
- Insulin lowers blood glucose by promoting uptake and storage as glycogen; glucagon raises it.
- A reflex arc runs: receptor → sensory neurone → relay neurone → motor neurone → effector.
- Negative feedback means the response reverses the original change.
Worked example
After sweating heavily, water loss raises the concentration of the blood. Osmoreceptors in the hypothalamus detect this, the posterior pituitary releases more ADH, the collecting ducts reabsorb more water, urine volume falls and its concentration rises. Blood water content returns towards normal - the original change is reversed, so this is negative feedback.
Traps that cost marks
- Saying ADH is “made by the pituitary”.
- Writing “insulin makes glucose” instead of “insulin lowers blood glucose”.
- Putting the effector before the motor neurone in a reflex arc.
- Calling a response “negative feedback” without stating what is being reduced.