Excretion and homeostasis
Excretion and homeostasis
This domain tests kidney function, water balance and temperature balance.
Core rules
- Excretion is removal of metabolic waste; egestion is removal of undigested food. Kidneys excrete urea, excess water and salts.
- Ultrafiltration in the glomerulus filters small molecules into the nephron; useful glucose, amino acids and some water and salts are reabsorbed; urine remains.
- ADH from the pituitary controls water reabsorption. High blood solute concentration triggers more ADH, more water reabsorption, smaller volume of concentrated urine. Low solute concentration reduces ADH.
- Thermoregulation: sweating and vasodilation increase heat loss; vasoconstriction and shivering reduce heat loss or increase heat production. Body temperature is controlled by negative feedback.
Worked example
After heavy exercise on a hot day, a person sweats a lot and blood becomes more concentrated. Osmoreceptors detect this; the pituitary releases more ADH; the kidney collecting ducts become more permeable to water; more water is reabsorbed; urine volume falls and urine is more concentrated. Reasoning: the response opposes the change, so it is negative feedback. Drinking a lot of water reverses the sequence.
Traps
- Calling sweating excretion of water as the main point; sweat is mainly for cooling.
- Saying ADH increases urine volume.
- Confusing urea with glucose in urine.
- Saying vasodilation cools blood by narrowing vessels.
- Ignoring negative feedback wording.