Excretion and Osmoregulation
Excretion and Osmoregulation tests how nitrogenous waste is removed and water-salt balance is held. Focus nephron function, countercurrent multiplication, hormones and animal osmoregulatory strategies.
Core rules
- Filtration occurs in the renal corpuscle; reabsorption occurs mainly in the proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct; secretion adds substances to filtrate.
- The loop of Henle countercurrent multiplier creates the medullary osmotic gradient.
- ADH increases water permeability of the collecting duct; aldosterone increases sodium reabsorption; ANF promotes sodium excretion.
- Ammonia is ammonotelic and needs much water; urea is ureotelic; uric acid is uricotelic and conserves water.
- Osmoregulators maintain internal osmolarity; osmoconformers match the environment.
Worked example
A person produces 180 L of glomerular filtrate per day and 1.5 L of urine. How much is reabsorbed?
Reasoning: reabsorbed = 180 − 1.5 = 178.5 L, about 99%. If ADH is high, urine volume falls and concentration rises. If ADH is absent, a large dilute urine is produced. Answer: 178.5 L reabsorbed.
Traps that cost marks
- Glucose is normally absent from urine because it is fully reabsorbed.
- ADH increases water reabsorption, not sodium reabsorption.
- Aldosterone acts on sodium, not water directly.
- Filtration is not the main function of the loop of Henle.
- Ammonia excretion requires abundant water.