Excretion and Osmoregulation
What this domain tests
Expect nephron structure and function, the steps of urine formation, hormonal control of water balance, accessory excretory organs, dialysis, and plant excretory routes.
Core rules
- Excretion removes nitrogenous wastes and regulates water and salt balance (osmoregulation).
- Ultrafiltration in the glomerulus: high pressure drives water, glucose, amino acids, urea and salts into Bowman's capsule; proteins and blood cells stay behind.
- Selective reabsorption in the proximal convoluted tubule returns all glucose and amino acids and most water and sodium by active transport.
- Loop of Henle maintains the concentration gradient; the collecting duct's water permeability is set by ADH from the posterior pituitary.
- Tubular secretion adds H+, K+ and drugs into the filtrate, helping acid–base balance.
- Urine: about 95% water plus urea, uric acid, creatinine and mineral salts.
- Liver deaminates surplus amino acids to make urea; also detoxifies.
- Skin (sweat), lungs (CO2) and liver act as accessory excretory organs.
- Plants: stomata and lenticels lose gases and water, hydathodes release water droplets; gums, resins, latex and calcium oxalate crystals are stored in dead tissues.
Worked example
Two patients pass large volumes of urine. Patient A's urine is sugary and blood glucose is high; glucose exceeded the tubular maximum for reabsorption, so it spilled over - this is diabetes mellitus. Patient B's urine is dilute with no sugar; ADH deficiency means the collecting duct reabsorbs little water - diabetes insipidus. Same symptom, different hormone and tubule cause.
Traps that cost marks
Saying the filtrate lacks glucose; claiming the kidney makes urea; writing that ADH increases urine volume; confusing diabetes mellitus with diabetes insipidus; forgetting the loop of Henle.