Excretion and Homeostasis
IGCSE (Year 10โ11) ๐ซ Physiology & Health Kidneys, liver, thermoregulation, blood glucose control.
๐ก๏ธ The Kidney โ Structure
The kidneys filter blood and produce urine, regulating water content (osmoregulation) and removing nitrogenous waste (urea).
Urea production
Excess amino acids โ deamination in liver โ ammonia (toxic) โ converted to urea โ transported in blood to kidneys โ excreted in urine- Nephron โ the functional unit (~1 million per kidney)
- Ultrafiltration โ high pressure in glomerulus forces small molecules (glucose, urea, water, salts) into Bowman's capsule
- Selective reabsorption โ all glucose, most water and salts reabsorbed in PCT by active transport
- Tubular secretion โ additional waste added in DCT
๐ก๏ธ ADH and Osmoregulation
The pituitary gland releases ADH (antidiuretic hormone) to control water reabsorption in the kidney.
- Blood too concentrated (dehydrated) โ osmoreceptors in hypothalamus โ more ADH released โ collecting duct more permeable to water โ more water reabsorbed โ small volume of concentrated urine
- Blood too dilute (overhydrated) โ less ADH โ collecting duct less permeable โ less water reabsorbed โ large volume of dilute urine
- This is a negative feedback loop
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