A level Biology B · Edexcel 9BI0 · Topic 9C
The kidney and osmoregulation
Key points
- Each kidney has about a million nephrons.
- Ultrafiltration in the glomerulus: high pressure forces water, glucose, ions and urea into Bowman's capsule; blood cells and large proteins stay behind.
- Selective reabsorption in the proximal convoluted tubule: all glucose and much water and sodium return to the blood by co-transport and osmosis; cells have microvilli and many mitochondria.
- The loop of Henle creates a high solute concentration (low water potential) in the medulla, so water can be reabsorbed from the collecting duct.
- Osmoregulation: osmoreceptors in the hypothalamus detect low water potential; the posterior pituitary releases ADH, which adds aquaporins to the collecting duct so more water is reabsorbed and urine is more concentrated.
- Animals in dry habitats have longer loops of Henle.
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The kidneys remove waste such as urea from the blood and keep its water potential constant. Each kidney does this through about a million nephrons, using filtration, selective reabsorption and a clever concentrating mechanism.
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