The kidneys

ultra filtration, selective reabsorption, reabsorption of water,osmoregulation

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  • Created by: laura
  • Created on: 17-05-11 11:23


What? water, amino acids, glucose, urea and inorganic ions are filtered out of the blood into the bowmans capsule

How? pressure in the glomerulus is higher than in the bowmans capsule so fluid and small molecule is pushed into the bowmans capsule. Barriers between the bowmans capsule and the capillarys are adapted to allow ultrafiltration. (endothelium has narrow gaps, basement membrane consists of fine mesh, epithelial cells have a specialised shape)

Why? it leaves a low water potential in the blood which important to help reabsorb water at a later stage

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Selective Reabsorption

What? glucose , amino acids and some salts are removed from the filtrate and reabsorbed back into the blood


  • sodium ions, glucose and amino acid molecules are transported into the cell.
  • as glucose and amino acid concentrations increase inside the cell, they can difuse out of the opposite side of the cell into the tissue fluid
  • these substances then diffuse into the blood from the tissue fluid and are carried away.

Why? so that useful substances aren't excreted and can be re-used elswhere in the body.  

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Water Reabsorption

What ? water is reabsorbed from collecting duct


  • As fluid moves deeper into medulla its water potential becomes lower because water is lost via osmosis and sodium and chloride ions diffuse into the tubule.
  • As the fluid ascends back up towards the cortex, its water potential becomes higher because sodim and chloride ions diffuse out and are activly transported out of the tubule, the wall of the ascending limb is impermeable to water.
  • Fluid has high WP in collecting duct and so water moves out by osmosis. It then enters the blood cappillaries by osmosis and is carried away.

Why? To prevent the body loosing too much water as the individual would become dehydrated.

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what? its the control of water and salt levels in the body.


  • decrease in water potential=more ADH is released = walls of collecting duct become more permeable = more water is reabsorbed into blood and less urine is produced.
  • inctrease in water potential=less ADH released= walls of collecting duct become less permeable=less water is reabsorbed into blood and so more urine is produced.

why? the water balance between cells and the surrounding fluids must be maintained to prevent problems with osmosis.

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