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Applying starling's law at the glomerulus?

  1. Jul 6, 2010 #1
    Hello everyone,

    Since filtration occurs at the glomerulus, starling's law can be applied. Now I'm very confused with this. I understand the concept but starling's law is only for water molecules right? Now this is my question if water moves out of the glomerulus to be filtered, do solutes dissolved in it move out as well or is it just the water? Thanks :smile:
     
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  3. Jul 6, 2010 #2
    I think it's the high pressure that squeezes stuff out.In addition to water, urea, ions and sugar pass into the Bowmans capsules.
     
  4. Jul 6, 2010 #3
    Thanks a lot for the reply :smile: In the definitions however they say these forces are for the movement of water. Also the proteins inside the capillary apply colloid osmotic pressure, colloid osmotic pressure only applies to water molecules right? So do the solutes come in because of their gradients, or when water comes in do they carry the solutes with them.
     
  5. Jul 6, 2010 #4
    Hello sameeralord.Basically the filtering system has three layers one of which is negatively charged and repels negatively charged molecules.The layers and slits are so constructed that anything with a diameter of less than about 4nm passes,through larger material being blocked.In answer to your question,therefore,I would say that the water carries the solutes with it.
    My knowledge of biology is extremely limited and I got the above information from a web site entitled PeteSmif.org.uk.Have a look.
     
  6. Jul 6, 2010 #5

    Andy Resnick

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    The driving force is blood pressure- that is correct. In the glomerulus, the podocytes which wrap around the capillary (foot processes) have slit diaphragms, which allow nearly all small molecules through into Bowman's capsule. IIRC, virii are too large to pass through the slit diaphragm.

    I don't have a reference handy, but the pressure jump across the slit diaphragm is fairly large, considering the area of the slit and the volume of fluid that passes through.

    Once in the nephron, the components of the ultrafiltrate are selectively resorbed into the blood. Different portions of the nephron do different things- for example, the descending limb of Henle is highly permeable to water but not salt, while the ascending limb is salt permeable and water impermeable. The nephron (well, all 2 million of them) controls the total amount of salt and water in your body. There's also feedback along the length of the nephron- the distal convoluted tubule, IIRC, communicates with the juxtaglomerular apparatus to control overall fluid volume within the nephron via the renin-angiotensin mechanism.
     
  7. Jul 6, 2010 #6

    Andy Resnick

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    It can be tricky to sort it all out; osmotic/oncotic pressure jumps are formally identical to hydrostatic pressure jumps, only in terms of solute concentration instead of fluid height. Even worse, selectively permeable membranes mean that there can be different driving forces on (say) sodium ions or calcium ions. Then, hteres issues of charge conservation- the Na-gluc transporter does not maintain charge neutrality, so there must be other channels which allow ions to 'complete the circuit'.

    The rule of thumb is "water follows sodium".
     
  8. Jul 6, 2010 #7
    First of all I'm very glad to recieve help from you on this matter :smile: However I think I'm unable to express my question properly. Ok now I read about starling law and everything there is for the movement of water. Now in the kidneys other solutes move out as well right, how are they affected by starling forces. I can understand how hydrostatic pressure would help them move out but I don't understand how colloid osmotic pressure has an effect on solutes. I understand water follows sodium. So bascially my question is should starling law be applied only to water or (water+solutes). So in the kidney does water move out due to starling forces and solutes just undergo simple diffusion or does movement of water due to starling forces carry solutes with it. So lets say if blood pressure was lowered would this only affect the movement of water or solutes as well. Thanks :smile:
     
  9. Jul 7, 2010 #8

    Andy Resnick

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    It's not that simple- ion transport in the kidney occurs by transporter proteins- this is not the same thing as a simple pore. Additionally, it's not so easy to relate blood pressure and transepithelial driving forces in the nephron.

    In the glomerulus, the pressure is about 60 mmHg below the blood, this promotes outward flow. However the capsular hydrostatic and plasma osmotic pressures oppose this flow by about 18 mmHg and 32 mmHg respectively- the net outward pressure jump across the slit diaphragm is about 10 mmHg.

    Within the nephron, many coordinated processes occur:

    http://phsgirard.org/Anatomy/ExcretorySystem/nephron.jpg

    Driving forces on ion transport at the molecular level result from the Nernst-Planck equation.
     
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