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    I am having trouble learning the kidney. I know the structure and stuff, but I can't seem to understand/remember ultrafiltration and reabsorption.

    Could someone please explain it to me in simple language?
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    Urea, along with salt, water and glucose, etc., is extracted from the blood in the kidney by a process called ultrafiltration. Blood passing the top of the nephron is under high pressure, so fluid is forced through the sieve-like capillaries and into the capsule. This fluid is called the filtrate. It does not contain any blood cells or larger proteins, as they are too big to pass out of the capillaries and into the capsule.

    Much of what has been filtered out needs to be returned to the blood - they are too precious to lose - so the next process is called selective reabsorption.


    I got that from s-cool if that is any good - i thought it was a little more english than most !
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    Thank you, I will check out the website!!
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    ultrafiltration came up in jan04 cc heres the marking points for it (if it helps)

    endothelium of, blood capillaries / glomerulus;
    more / larger, gaps in / between, endothelial cells;
    basement membrane made of, collagen / glycoproteins;
    podocytes;
    large gaps between them / filtration slits;
    basement membrane, selective barrier / filter;
    prevents large proteins passing through;
    no cells pass through;
    one example of molecule which is filtered;
    high, blood / hydrostatic, pressure in capillary;
    afferent arteriole wider than efferent arteriole;
    lower (hydrostatic) pressure in renal capsule;
    water potential / AW, lower in glomerulus;
    ref to effective filtration pressure;
    AVP; e.g. glomerular filtrate is identical to blood plasma minus proteins filtration is by charge and size
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    Ultrafiltration refers to the filtration at the glomerulus occuring due to pressure. (get hold of a diagram)

    Proteins are too big to diffuse across from the capilaries into the renal capsule filtrate, so they remain in the blood increasing its solute potential (or alternatively, decreasing the water potential). This would NORMALLY cause water to move by osmosis down its water potential gradient from the filtrate/renal capsule into the blood.

    But in the capilaries, the afferent arteriole is wider than the efferent arteriole and this causes the water potential of the blood plasma (in the afferent) to become higher than that of the renal capsule. This pressure overcomes the concentration difference, and ultimately it's this pressure that causes water to move by osmosis from the capillaries into the renal capsule.
 
 
 
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Updated: June 19, 2005
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