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Difference between hydrostatic/osmotic pressure

Discussion in 'MCAT Discussions' started by axp107, 08.13.07.

  1. axp107

    axp107 UCLA 09': Italian Pryde

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    Can anyone clarify the difference? I got a kidney passage where I confused the two =(
  2. xlr8

    xlr8 New Member

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    both are present everywhere, but i think it matters the most in blood vessels:

    hydrostatic: the outward radial pressure in the blood vessels, generated in most part by the heart, wants to force the stuff out of the vessels. tends to be variable throughout the body (greatest in arteries)

    osmotic: the "inward" pressure, the tendency to allow water in. remains relatively constant throughout the body (unless you have a disorder where you have more solutes in your blood, ie more albumin or something. where it will then change the osmotic pressure)
    osmotic pressure = (Molarity)(R)(T)

    hydrostatic and osmotic are about equal in the capillaries, and osmotic usually overtakes in the venous end.
  3. halekulani

    halekulani Member

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    hydrostatic pressure - drives fluid out
    osmotic/oncotic pressure - solute/protein concentration that draws fluid in

    so if you have a capillary, high hydrostatic pressure drives fluid out (filtration) into the tissue. so towards the end of the capillary, you should have a higher oncotic pressure. when your oncotic pressure is high at the vein (after moving fluid into surrounding tissue you have lots of protein left), and is greater than your hydrostatic pressure, you start to reabsorb fluid.

    dang xlr8 beat me
  4. DieselD

    DieselD ASA Member

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    sweet...nice refresher... Thanks
  5. axp107

    axp107 UCLA 09': Italian Pryde

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    it doesn't only have to be fluids right? you're saying osmotic takes over near the "vein" end b/c CO2 is being "pulled" in right?
  6. habman6

    habman6

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    Osmotic deals with the solvent, not the solute (i.e. CO2). CO2 moves due to passive diffusion down concentration gradients.
  7. xlr8

    xlr8 New Member

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    its generally fluids, but there can be exceptions, like in cases where vascular permeability increases and proteins/solutes go out (ie inflammation). if for instance solutes go out of the vessels into tissue, the tissue will have more solutes, so MRT (osmotic pressure) increases in the tissues, but decreases in the blood (less solute and less MRT).

    osmotic pressure generally takes over because the hydrostatic force from the heart basically diminishes by the time it gets to the veins, they have those things that close to prevent black flow and use skele muscles to get back to the heart.

    The CO2 being pulled in has more to do with RBCs, in that the RBCs have no oxygen (since they just dropped it off) and pick up CO2 (in the form of HCO3- in exchange for Cl-), i think its called the haldane effect.

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