Medical What exactly is the difference between crystalloids and colloids?

  1. Hello guys,

    I'm bit confused here, hope you guys can help me out.

    1. Crystalloid- Ok they contain water soluble molecules. So eg is Saline (NaCl). Now book says if I infuse crystalloids then only 1/3 of of my infusion would remain in the intravascular compartment. Now my question is why is this? If I add lot of salt to intravascular compartment eg by giving saline, shouldn't I suck more water from extracellular compartment, why am I losing 2/3 instead.

    What I think: Is it because Saline contains like 0.9% Saline. So this is actually more like you are adding more water to the intravascular compartment rather than salts, so some water may leak out. Does this mean if I give 5% Saline solution more water would stay in the intravascular compartment than 0/9%.

    2. Colloid- Ok why are they called plasma expanders

    What I think: Ok they are big molecules, increasing colloid osmotic pressure and sucking water from extracellular environment into blood vessels. But crystalloids should also then be considered plasma expanders because add the end, they also increase the fluid even if it is not as great as colloids.

    Thanks a lot in advance. :smile:
  2. jcsd
  3. Normal saline contains roughly the same osmolarity of NaCl as blood. It is a crystalloid because it contains a water-soluble molecule.

    If you used a saline solution with a larger concentration of salt then it would act to induce a shift in fluid from the intracellular to the extracellular space due to the osmotic gradient (hypertonic saline is therefore effective at reducing intracranial pressure as it decreases brain water).

    Colloids contain larger molecules that are (for the most part) not water-soluble e.g. toothpaste (sorry for the non-medical example, I'm a chemical engineer ^_^).

    They are both volume expanders and plasma expanders. More information on the respective advantages / disadvantages of each can be found here:

    Hope this helps!
  4. Hey, Thanks for the reply :smile: However in this scenario if I add salt into intravascular compartment, wouldn't that make it more hypertonic, so shouldn't water from outside come in, instead. Why have you stated the opposite? Did I missunderstand something.

    EDIT: Oh wait are u saying? If I add salt to blood vessels, water from red blood cells for example would come out increasing intravascular compartment, then this excess water would go out of the intravascular compartment due to increase hydrostatic pressure inside the vessels, so you lose some water from crystalloids.
  5. The following is how I understand it. If I am wrong then someone please correct me.

    Colloids are iso-oncotic with blood plasma, which means that they do not affect the force causing fluid transfer from the interstitial fluid to the blood plasma. Giving someone a colloid solution will cause the plasma to expand, expanding the volume of the blood. Some hemodilution will occur and the increased hydrostatic pressure (the force causing fluid transfer from the blood plasma to the interstitial fluid) may cause some fluid to leak out into the interstitial and transcellular spaces, but for the most part with colloid solutions whatever you put in stays in the blood.

    Keeping it simple, isotonic crystalloids have roughly the same osmotic pressure as the extracellular fluid in general, but are NOT iso-oncotic with blood plasma. Therefore when you give someone a crystalloid solution the hydrostatic / osmotic pressure difference between the interstitial fluid and blood plasma is changed, and the replacement fluid will distribute between the two. Therefore crystalloid solutions require a larger volume of replacement solution to achieve the same degree of blood volume expansion achievable with a colloid solution, thus also increasing the degree of hemodilution.

    You can alter the amount of water within cells by using hypo/hypertonic solutions (i.e. my point about intracranial pressure). However I now realise that was a digression from your original question, so apologies if this caused any confusion. The main difference between colloids and crystalloids that I think you are concerned about is that colloids are iso-oncotic, whereas crystalloids are typically merely isotonic. For the physical differences, see my previous post.
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