Relation between Spherocytosis and High Oncotic pressure??? - USMLE Forums
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Old 02-11-2013
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RBC Relation between Spherocytosis and High Oncotic pressure???

Need to understand this :
the first important diagnosis for Hereditary spherocytosis ( as mentioned in step up to medicine 3rd) is :
"l. RBC osmotic fragility to hypotonic saline
a. Tests the ability of RBCs to swell in a graded series of hypotonic solutions
b. Because of their shape, spherocytes tolerate less swelling before they rupture; thus,
they are osmotically fragile. The RBCs undergo lysis at a higher (thus earlier)
oncotic pressure."

I want to understand what this phrase means "The RBCs undergo lysis at a higher (thus earlier) oncotic pressure."
  • Do RBCs refer to spherocytes ??
  • "(thus earlier)" earlier of what??
  • What is the relation of this to high oncotic pressure and how that can be achieved in the investigation??

Thanks
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Old 02-11-2013
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The RBCs in spherocytosis are spherocytes due to spectrin/ankyrin defect/deficiency.
Spherocytes have less membrane & therefore less capacity to expand when placed in a hypotonic solution. Due to this, they attain higher intracellular pressure earlier as compared to normal biconcave RBCs.
Fluid tens to go from areas of low solute concentration to higher concentration. So, when placed in a hypotonic solution the fluid moves from outside to inside of spherocytes due to osmosis thereby creating high intracellular presuure. Same process happens when you place normal RBCs in a hypotonic solution but they can tolerate it better due to their higher capacity to expand.
As a crude analogy, think of them as a completely filled balloon (spherocytes) as compared to partially filled balloon (normal RBCs). If you try to fill the completely filled ballon it will burst earlier due to attaining high pressure first as compared to a partially filled ballon.

Hope this helps.
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Old 02-11-2013
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Info Thank u very much Dr Novobiocin but .....

Quote:
Originally Posted by Novobiocin View Post
The RBCs in spherocytosis are spherocytes due to spectrin/ankyrin defect/deficiency.
Spherocytes have less membrane & therefore less capacity to expand when placed in a hypotonic solution. Due to this, they attain higher intracellular pressure earlier as compared to normal biconcave RBCs.
Fluid tens to go from areas of low solute concentration to higher concentration. So, when placed in a hypotonic solution the fluid moves from outside to inside of spherocytes due to osmosis thereby creating high intracellular presuure. Same process happens when you place normal RBCs in a hypotonic solution but they can tolerate it better due to their higher capacity to expand.
As a crude analogy, think of them as a completely filled balloon (spherocytes) as compared to partially filled balloon (normal RBCs). If you try to fill the completely filled ballon it will burst earlier due to attaining high pressure first as compared to a partially filled ballon.

Hope this helps.

Thank u very much but I need to understand this sentence
"The RBCs undergo lysis at a higher (thus earlier) oncotic pressure."
from your explanation I think that RBCs in these sentence refer to Spherocytes as they lysis earlier??? ........but what means by high oncotic pressure (does it mean increase the tonicity of the solution or what)??? (sorry i have defect in these concepts as i prepare for step 2 ck first )....Thank u very much for your help
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Old 02-11-2013
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Please don't take it the wrong way but I strongly suggest doing the Step 1 first. That way you will build up these concepts and will not have any problems in understanding & using these basic concepts for Step 2 CK.
There is no advantage in doing Step 2 CK first unless someone has years of clinical experience.
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