Etorphine

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One thing I have never been able to understand is why petechiae occur in various disease processes. You always hear about it associated with diseases like Hemolytic Uremic syndrome, Leukemia, thrombocytopenic diseases, but why? What is going on at the microscopic level, and what is the linear sequence of events?
 
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micro hemorragic surfacce vessels. The reason this occurs can be different from disease to disease and i think in some diseases it is unknown.
 

type b pharmD

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One thing I have never been able to understand is why petechiae occur in various disease processes. You always hear about it associated with diseases like Hemolytic Uremic syndrome, Leukemia, thrombocytopenic diseases, but why? What is going on at the microscopic level, and what is the linear sequence of events?
Most are easily explainable through usual analysis of the physiological imbalance. Immune mediated attack on microvascular surfaces, inappropriate levels of clotting factors, changes in hemodynamics, even foreign particle deposition.

I find aHUS to be much more interesting than standard uremic hemolysis.. why don't you pick up a basic pathophys book and reason out these mechanisms ? They are not rocket science.. a good deal is known about the physiology of the microvasculature as well as clotting (or lack therof)
 

Anastomoses

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Micro hemmorhage; usually due to platelet insufficiency. In Leukemia, the cancer cells suck up all the nutrients squeezing the platelets out for nutrients along with other WBCs --> again platelet insufficiency.
 

Apoplexy__

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One thing I have never been able to understand is why petechiae occur in various disease processes. You always hear about it associated with diseases like Hemolytic Uremic syndrome, Leukemia, thrombocytopenic diseases, but why? What is going on at the microscopic level, and what is the linear sequence of events?
The previous posters already explained your question, but I'd just like to throw in that Goljan audio does a fantastic job of explaining this whole concept - when you see petechiae, what the associated symptoms of platelet insufficiency are, how this constellation of symptoms differs from clotting factor-based hypocoaguability, etc.