Norepinephrine Spillover
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Background 
One suggested reason for why the blood pressure control system has changed is that it is underactive in space. The autonomic nervous system releases chemical messengers when it operates (norepinephrine is one). There has been some disputed evidence from previous space flights that the levels of norepinephrine are lowered in space and that this indicates a low level of activity in the blood pressure control system. Looking only at the blood levels of norepinephrine, however, can lead to false conclusions, since the blood level depends not only on how much is released, but also on how fast it is removed from the circulation. 

Methodology 
To determine the rate at which norepinephrine is removed from the system, Neurolab scientists will use the "spillover" method. In this method the rate and concentrations are measured as the agent enters and exits the system. Using a "spillover" formula, an accurate reading of the system release rate can be found. 

 

Norepinephrine Spillover Formula

 
    When the incoming rate([NE]1) multiplied by the  
    incoming concentration(F1) equals the outgoing  
    rate([NE]2) multiplied by the outgoing  
    concentration(F2), then the outgoing rate equals  
    the system rate. This rate is known as "clearance"