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.
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"