From a evolutionary standpoint, vertebrates, such as humans, developed under standard gravity. However, the cardiovascular systems of humans and primates can rapidly adapt to hypergravitational conditions. Given that hyper gravity is encountered only under extreme conditions, how can this be that +Gz naive organisms can rapidly adapt rapidly to a completely non-physiological condition, such as hyper G. Is this reaction a built in reflex arc? How do invertebrates respond to hyper G?
The point is that the reflexes you refer to are "negative feedback systems"- i.e. they continue to be ramped up until the signal to be activated is diminished.
There reflexes are the same ones that stop you passing out when you standup quickly!
I.e. +Gs= venous pooling= reduced venous return= reduced cardiac output= reduced BP=activated baroreceptor= venous contraction= reduced venous pooling--- back to start.... and around and around it goes.Until it reaches the asymptote of the baroreceptor...
Inverts-no idea!
to answer your question with more questions: from haemodynamic adaptation standpoint...would the receptors simply increase its numbers or its plasticity , please see http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120016784.pdf
OR would they be RE-located to different areas of vertebrate's circulation under different G's that the organism would experience for not short but rather extended period of time? I think this would be evolutionary slow adaptation for vertebrate.... but possible.
The compensatory mechanisms that allow humans to function in high +Gz are also stimulated by more mundane experiences, such as changing position, emerging from immersion in water, or responding to the dehydration associated with gastrointestinal illness. Yes, the reaction is a built in reflex arc, involving baroreceptors in the carotids and aortic arch. That reflex serves to maintain perfusion of the brain.
The Baroreflex controls the cardiovasular response to altered gravity, initially due to the cephalic fluid shift in microgravity. As well as examples shown above this would have been useful to our ancestors doing activities such as hanging upside down in trees to pick fruit, or to fetch honey. If every time our ancestors tried to eat nutrient and energy rich food like this they passed out and fell, we would have learnt to avoid trees, so would have had less access to calories or safe hiding spaces. So while the baroreflex response did not evolve for space, it conveniently allows us to function well in the space environment because we experienced similar problems during our evolution
If you look in the Erasmus Experiment Archive: http://eea.spaceflight.esa.int/portal/
there are lots of experiments dealing with autonomic cardiovascular control.
On the semantic viewpoint a "reflex" "reflects" a reaction to a stimulus...a lot ca be considered to be somehow related, it is usually also considered that little adjustments can be added to the "reflected" action it is more a all or nothing answer. So I see no objection to add your questioned mechanism to the big family of reflexes.
All the best!