"nitric oxide last week - it helps open blood vessels to prevent high blood pressure"
It's used for a number of control mechanisms in the body. AFAIK (it's mostly not my fields of expertise), there are 3 main mechanisms. One form of the enzyme eNOS (Nitric oxide synthetase ) is activated in blood vessels, I assume under some control, and converts the amino-acid arginine into nitric oxide (+citrulline) . This relaxes the blood vessels. Another NOS, iNOS generates NO by the same mechanism to generate an inflammatory reaction as part of the response to infection etc.
I think, also, that nitrates/nitrites in the diet can generate a background level of NO which may help blood vessel control.
Synthetic nitrites are used to produce a rapid rise in NO levels to alleviate angina symptoms.
But its nature - it modifies the activity of lots of proteins and has a number of recorded effects ( on insulin & airways for example) may mean there are plenty of unknown functions (good & bad)