You can't get an erection in space.
I expect that's not true.
If the normal blood flow and pressure through my body required to keep me alive still works, then I can't see why other functions of blood pressure would not also continue to work.
All you lose is gravity, and the loss of h-rho-g hydrostatic head.
But if you are on Earth, lying flat on a bed, the h term in the equation becomes zero, and so g becomes irrelevant.
The pump ( heart ) still provides sufficient head for the purpose.
Now, if you were thrown out indo deep space ( perhaps after listening to Vogon poetry ), the pressure differential may indeed enhance your erection as in the manner of the manhood-enhancing vacuum pumps offered on the Internet (or is that just my Internet? ).
Anyways, enough of this...
Your blood pressure is much lower in zero-g. That's one factor. And the flow of blood and other fluids is significantly altered. Think about it: at 1G your heart needs to work against gravity to perfuse your brain. It doesn't need to work nearly so hard in zero G. During the first few days in space astronauts get facial and eye oedema, as the fluid pumping systems (you have more than one, and they interact) adapt to zero G. All sorts of other stuff happens too that I can't remember.
No, I don't agree.
( My background is in an industry where we design hydraulic systems to work on the surface and also and extreme depths of 20k psi, so there may indeed be differences I have not appreciated. )
At 1G whilst standing up, you have a closed circuit of fluid going up one way from the pump, down to the bottom, and then back up to the pump.
A loop of fluid in this system is entirely in equilibrium.
The hydrostatic head on the down leg exactly equals the hydrostatic head on the up leg.
So H-rho-g on one side of the system equals h-rho-g on the other,
The pump really does not have to work too hard: it does not need to work against gravity; because gravity both helps and hinders in equal measure. It helps by providing a down-force on one side of the pump, and hinders on the up-ward suction side of the pump.
So lying down on the job makes no difference.
And for the same reason, I think zero G is the same.
Someone should tweet that astronaut chap who was in the space thing and ask him.