why does venodilation reduce venous return to the heart rather than increase it, according to Poiseuille law?

[Moxible]

I'm aware that veins act as a reservoir for blood but laws of flow should still hold and increasing the internal radius should reduce resistance and increase flow. Am I missing something here?

[TheVat]

Venodilation relaxes the smooth muscle in venous walls,  which makes them enlarge and act as larger reservoirs.  It's not a matter of reduced resistance,  it's that the volume of blood now returning to the heart is LESS,  because more blood is being held in "reserve" in veins.   When venodilation decreases,  it essentially means that the smooth muscles in vein walls are tightening and engaging as secondary pumps -- which steps up venous return. 

You may be interested to know (or already know)  that an extra liter of blood is in the veins, which means that rapid venoconstriction  can add that extra blood to the circulation in the case of an emergency, i. e. sudden blood loss and bp drop. 

Edited August 5, 2021 by TheVat
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