Venoconstriction raises pressure inside a vein and raises flow as well. What explains the rise in flow?

1. Constricting an artery narrows its lumen, raising resistance and pressure while flow falls. 2. A vein behaves differently because its wall is thin and its resting lumen is irregular and flattened. 3. When venous smooth muscle contracts, that irregular lumen becomes more rounded. 4. A rounder lumen presents less surface area to the blood, so friction and resistance fall. 5. Pressure rises and resistance falls together, so venous flow increases. 6. Since atrial pressure approaches zero during relaxation, that extra flow becomes venous return and raises preload. 7. Viscosity is set by formed elements and plasma proteins, so constriction cannot thin the blood. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 20 "The Cardiovascular System: Blood Vessels and Circulation", section 20.2 Blood Flow, Blood Pressure, and Resistance_