What causes blood velocity to increase when blood goes from the capillaries to the veins?

Question

What causes blood velocity to increase when blood goes from the capillaries to the veins?

Answer 1

energy, mass and momentum are conserved even as the vessels norrow, so to make up for the decreased area, the blood picks up speed. it can be shown by the equation A1V1=A2V2

A is area

V is velocity.

the more you narrow the vessel the faster it is going to go. It is kinda of like when you have a hose, and then you put your finger to restrict the flow, the water shoots out at a much greater velocity and distance.

Answer 2

Capillaries are narrow, veins have a much greater diameter.  Think of traffic: a fixed number of cars moves slowly on narrow side street but can move much more rapidly on wider highways.  In addition, there is greater "frictional" interaction between blood cells and the walls of capillaries than between these cells and the walls of veins.