Why do veins have higher velocity compared to capillaries? Are we saying fluids are moving fast?

By Crystal Bishop

You may wonder why blood in veins has a higher velocity than in capillaries.  That answer comes down to the work of the veins compared to the work of the capillaries.  When I say work, it is a type of force. Remember, force is the push or pull applied on an object. When force is applied to or on an object, and the object moves, as a result, we say “work” has been accomplished. Another way to look at “work” is it is an outcome or result of an action.

Regarding fluids, are we saying if it has velocity, it is moving fast?

When we say a fluid’s velocity, we do not indicate something is moving fast. Let me clarify, speed is the rate at which something can move.  Velocity expresses an object moving at a particular rate (speed) in a particular direction. 

The range of velocity and pressure can vary widely.  Using the previous example; you can breathe out very, very slowly, and although the velocity of the air exceeds the pressure, it may not be by much; but the velocity is still higher than the pressure.  The other example is while holding your pee, you slowly leak urine; momentarily, the velocity began exceeding the pressure until you force your muscles to stop.  As pressure builds, the urgency will continue until the leaking occurs again.  In both cases, the velocity only just exceeded the pressure. 

Simply put, the movement of these fluids through velocity is the body’s only way of relieving or lowering the pressure in the organs, it doesn’t necessarily mean it happens fast or quickly.